Patent Publication Number: US-11656985-B2

Title: External memory as an extension to local primary memory

Description:
This application is a non-provisional of, and claims priority under 35 USC § 119(e) to, U.S. provisional application 62/968,394, filed Jan. 31, 2020, the entire contents of which are incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     This application relates to memory and, in particular, to external memory. 
     2. Related Art 
     Under some circumstances, computing systems may encounter a scenario where memory usage nearly equals, equals, or exceeds available local primary memory. Such computing systems suffer from a variety of drawbacks, limitations, and disadvantages. Accordingly, there is a need for inventive systems, methods, components, and apparatuses described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views. 
         FIG.  1    illustrates a hardware diagram of an example external memory system; 
         FIG.  2    illustrates an example memory appliance; 
         FIG.  3    illustrates an example client; 
         FIG.  4    illustrates an example management server; 
         FIG.  5 A  and  FIG.  5 B  illustrate an example scenario of a request to migrate a region; 
         FIG.  6    illustrates an example external memory allocation system; 
         FIG.  7    illustrates an example flowchart for creation of an external memory allocation; 
         FIG.  8    illustrates an example of memory-allocation data structure(s); 
         FIG.  9 A ,  FIG.  9 B ,  FIG.  9 C , and  FIG.  9 D  illustrate an example scenario of coordinated allocation of external memory; 
         FIG.  10    illustrates an example flowchart for selecting between multiple types of memory in response to a request to allocate memory; 
         FIG.  11    illustrates an example flowchart for converting anonymous memory to file-backed memory; 
         FIG.  12    illustrates an example flowchart for converting file-backed memory to anonymous memory; 
         FIG.  13 A  and  FIG.  13 B  illustrate example flow diagrams for handling a page fault for portions of an address space and/or for reclaiming portions of local primary memory using external primary memory; 
         FIG.  14 A  and  FIG.  14 B  illustrate example mappings and/or associations between address spaces, file data, local primary memory, and/or external primary memory for example systems in which some portions of an address space are mapped to local primary memory and/or some portions of the address space are associated with external primary memory; 
         FIG.  15    illustrates an example flow diagram for relocating one or more portions of external primary memory; 
         FIG.  16    illustrates an example flowchart for migrating an application logic from a first client to a second client; 
         FIG.  17    illustrates a memory architecture diagram of an example system providing multiple tiers of memory; and 
         FIG.  18    illustrates a schematic diagram of an example of the system in which a type of memory to allocate is selected from a group of memory types based on an available memory determination. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a technical solution to solve a technical problem of providing scalable primary memory to a computing system. The primary memory may scale on demand. The primary memory may be external to the computing system. Further, a technical solution is described to solve a technical problem of efficiently using available primary memory capacity. Various other technical problems and their respective technical solutions are also provided and will be evident to persons skilled in the art. For example, the technical solution may enable multiple clients to share a single region and/or external memory allocation when using a memory allocation interface. Alternatively or in addition, the technical solution may enable applications that use a memory allocation interface to be migrated from one physical machine to another, without losing metadata related to allocated portions. 
     For example, it may be beneficial to provide primary memory to a local machine from an aggregated “pool” of memory, which may be referred to as a ‘memory pool’. The memory pool may be external to the local machine. The memory pool may involve multiple memory appliances, and the memory pool may scale to an infinite or arbitrarily large number of memory appliances without performance irregularities due to the scaling. The technical solutions described may enable an installation, such as a server cluster, or an administrator to provision primary memory to servers or persons, associated with the installation, with dynamic policies like quotas. Further, the technical solutions described may enable dynamic allocation of memory by applications from the memory pool on demand, whenever needed. The technical solutions described may further enable primary memory of a local machine, such as a single server to balloon to any size needed and shrink back to original size when the larger memory capacity is no longer needed, irrespective of the original memory capacity of the server and substantially without a limit on how large the memory pool may become. 
       FIG.  1    illustrates a hardware diagram of an example external memory system  100 . The external memory system may include a memory appliance  110 , a management server  120 , a client  130 , and one or more interconnects  140 . The external memory system may include more, fewer, or different elements. For example, the external memory system may include multiple clients, multiple memory appliances, and/or multiple management servers. Alternatively, the external memory system may include just the client, just the memory appliance, and/or just the management server. 
     The memory appliance  110  may include memory that may be externally allocatable as primary memory. Henceforth, throughout this disclosure, unless specified otherwise, “memory” refers to primary memory. The management server  120  may be a memory pool manager, responsible to allocate and/or manipulate memory allocations for the client  130  using the memory appliance  110 . The client  130  may be a machine or a device requesting external memory. The client  130  may contain local memory that operates as the primary memory of the client  130 . However, the external memory allocation may be requested by the client to scale the capacity of the primary memory available locally. Alternatively, or in addition, the client  130  may operate the locally available primary memory as a cache memory when accessing the externally allocated memory from the memory appliance  110 . For example, cache memory may be used by the client to reduce average time to access data from the externally allocated memory. The locally available primary memory may be faster than the externally allocated memory and may be used to store copies of data from frequently used memory locations of the externally allocated memory. For example, the client may read data from or write data to a location in the externally allocated memory. The client may first check whether a copy of the data is in the cache memory, such as the locally available memory. If so, the client may read the data from or write the data to the cache memory, which may be faster than reading from or writing to the externally allocated memory. 
     The memory appliance  110 , the management server  120 , and the client  130  may communicate with each other over the interconnects  140 . The communication may be unidirectional or bi-directional. An interconnect may electrically couple the memory appliance  110 , the management server  120 , and/or the client  130 . Each of the interconnects  140  may include a physical component that transports signals between two or more devices. For example, an interconnect may be a cable, a wire, a parallel bus, a serial bus, a network, a switched fabric, a wireless link, a point to point network, or any combination of components that transport signals between devices. Alternatively or in addition, the memory appliance  110 , the management server  120 , and the client  130  may communicate over a communication network, such as a switched fabric, a Storage Area Network (SAN), an InfiniBand network, a Local Area Network (LAN), a Wireless Local Area Network (WLAN), a Personal Area Network (PAN), a Wide Area Network (WAN), a circuit switched network, a packet switched network, a telecommunication network or any other now known or later developed communication network. The communication network, or simply “network”, may enable a device to communicate with components of other external devices, unlike buses that only enable communication with components within and/or plugged into the device itself. Thus, a request for primary memory made by an application executing on the client  130  may be sent over the interconnect  140 , such as the network. The request may be sent to devices external to the client  130 , such as the management server  120  and/or the memory appliances  110 . In response to the request, the application that made the request may be allocated memory from memories of one or more memory appliances that are external to the client  130 , instead of being allocated a portion of memory locally available inside the client  130  itself. 
     The management server  120  may dynamically allocate and/or manipulate external memory allocations for the client  130 . An external memory allocation may reference one or more regions in the memory appliance  110 . The management server  120  may allocate and/or manipulate the regions in the memory appliance  110  using region access logic requests. The client  130  may allocate and/or manipulate external memory allocations and/or regions using allocation logic requests. 
     Multiple memory appliances may be “pooled” to create a dynamically allocatable, or allocable, external memory pool. For example, new memory appliances may be discovered, or as they become available, memory of, or within, the new memory appliances may be made part of the memory pool. The memory pool may be a logical construct. The memory pool may be multiple memory appliances known to and/or associated with the management server  120 . The memory appliances involved in the memory pool may not know about each other. As additional memory appliances are discovered, the memory of the memory appliances may be added to the memory pool, in other words, the portions of the memory of the memory appliances is made available for use by the requesting client  130 . The client  130  may be able to request dynamically allocatable external memory from the memory pool which may be available for use, even though the external memory exists on other machines, unknown to the client  130 . The client  130 , requesting memory, at time of requesting the memory, may be unaware of the size of the memory pool or other characteristics related to configuration of the memory pool. The memory pool may increase or decrease at any time without a service interruption of any type to the memory consumers, such as the machines requesting memory. 
     The external memory allocations may span multiple memory appliances. Thus, the external memory system  100  makes available memory capacity, larger than what may be possible to fit into the requesting client  130 , or a single memory appliance  110 , or a single server. The memory capacity made available may be unlimited since any number of memory appliances may be part of the memory pool. The memory pool may be expanded based on various conditions being met. For example, the maximally price-performant memory available may be selected to grow the memory pool in a maximally cost-efficient manner. Alternatively, or in addition, memory appliances may be added at any moment to extend the capacity and performance of the aggregate pool, irrespective of characteristics of the memory appliances. In contrast, the individual client  130 , such as a server computer, may be limited in physical and local memory capacity, and moreover, in order to achieve the largest memory capacity, expensive memory may have to be used or installed in the individual client  130  absent dynamically allocatable external memory. 
     Instead, with dynamically allocatable external memory, such as the memory pool, one no longer needs to buy expensive large servers with large memory capacity. One may instead buy smaller more energy-efficient and cost-effective servers and extend their memory capacity, on demand, by using dynamically allocatable external memory. 
     The memory pool may be managed by the management server  120 . The management server  120 , using various components, may provision external primary memory to the client  130  or multiple clients that request external memory allocation. The memory pool manager may provision external memory to different clients at different times according to different policies, contracts, service level agreements (SLAs), performance loads, temporary or permanent needs, or any other factors. 
     For example, the client  130  may be server cluster. By using external memory allocation and provisioning, the server cluster need not require servers to have sufficient pre-existing local memory in order to process all anticipated loads. A typical approach to have each individual server to have full capacity memory leads to over-purchasing memory for all servers in order to satisfy exceptional cases needed by some servers, some of the time. Instead, with external memory, the server cluster may provision portions of external memory where and when needed, thereby saving money, space, and energy, by providing on-demand memory to any capacity. The server cluster may even support memory capacities impossible to physically fit into a single machine. 
     In another example, external memory may be dynamically allocated according to performance ratings of the external memory. For example, higher-performance external memory may be provisioned for some purposes, and/or lower-performance, but larger capacity and/or lower cost, external memory for other purposes. 
     The memory pool may provide dynamic memory allocation so that the client  130  may request to receive external memory, and when the external memory is no longer needed, the client  130  may release the external memory back to the memory pool. The dynamic memory allocation may enable the client  130  to allocate a provisioned amount of external memory for various purposes on the client  130  at various times, on-the-fly, according to client-logic needs rather than based on an installation policy, or local, internal memory of a particular server. 
     The client  130  may access the dynamically allocatable external memory through one or more of a variety of interfaces. The different interfaces to access the external memory may vary the lowest level addressing used to address the external memory. The client  130  may be provided with different sub-interfaces for each respective access interface. For example, the access interface(s) may provide physical mapping, programmatic APIs, or any other application-specific interface, to use the external memory so as to solve a multitude of diverse problems in optimal ways for every case. The different access interfaces may even be employed at the same time, and even against the same external memory allocation. 
     Depending upon the interface used, external memory operations may not be constrained to memory page size. For some interfaces, external memory operations may be as small as a single byte or character and scale to any degree. 
     In an example, the dynamically allocatable external memory may enable multiple clients to share an external memory allocation. The multiple clients, in this case, may access and/or operate on the data in the shared external memory allocation at the same time. Thus, external and scalable shared memory may be provided to the multiple clients concurrently. 
     One or more clients may be logically grouped together and/or may be operated upon as a group. A group of one or more clients may be considered a client group. Accordingly, actions described throughout this disclosure as being performed upon and/or by one or more clients may alternatively or in addition be performed upon and/or by one or more client groups. 
     As described throughout this disclosure, external memory operations may be carried out via direct communication, referred to as a client-side memory access, between the client  130  and the memory appliance  110  that is part of the memory pool. The client-side memory access provides a consistent low latency, such as at least one of: one round-trip time, switching time, and/or communication interface processing time. The client-side memory access also provides determinacy, or in other words a predictable performance, such as a determinate amount of time for a given memory operation to be performed. Thus, by using the client-side memory access, the dynamically allocatable external memory provides a high level of determinacy and consistent performance scaling even as more memory appliances and external memory clients are deployed and/or used for dynamic load balancing, aggregation, and/or re-aggregation. 
     Throughout this text, the terms hypervisor, operating system-level virtualization logic, container hosting logic, jail hosting logic, and/or zone hosting logic may be used interchangeably to refer to a portion of the client logic responsible for providing interfaces and/or abstractions to facilitate sharing access to the physical resources of the client with one or more application logics. Also, the terms virtual machine, operating system-level virtualization, container, jail, and/or zone may be used interchangeably to refer to the portion of the application logic which utilizes the interfaces and/or abstractions provided by the client logic. The virtual machine, operating system-level virtualization, container, jail, and/or zone are examples of a virtualization instance. The hypervisor and the operating system-level virtualization logic are examples of virtualization logic that may execute one or more virtualization instances. 
     Dynamically allocatable external memory may also be persistent, meaning the data stored in the external memory is durable over time. This extends the memory paradigm to include the persistence aspects of external storage while retaining the performance of memory. This provides performance of memory with conveniences of a storage paradigm. 
       FIG.  2    illustrates the example memory appliance  110 . By way of example, the system  100  for dynamically allocatable external memory may store data of one or more regions in one or more memory appliances. The memory appliance  110  may be a server, a device, an embedded system, a circuit, a chipset, an integrated circuit, a field programmable gate array (FPGA), an application-specific integrated circuit, a virtual machine, a virtualization instance, a container, a jail, a zone, an operating system, a kernel, a device driver, a device firmware, a hypervisor service, a cloud computing interface, and/or any other hardware, software, and/or firmware entity which may perform the same functions as described. The memory appliance  110  may include a memory  210 , a memory controller  220 , a communication interface  230 , a processor  240 , a storage controller  250 , and a backing store  260 . In other examples, the memory appliance may contain different elements. For example, in another example, the memory appliance  110  may not include the storage controller  250  and the backing store  260 . The memory  210  may further include a region access logic  212 , one or more regions  214 , region metadata  215 , and an observer logic  218 . The observer logic  218  may not be present in other example memory  210 . The region access logic  212  and/or the observer logic  218  may be referred to as a region access unit and/or an observer unit respectively. The memory appliance may include more, fewer, or different elements. For example, the memory appliance  110  may include multiple backing stores, multiple storage controllers, multiple memories, multiple memory controllers, multiple processors, or any combination thereof. The memory appliance  110  may store data received over the one or more interconnects  140 . 
     The region access logic  212  in the memory appliance  110  may register the regions  214  or portions of the regions with one or more communication interfaces  230 . Alternatively, or in addition, the region access logic  212  may provide and/or control access to the region  214  by one or more clients and/or one or more management servers. A communication interface in the client  130  may provide client-side memory access to the memory  210  of the memory appliance  110 , to the regions  214 , and/or to portions of the regions in the memory appliance  110 . One or more interconnects or networks may transport data between the communication interface of the client  130  and the communication interface  230  of the memory appliance  110 . For example, the communication interfaces may be network interface controllers or host controller adaptors. 
     A client-side memory access may bypass a processor, such as a CPU (Central Processing Unit), at the client  130  and/or may otherwise facilitate the client  130  accessing the memory  210  on the memory appliance  110  without waiting for an action by the processor included in the client  130 , in the memory appliance, or both. For example, the client-side memory access may be based on the Remote Direct Memory Access (RDMA) protocol. The RDMA protocol may be carried over an InfiniBand interconnect, an iWARP interconnect, an RDMA over Converged Ethernet (RoCE) interconnect, an Aries interconnect, a Slingshot interconnect, and/or any other interconnect and/or combination of interconnects known now or later discovered. Alternatively, or in addition, the client-side memory access may be based on any other protocol and/or interconnect that may be used for accessing memory. A protocol that may be used for accessing memory may be a CPU protocol/interconnect, such as HyperTransport, Quick Path Interconnect (QPI), Ultra Path Interconnect (UPI), and/or Infinity Fabric. Alternatively, or in addition, a protocol that may be used for accessing memory may be a peripheral protocol/interconnect, such as Peripheral Component Interconnect (PCI), PCI Express, PCI-X, ISA, Gen-Z, and/or any other protocol/interconnect used to interface with peripherals and/or access memory. The communication interfaces may provide reliable delivery of messages and/or reliable execution of memory access operations, such as any memory access operation carried out when performing the client-side memory access. Alternatively, or in addition, delivery of messages and/or execution of memory access operations may be unreliable, such as when data is transported between the communication interfaces using the User Datagram Protocol (UDP). The client  130  may read, write, and/or perform other operations on the memory  210 , to the regions  214  within the memory  210 , and/or to portions of the regions using client-side memory access. In providing client-side memory access, the client  130  may transmit requests to perform memory access operations to the memory appliance  110 . In response, the memory appliance  110  may perform the memory access operations. Similar to as done by the storage device of U.S. patent application Ser. No. 13/036,544, filed Feb. 28, 2011, entitled “High performance data storage using observable client-side memory access” by Stabrawa, et al., which published as US Patent Application Publication US2012/0221803 A1, and which is hereby incorporated by reference, the memory appliance  110  may observe or otherwise identify the memory access operations. In response to identifying the memory access operations, the memory appliance  110  may, for example, copy the data of the region  214  to one or more backing stores  260  independently of performing the memory access operations on the memory  210 . A backing store  260  may include one or more persistent non-volatile storage media, such as flash memory, phase change memory, 3D XPoint memory, memristors, EEPROM, magnetic disk, tape, or some other media. The memory  210  and/or the backing store  260  (if included) may be subdivided into regions. 
     The memory appliance may be powered by a single power source, or by multiple power sources. Examples of the power source include a public utility, internal or external battery, an Uninterruptible Power Supply (UPS), a facility UPS, a generator, a solar panel, any other power source, or a combination of power sources. The memory appliance may detect the condition of the one or more power sources that power the storage device. 
     The memory  210  may be any memory or combination of memories, such as a solid state memory, a random access memory (RAM), a dynamic random access memory (DRAM), a static random access memory (SRAM), a flash memory, a read-only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), a phase change memory, 3D XPoint memory, a memristor memory, any type of memory configured in an address space addressable by the processor, or any combination thereof. The memory  210  may be volatile or non-volatile, or a combination of both. 
     The memory  210  may be a solid state memory. Solid state memory may include a device, or a combination of devices, that stores data, is constructed primarily from electrical conductors, semiconductors and insulators, and is considered not to have any moving mechanical parts. Solid state memory may be byte-addressable, word-addressable or block-addressable. For example, most dynamic RAM and some flash RAM may be byte-addressable or word-addressable. Flash RAM and other persistent types of RAM may be block-addressable. Solid state memory may be designed to connect to a memory controller, such as the memory controller  220  in the memory appliance  110 , via an interconnect bus  270 , such as the interconnect  270  in the memory appliance  110 . 
     Solid state memory may include random access memory that permits stored data to be read and/or written in any order (for example, at random). The term “random” refers to the fact that any piece of data may be returned and/or written within a constant time period, regardless of the physical location of the data and regardless of whether the data is related to a previously read or written piece of data. In contrast, storage devices such as magnetic or optical discs rely on the physical movement of the recording medium or a read/write head so that retrieval time varies based on the physical location of the next item read and write time varies based on the physical location of the next item written. Examples of solid state memory include, but are not limited to: DRAM, SRAM, NAND flash RAM, NOR flash RAM, V-NAND, Z-NAND, phase change memory (PRAM), 3D XPoint memory, EEPROM, FeRAM, MRAM, CBRAM, PRAM, SONOS, RRAM, Racetrack memory, NRAM, Millipede, T-RAM, Z-Ram, TTRAM, and/or any other randomly-accessible data storage medium known now or later discovered. 
     In contrast to solid state memory, solid state storage devices are systems or devices that package solid state memory with a specialized storage controller through which the packaged solid state memory may be accessed using a hardware interconnect that conforms to a standardized storage hardware interface. For example, solid state storage devices include, but are not limited to: flash memory drives that include Serial Advanced Technology Attachment (SATA) or Small Computer System Interface (SCSI) interfaces; Flash or DRAM drives that include SCSI over Fibre Channel interfaces; DRAM, Flash, and/or 3D XPoint memory drives that include NVMe interfaces; DRAM drives that include SATA or SCSI interfaces, USB (universal serial bus) flash drives with USB interfaces, and/or any other combination of solid state memory and storage controller known now or later discovered. 
     The memory  210  may include the region access logic  212 , the region  214 , and the region metadata  215 . In an example, each portion of the memory  210  that includes a corresponding one of the region access logic  212 , the region  214 , and the region metadata  215  may be of a different type than the other portions of the memory  210 . For example, the memory  210  may include a ROM and a solid state memory, where the ROM includes the region access logic  212 , and the solid state memory includes the region  214  and the region metadata  215 . The memory  210  may be controlled by the memory controller  220 . The memory  210  may include more, fewer, or different components. For example, the memory may include the observer logic  218 . 
     The processor  240  may be a general processor, a central processing unit (CPU), a server, a microcontroller, an application specific integrated circuit (ASIC), a digital signal processor, a field programmable gate array (FPGA), a digital circuit, an analog circuit, or any combination thereof. The processor  240  may include one or more devices operable to execute computer executable instructions or computer code embodied in the memory  210  or in other memory to perform features of the external memory system. For example, the processor  240  may execute computer executable instructions that are included in the observer logic  218  and the region access logic  212 . 
     The processor  240 , the memory controller  220 , and the one or more communication interfaces  230  may each be in communication with each other. Each one of the processor  240 , the memory controller  220 , and the one or more communication interfaces  230  may also be in communication with additional components, such as the storage controller  250 , and the backing store  260 . The communication between the components of the memory appliance  110  may be over an interconnect, a bus, a point-to-point connection, a switched fabric, a network, any other type of interconnect, or any combination of interconnects  270 . The communication may use any type of topology, including but not limited to a star, a mesh, a hypercube, a ring, a torus, a fat tree, a dragonfly, or any other type of topology known now or later discovered. Alternatively or in addition, any of the processor  240 , the memory  210 , the memory controller  220 , and/or the communication interface  230  may be logically or physically combined with each other or with other components, such as with the storage controller  250 , and/or the backing store  260 . 
     The memory controller  220  may include a hardware component that translates memory addresses specified by the processor  240  into the appropriate signaling to access corresponding locations in the memory  210 . The processor  240  may specify the address on the interconnect  270 . The processor  240 , the interconnect  270 , and the memory  210  may be directly or indirectly coupled to a common circuit board, such as a motherboard. In one example, the interconnect  270  may include an address bus that is used to specify a physical address, where the address bus includes a series of lines connecting two or more components. The memory controller  220  may, for example, also perform background processing tasks, such as periodically refreshing the contents of the memory  210 . In one example implementation, the memory controller  220  may be included in the processor  240 . 
     The one or more communication interfaces  230  may include any one or more physical interconnects used for data transfer. In particular, the one or more communication interfaces  230  may facilitate communication between the memory appliance  110  and the client  130 , between the memory appliance  110  and the management server  120 , between the memory appliance  110  and any other device, and/or between the management server  120  and any other device. The one or more communication interfaces  230  may communicate via the one or more interconnects. The one or more communication interfaces  230  may include a hardware component. In addition, the one or more communication interfaces  230  may include a software component. Examples of the communication interface include a Direct Memory Access (DMA) controller, an RDMA controller, a Network Interface Controller (NIC), an Ethernet controller, a Fibre Channel interface, an InfiniBand interface, a SATA interface, a SCSI interface, a USB interface, an Ethernet interface, an Aries interface, a Slingshot interface, an Omni-Path interface, a Gen-Z interface, a PCI Express interface, a PCI-X interface, a PCI interface, a Silicon Photonics interface, an optical communications interface, a wireless communications interface, and/or any other physical communication interface known now or later discovered. The one or more communication interfaces  230  may facilitate client-side memory access, as described throughout this disclosure. 
     The region  214  may be a configured area of the memory  210  that is accessible via a memory access protocol and/or storage protocol now known or later discovered. Storage protocols and memory access protocols are described elsewhere in this disclosure. The region  214  may be a logical region which maps a sequence of data blocks to corresponding memory locations in the memory  210 . Therefore, in addition to the data blocks themselves, the region  214  may include region information, such as a mapping of data blocks to memory locations or any other information about the data blocks. The data blocks of the region  214 , which may be configured by the region access logic  212 , may all be stored in the memory  210 . The volume information may or may not be included in the memory  210 . Accordingly, when the region  214  is said to be included in the memory  210 , at least the data blocks of the region  214  (the data stored in the region) are included in the memory  210 . Alternatively, or in addition, the volume information may be included in the region metadata  215 . 
     The region metadata  215  may include properties, configuration parameters, and/or access parameters related to the region  214 . 
     Properties may include the size of the region, references to portions within the memory allocated to the region  214 , and/or any other aspects describing the region  214 , its data, its memory, and/or its backing store. 
     Configuration parameters may include an indication of whether or not the region  214  may be persisted to the backing store  260 , an indication of what logic and/or method may be used to persist the region  214  to the backing store  260 , an identifier which may be used to locate persisted data related to the region  214 , and/or any other parameters used to specify how the region  214  may behave or be treated. 
     Access parameters may include a list of zero or more communication interfaces  230  included in the memory appliance  110  which may be used to access the region  214 , a list of zero or more clients, memory appliances, and/or management servers which are allowed to access the region  214 , a list of zero or more communication interfaces of clients, memory appliances, and/or management servers which are allowed to access the region  214 , a password which may be used to authenticate access to the region  214 , an encryption key which may be used to authenticate access to the region  214 , access permissions, and/or any other parameters used to specify how the region may be accessed. 
     Access permissions may include a mapping of access method to permissions granted and/or revoked. Access methods may include: via a specified communication interface  230  included in the memory appliance  110 ; via a specified communication interface of a client, memory appliance, and/or management server; by a specified client; by a specified memory appliance; by a specified management server; using a specified password; using a specified encryption key; and/or any other identifiable method used to access the region. 
     Permissions may include data read access, data write access, metadata read access, metadata write access, destroy access, and/or any other capability that may be selectively granted and/or revoked to a client, a memory appliance, and/or a management server. For example, the access parameters may include access permissions that indicate that a particular management server may read the metadata for the region  214 , but may not read and/or write the data of the region  214 . In a second example, the access parameters may include access permissions that indicate that a particular client may read the data for the region  214 , but may not write the data for the region  214 . 
     The storage controller  250  of the memory appliance  110 , of the management server  120 , and/or of the client  130  may include a component that facilitates storage operations to be performed on the backing store  260 . A storage operation may include reading from or writing to locations within the backing store  260 . The storage controller  250  may include a hardware component. Alternatively or in addition, the storage controller  250  may include a software component. 
     The backing store  260  of the memory appliance  110 , of the management server  120 , and/or of the client  130  may include an area of storage comprising one or more persistent media, including but not limited to flash memory, phase change memory, 3D XPoint memory, Memristors, EEPROM, magnetic disk, tape, or other media. The media in the backing store  260  may potentially be slower than the memory  210  on which the region  214  is stored. 
     The storage controller  250  and/or backing store  260  of the memory appliance  110  may be internal to the memory appliance  110 , a physically discrete component external to the memory appliance  110  and coupled to the backing store  260 , included in a second memory appliance or in a device different from the memory appliance  110 , included in the management server  120 , included in the client  130 , part of a server, part of a backup device, part of a storage device on a Storage Area Network, and/or part of some other externally attached persistent storage. Alternatively, or in addition, a region included in a different memory appliance may be used as the backing store for the memory appliance  110 . 
     One or more memory appliances may be logically grouped together and/or may be operated upon as a group. A group of one or more memory appliances may be considered a memory appliance group. Accordingly, actions described throughout this disclosure as being performed upon and/or by one or more memory appliances may alternatively or in addition be performed upon and/or by one or more memory appliance groups. 
       FIG.  3    illustrates the example client  130 . The client  130  may include a memory  310 , a memory controller  320 , a processor  340 , and a communication interface  330 , similar to the memory  210 , the processor  240 , the communication interface  230 , and the memory controller  220  of the memory appliance  110 . The client  130  may include more, fewer, or different components. For example, the client  130  may include a storage controller  350 , a backing store  360 , multiple storage controllers, multiple backing stores, multiple memories, multiple memory controllers, multiple processors, or any combination thereof. Alternatively, the client  130  may just include a process executed by the processor  340 . 
     The storage controller  350  and/or backing store  360  of the client  130  may be internal to the client  130 , a physically discrete device external to the client  130  that is coupled to the client  130 , included in a second client or in a device different from the client  130 , included in the management server  120 , included in the memory appliance  110 , part of a server, part of a backup device, part of a storage device on a Storage Area Network, and/or part of some other externally attached persistent storage. Alternatively, or in addition, the region  214  included in the memory appliance  110  may be used as the backing store  360  for the client  130 . 
     The memory  310  of the client  130  may include a client logic  312 . The memory  310  of the client  130  may include more, fewer, or different components. For example, the memory  310  of the client  130  may include an application logic  314 , the region metadata  215 , a data interface  316 , and/or external memory allocation metadata  318 . The processor  340  may execute computer executable instructions that are included in the client logic  312  and/or the application logic  314 . Alternatively, or in addition the client logic  312 , the application logic  314 , and/or the data interface  316  may be referred to as a client logic unit  312 , an application logic unit  314  and/or a data interface unit, respectively. The components of the client  130  may be in communication with each other over an interconnect  370 , similar to the interconnect  270  in the memory appliance  110  or over any other type of interconnect. 
     The application logic  314  and/or the client logic  312  may include a user application, an operating system, a kernel, a device driver, a device firmware, a virtual machine, a hypervisor, a container, a jail, a zone, a cloud computing interface, a circuit, a logical operating system partition, or any other logic that uses the services provided by the client logic  312 . A container, a jail, and a zone may be technologies that provide userspace isolation or compartmentalization. Any process in the container, the jail, or the zone may communicate only with processes that are in the same container, the same jail, or the same zone. The application logic  314  and/or the client logic  312  may be embedded in a chipset, an FPGA, an ASIC, a processor, or any other hardware device. 
       FIG.  4    illustrates the example management server  120 . The management server  120  may be a server, a device, an embedded system, a circuit, a chipset, an integrated circuit, a field programmable gate array (FPGA), an application-specific integrated circuit, a virtual machine, an operating system, a kernel, a device driver, a device firmware, a hypervisor service, a cloud computing interface, and/or any other hardware, software, and/or firmware entity which may perform the same functions as described. The management server  120  may include a memory  410 , a processor  440 , a communication interface  430 , and a memory controller  420 , similar to the memory  210 , the processor  240 , the communication interface  230 , and the memory controller  220  of the memory appliance  110 . The management server  120  may include more, fewer, or different components. For example, the management server may include a storage controller  450 , a backing store  460 , multiple storage controllers, multiple backing stores, multiple memories, multiple memory controllers, multiple processors, or any combination thereof. Alternatively, the management server  120  may just include a process executed by the processor  440 . 
     The storage controller  450  and/or backing store  460  of the management server  120  may be internal to the management server  120 , a physically discrete device external to the management server  120  that is coupled to the management server  120 , included in a second management server or in a device different from the management server  120 , included in the client  130 , included in the memory appliance  110 , part of a server, part of a backup device, part of a storage device on a Storage Area Network, and/or part of some other externally attached persistent storage. Alternatively, or in addition, the region  214  included in the memory appliance  110  may be used as the backing store  460  for the management server  120 . 
     The memory  410  of the management server  120  may include an allocation logic  412  and/or external memory allocation metadata  414 . The memory  410  of the management server  120  may include more, fewer, or different components. For example, the memory  410  of the management server  120  may include region metadata  215 . The processor  440  may execute computer executable instructions that are included in the allocation logic  412 . The allocation logic  412  may be referred to as an allocation logic unit. The components of the management server  120  may be in communication with each other over an interconnect  470 , such as the interconnect  270  in the memory appliance  110  or over any other type of interconnect. 
     During operation of the external memory system, the region access logic  212  may provide the client  130  and/or management server  120  with client-side memory access to the region  214 . Alternatively, or in addition, the region access logic  212  may provide other memory appliances with client-side memory access to the region  214 . Client-side memory access may include a memory access operation. A memory access operation may include, for example, a read memory operation or a write memory operation. The memory access operation may be performed by the memory appliance  110  in response to receiving a request from the client  130  and/or management server  120  at the communication interface  230  of the memory appliance  110 . The request may include, for example, a starting memory offset, a size of memory allocation, a starting memory location, a number of units of memory to access, or any other attribute relating to the requested memory access operation. The request may address the memory  210  on a block-addressable basis, a word-addressable basis, a byte-addressable basis, or on any other suitable unit of memory basis. 
     The region access logic  212  may register the region  214  with the communication interface  230  and/or with a device other than the memory appliance, such as with the client  130  and/or management server  120 . Alternatively or in addition, the region access logic  212  may determine a location or locations in the memory  210  of the memory appliance  110  where the region  214  is located. The region access logic  212  may register the location or locations with the communication interface  230  and/or with a device other than the memory appliance  110 , such as with the client  130  and/or management server  120 . 
     The region access logic  212  may control and/or specify how the region  214  may be accessed. For example, the region access logic  212  may control which regions are available on the memory appliance  110  and/or which operations may be performed. In one example, the region access logic  212  may control access based upon the current time, day, month or year; an identity or a location of the communication interface, an identity or a location of the client and/or management server; or some other attribute of the client  130 , the memory appliance  110 , the management server  120 , the interconnect  140 , or of the surrounding environment that is detectable by the region access logic  212 , such as the condition of the power source that powers the memory appliance  110 . Alternatively or in addition, the region access logic  212  may control access based on an authentication mechanism, including but not limited to a password, a key, biometrics, or a cryptographic authentication. 
     The region access logic  212  or the communication interface  230  may provide client-side memory access using any memory access protocol now known or later discovered. The memory access protocol may be any communication protocol used to transfer data between a memory in a first device, such as the memory  310  in the client  130 , and a memory in a second device, such as the memory  210  in the memory appliance  110 , where the data is transferred independently of CPU&#39;s in the first and second devices, such as the processor  340  in the client  130  and the processor  240  in the memory appliance  110 . Therefore, in examples where the first device includes an operating system, the data may be transferred from the memory of the first device to the memory of the second device without involvement of the operating system. Although instructions executed by the CPU may direct a hardware data controller to transfer the data from the memory of the first device to the memory of the second device, the actual transfer of the data between the memories may be completed without involvement of the CPU and, if the first device includes an operating system, without involvement of the operating system. The memory access protocol may describe, for example, a format of the request for the memory access operation to be performed on the memory in the second device or system. 
     The memory access protocol may be implemented, for example, using one or more hardware controllers, such as the communication interface  230  in the memory appliance  110  and the communication interface  330  in the client  130 . The memory access protocol and electrical characteristics of the hardware controller may be part of a common standard. Accordingly, the memory access protocol and electrical characteristics of the communication interfaces may be part of one standard. In one example, the access protocol may be the RDMA protocol implemented in the communication interfaces, where the memory access protocol and the communication interfaces conform to an InfiniBand standard. In a second example, the memory access protocol may be Internet Wide Area RDMA Protocol (iWARP), where iWARP is implemented in the communication interfaces, and where the communication interfaces conform to an iWARP standard. The iWARP standard, which is an Internet Engineering Task Force (IETF) protocol, is RDMA over TCP (Transport Control Protocol). In a third example, the memory access protocol may be RDMA over Converged Ethernet (RoCE), where RoCE is implemented in the communication interfaces, and where the communication interfaces conform to RoCE and Ethernet standards. In a third example, the memory access protocol may be a PCI bus-mastering protocol implemented in the communication interfaces, where the communication interfaces conform to a PCI standard. The memory access protocol, such as RDMA, may be layered directly over a transport protocol, such as TCP. 
     The region access logic  212 , the client logic  312 , and/or the allocation logic  412  may utilize multiple communication interfaces to provide resiliency against various communication failure modes. Communication failure modes may include failure of one or more communication interfaces, failure of one or more ports included in one or more communication interfaces, failure of a portion of the interconnect, such as an interconnect cable or interconnection fabric switch, and/or any other failure that may sever a network link between any two communication interfaces. The region access logic  212  may provide resiliency against communication failure modes using features of the communication interfaces. In a first example, the region access logic  212  may configure the communication interfaces to use an alternate path if a primary path experiences interconnect errors, such as using InfiniBand Automatic Path Migration. In a second example, the region access logic  212  may provide resiliency against communication failure modes by choosing communication modes that are by design resilient against interconnect errors, such as InfiniBand reliable connections, TCP connections, etc. Alternatively, or in addition, the region access logic  212  may provide resiliency against communication failure modes by establishing multiple active network links, and using one or more of the non-failing network links to provide connectivity. The multiple active network links may be selected to optimize redundancy versus failures. For example, the multiple network links may utilize different ports on different communication interfaces, such that a failure of one port or one communication interface may only cause one of the multiple active network links to fail. 
     In one or more examples, the region access logic  212  may additionally provide block-level access to the region  214  using any storage protocol now known or later discovered. A storage protocol may be any communications protocol used to transfer data between a block storage device or system, such as the memory appliance  110 , and a device or system, such as the client  130 , that stores data in, and/or retrieves data from, the block storage device or system. A storage protocol may be implemented, for example, using one or more software and/or hardware storage controllers. The storage protocol and electrical characteristics of the hardware storage controller may be part of a common standard. In one example, the storage protocol may be the universal serial bus mass storage device class (USB MSC or UMS), which is a set of computing communications protocols defined by the USB Implementers Forum that runs on a hardware bus, such as the interconnect, that conforms to the USB standard. In a second example, the storage protocol may be the SCSI command protocol. In a third example, the storage protocol may be the SATA protocol. Additional examples of the storage protocol include Serial Attached SCSI (SAS) and Internet Small Computer System Interface (iSCSI). Alternatively or in addition, the region access logic  212  may provide block-level access using any storage protocol that transfers data with a data transfer protocol, such as SCSI over Fibre Channel, SCSI RDMA Protocol (SRP) over Remote Direct Memory Access (RDMA), iSCSI over TCP/IP, or any other combination of storage protocol and data transfer protocol known now or discovered in the future. 
     Accessing the region  214  using the storage protocol may be slower than accessing the region  214  using the memory access protocol. In contrast to the memory access protocol, the processor  340  of the client  130  may interact with the storage controller  350  during the transfer of data to the block storage device  360  or system, where the storage controller implements the storage protocol. Therefore, the storage protocol is different from the memory access protocol. 
     By providing block-addressable client-side memory access and/or block-level access through the region access logic  212 , the memory appliance  110  may be considered, in an example implementation, a block storage device. A block storage device may also be referred to as a block device. A block device stores data in blocks of a predetermined size, such as 512 or 1024 bytes. The predetermined size may be configurable. A block device is accessed via a software and/or hardware storage controller and/or a communication interface, such as the communication interface  230 . Examples of other block devices include a disk drive having a spinning disk, a tape drive, a floppy disk drive, and a USB flash pen drive. 
     The region access logic  212  may subdivide the memory  210 , and/or the backing store  260  into one or more regions. Each one of the regions, such as the region  214  in the memory  210  of the memory appliance  110 , may be a configured area of storage that is accessible via any access protocol and/or storage protocol. Access protocols and storage protocols are described elsewhere in this disclosure. 
     The backing store  260  may include any block device. Examples of block devices include, but are not limited to, hard disks, CD-ROM drives, tape drives, solid state storage devices, flash drives, or any other mass storage device. 
     The client logic  312  and/or the allocation logic  412  may perform memory access operations on the region  214  in the memory  210  of the memory appliance  110  using client-side memory access over the memory access protocol. Alternatively or in addition, the client logic  312  and/or the allocation logic  412  may perform operations to discover the memory appliance  110  when connected, or to discover available regions that may be accessible on the memory appliance  110 . Alternatively or in addition, the client logic  312  and/or the allocation logic  412  may perform administration operations to modify attributes or metadata, such as the region metadata  215 , associated with the region  214 . The operations may include sending region access logic requests, described elsewhere in this disclosure. In an example, the client logic  312  and/or the allocation logic  412  may perform an administration operation to set a human readable label associated with the region  214 . In an example, the client logic  312  and/or the allocation logic  412  may perform an administration operation to change the operations that are available to the client  130  and/or to other clients. The administration operations may be used, for example, to coordinate shared access to the region by multiple clients. 
     The client logic  312 , the allocation logic  412 , and/or another logic may perform operations that communicate information to the observer logic  218  about a set of one or more memory access operations that were requested or that are to be requested by the client logic  312 , the allocation logic  412 , and/or another logic. For example, the client logic  312 , the allocation logic  412 , and/or another logic may transmit a notification message via the communication interface  330  of the client  130  and/or the communication interface  430  of the management server  120 . The observer logic  218  may receive the notification message via the communication interface  230  of the memory appliance  110 . The notification message may precede or follow the set of memory access operations requested by the client logic  312  and/or the allocation logic  412 . The notification message may identify attributes of the set of memory access operations. 
     Alternatively or in addition, the client logic  312 , the allocation logic  412 , and/or another logic may perform memory access operations that are directly observable or identified by the observer logic  218 . For example, the request to perform the memory access operation may include notification information, such as an RDMA write with immediate value operation. In addition to writing to the memory in the region  214 , the write with immediate value operation may cause the observer logic  218  to receive a notification that includes the immediate value specified by the client logic  312  and/or the allocation logic  412  in the RDMA write with immediate value operation. The value may include one or more attributes of the memory access operation. For example, the value may indicate what portion of the memory  210  is written to during the RDMA write with immediate value operation. Alternatively or in addition, the client logic  212  and/or the allocation logic  412  may perform operations that create a condition at the memory appliance  110  that the observer logic  218  may check for. For example, the client logic  312  and/or the allocation logic  412  may perform a client-side memory access operation to store information about a set of memory access operations in a particular portion of the memory on the memory appliance  110 . The information stored in the portion may include, for example, the offset, size, and/or type of each memory access operation performed. The observer logic may check the portion for updates in order to identify one or more attributes of the memory access operations. 
     The observer logic  218  may observe or otherwise identify the operations requested by the client logic  312 , the allocation logic  412 , and/or another logic that are performed on the region  214  and/or the memory appliance  110 . The observer logic  218  may identify the requested operations based on direct communication between the memory appliance  110  and any of: the client  130 , the management server  120 , and/or another memory appliance. For example, the observer logic  218  may listen for incoming notification messages at the communication interface  230 . Alternatively, or in addition, the observer logic  218  may passively monitor the operations requested by the client logic  312 , the allocation logic  412 , and/or another logic. For example, the observer logic  218  may listen for notification messages received as a result of operations performed by the client logic  312 , the allocation logic  412 , and/or another logic. 
     Alternatively, or in addition, the observer logic may check for conditions created by the client logic  312 , the allocation logic  412 , another logic, the communication interfaces, and/or another hardware component. For example, the observer logic  218  may read contents of one or more portions of the memory  210  that are accessible by the client  130  and/or the management server  120  using client-side memory access, by the communication interfaces, or by another hardware component. In an example, a first portion of the memory  210  may include one or more flags that indicate whether one or more second portions of the memory  210  have been updated by the memory access operations since the one or more second portions of the memory  210  were last copied to the backing store  260 . In a second example, a first portion of the memory  210  may include one or more flags that indicate whether one or more second portions of the memory  210  have been read or written by the memory access operations since the last time the flags have been checked by the observer logic  218 . In a third example, a first portion of the memory  210  may include one or more values that indicate how many times one or more second portions of the memory  210  have been read or written by the memory access operations since the last time the values have been checked by the observer logic  218 . 
     In response to identifying a set of memory access operations, the observer logic  218  may take further action. In an example, further action may include determining statistics related to the memory access operations (including but not limited to the type of operation, the number of operations, the size of the affected memory, and/or memory locations of each operation). In a second example, further action may include tracking or identifying regions of the memory  210  that have been written to or otherwise affected by the memory access operations. The observer logic  218  may persist the contents of the affected regions of the memory  210  to the backing store  260 , backing stores, and/or duplicate the contents of the affected regions of the memory  210  to another memory appliance, a block device, an external server, and/or a backup device. Alternatively, the observer logic  218  may take any other action related to the memory access operations. 
     The memory access operation may complete at the memory appliance  110  without waiting for the observer logic  218  to identify the memory access operation. Alternatively or in addition, the memory access operation may complete at the memory appliance  110  without waiting for the observer logic  218  to take any further action in response to identifying the memory access operation. Accordingly, the client logic  312  and/or the allocation logic  412  may perform a write operation to the region  214  in the amount of time that the request to perform the write operation travels over the interconnect  140  and the memory appliance  110  writes data to the memory. The overhead associated with storage protocols and/or writing the data to the backing store  260  may be avoided. 
     Mechanisms for observing or identifying the operations requested by the client logic  312  and/or the allocation logic  412  and the actions taken in response to identifying the operations may take any of numerous forms. A particular mechanism may balance tradeoffs between individual operation latency, operations per second from an individual client and/or management server, aggregate operations per second from multiple clients and/or management servers, demand placed upon compute resources of the clients, demand placed on compute resources of the management servers, and demand placed on compute resources of the memory appliance or on the observer logic, among others. 
     Alternatively or in addition the observer logic  218  may not observe or identify the memory access operations performed. Alternatively or in addition, the observer logic  218  may take one or more actions without specific knowledge of the memory access operations. For example, the observer logic  218  may persist the entire contents of the region  214  to the backing store  260 ; duplicate the entire contents of the region  214  to another storage device, external server, and/or backup device; and/or take some other action related to the region  214 . Alternatively or in addition, the observer logic  218  may compare the contents of the region  214  with the contents of the backing store  260 . Alternatively or in addition, the observer logic  218  may use computed hash values to determine which areas of the region  214  have been modified. A computed hash value may be a computed output which is expected with high probability to have a different value for two different input buffers and which may be smaller than one or both input buffers. Examples of computed hash values include checksums, cyclic redundancy check codes, and cryptographic hash codes. The observer logic  218  may perform actions without knowledge of the memory access operations periodically, prior to system shutdown, according to a schedule, or in response to a particular event, such as a hardware interrupt. 
     Alternatively, a client-side memory access may be performed as described in this disclosure, and then the client logic  312  may choose to wait for an additional notification from the observer logic  218  that the further actions are complete. For example, the client-side memory access may be a first client-side memory access, and the further actions may include replicating data from the affected regions to one or more additional memory appliances using additional client-side memory accesses between the memory appliances. Waiting for the additional notification for the first client-side memory access provides assurance to the client logic  312  that the affected regions have been synchronized between the multiple memory appliances. If an application is performing activities that may benefit from this assurance, it may be beneficial to wait for the additional notification. While waiting for the additional notification does increase the overall latency of the first client-side memory access by the time it takes for the observer logic  218  to be notified and replicate the affected regions and the time it takes to receive the additional notification, the client logic  312  still does not need to wait for the observer logic  218  of the additional memory appliances to be notified or take any action. 
     The application logic  314 , the client logic  312 , the allocation logic  412 , and/or another logic may perform data translation on the data being read and/or written to the region  214 . Alternatively, or in addition, the communication interfaces  230   330   430 , the memory controllers  220   320   420 , the storage controllers  250   350   450 , and/or the backing stores  260   360   460  may perform data translation. Data translation may include manipulating the data being read and/or written. 
     In a first example, data translation may include compressing the data being written to the region  214  and/or decompressing the data being read from the region  214 . Compression and/or decompression may be performed using any one or more compression schemes, such as Lempel-Ziv (LZ), DEFLATE, Lempel-Ziv-Welch (LZW), Lempel-Ziv-Renau (LZR), Lempel-Ziv-Oberhumer (LZO), Huffman encoding, LZX, LZ77, Prediction by Partial Matching (PPM), Burrows-Wheeler transform (BWT), Sequitur, Re-Pair, arithmetic code, and/or other method and/or scheme known now or later discovered which may be used to recoverably reduce the size of data. 
     In a second example, data translation may include encrypting the data being written to the region  214  and/or decrypting the data being read from the region  214 . Encryption and/or decryption may be performed using any one or more encryption schemes and/or ciphers, such as symmetric encryption, public-key encryption, block ciphers, stream ciphers, substitution ciphers, transposition ciphers, and/or any other scheme which may be used to encode information such that only authorized parties may decode it. One or more encryption keys for the one or more encryption schemes may be included in the access parameters for the region  214 . 
     In a third example, data translation may include performing error detection and/or error correction upon the data being written to the region  214  and/or the data being read from the region  214 . Error detection and/or error correction may be performed using any one or more error detection and/or error correction schemes, such as repetition codes, parity bits, checksums, cyclic redundancy checks, cryptographic hash functions, error correcting codes, forward error correction, convolutional codes, block codes, Hamming codes, Reed-Solomon codes, Erasure Coding-X (EC-X) codes, Turbo codes, low-density parity-check codes (LDPC), and/or any other scheme which may be used to detect and/or correct data errors. 
     Error detection and/or error correction may include performing additional calculations to confirm the integrity of the data written to and/or read from the region. For example, one or more digests, described elsewhere in this document, may be written to the region  214  and/or to the region metadata  215  for one or more corresponding portions of the region  214 . When reading the corresponding portion, if the stored digest does not match the digest which can be computed from the read data for the portion, then the read may be considered failed and/or the portion may be considered corrupted. Alternatively or in addition, the data may be corrected based upon the one or more digests and/or error correcting codes. 
     Further examples may include performing multiple types of data translation. For example, the client logic or another entity may encrypt the data being written to the region  214  and compute one or more error detecting and/or error correcting codes for the data and/or for the encrypted data. Alternatively or in addition, the client logic or another entity may decrypt the data being read from the region  214  and may perform error detection and/or error correction upon the data and/or encrypted data being read from the region. 
     The application logic  314 , the client logic  312 , the allocation logic  412 , and/or another logic may perform data monitoring on the data being read and/or written to the region  214 . Alternatively, or in addition, the communication interfaces, the memory controllers, the storage controllers, and/or the backing stores may perform data monitoring. Data monitoring may include observing the data being read and/or written. In an example, data monitoring may include performing virus scanning on data being read from and/or written to the region  214 . In a second example, data monitoring may include performing malware detection on data being read from and/or written to the region  214 . In a third example, data monitoring may include performing policy enforcement, such as monitoring for forbidden data patterns and/or strings, on data being read from and/or written to the region  214 . In a fourth example, data monitoring may include performing data indexing on data being read from and/or written to the region  214 . For example, an index for a first region may be created in a second region, the index providing fast lookup of data in the first region. 
     Presence of management servers, memory appliances, and/or clients may be detected automatically by the allocation logic  412 , the region access logic  212 , and/or the client logic  312 . When the management server  120 , the memory appliance  110 , and/or the client  130  is detected by the allocation logic  412 , the region access logic  212 , and/or the client logic  312 , it may become known to the allocation logic  412 , the region access logic  212 , and/or the client logic  312  that detected it. To facilitate being detected, the allocation logic  412 , the region access logic  212 , and/or the client logic  312  may transmit a hello message upon one or more interconnects  140  upon startup, periodically, and/or upon receiving a presence detection request message. Upon receiving a hello message, the allocation logic  412 , the region access logic  212 , and/or the client logic  312  may detect the management server  120 , the memory appliance  110 , and/or the client  130  that sent the hello message. To facilitate detecting management servers, memory appliances, and/or clients, the allocation logic  412 , the region access logic  212 , and/or the client logic  312  may send a presence detection request message. A presence detection request message may include information about the characteristics or configurations of the management servers and/or memory appliances including the allocation logic  412  and/or region access logic  212  that may respond. Alternatively or in addition, a presence detection request message may include an indication of whether only management servers, only memory appliances, only clients, or some combination of these may respond. 
     Alternatively, or in addition, the allocation logic  412 , the region access logic  212 , and/or the client logic  312  may register the presence of the corresponding management servers, memory appliances, and/or clients with one or more registration servers. A registration server may be an InfiniBand subnet administrator, a Domain Name System (DNS) server, a Multicast DNS (mDNS) server, Service Location Protocol (SLP) directory agent, an Active Directory Server, or any other server capable of receiving and/or distributing information about management servers, memory appliances, and/or clients. Alternatively, or in addition, the allocation logic  412 , the region access logic  212 , and/or the client logic  312  may include information about the characteristics and/or configuration of the corresponding management servers, memory appliances, and/or clients when registering their presence with the registration server. The allocation logic  412 , the region access logic  212 , and/or the client logic  312  may detect management servers, memory appliances, and/or clients by querying the one or more registration servers. 
     Alternatively, or in addition, presence of management servers and/or memory appliances may be specified by an administrator using a user interface. The user interface may be a graphical user interface, a web interface, a command-line interface, an application programming interface (API), and/or any other type of interface or combination of interfaces known now or later discovered. 
     Management servers may be associated with one or more memory appliances. Memory appliances may be associated with one or more management servers. Management servers may additionally be associated with zero or more other management servers. For example, the management server  120  may be associated with another management server that may function as a backup management server in case the management server  120  fails. The backup management server may maintain copies of data of the management server  120 , including, but not limited to, the external memory allocation metadata  414  and/or the region metadata  215 . The backup management server may have a copy of the backing store  460  of the management server  120 . The backup management server may obtain such copies of data at a predetermined schedule. Alternatively, or in addition, the backup management server may obtain a copy of the data in response to an event, such as modification of the data of the management server  120 . Alternatively, or in addition, the backup management server may obtain a copy of the data from the management server  120  in response to a request from an administrator, such as via the user interface. The backup management server  120  may obtain data of the management server  120  as described elsewhere in this disclosure. 
     Associations between management servers and memory appliances may be specified by an administrator using a second user interface, which may be part of the user interface described earlier. The second user interface may be a graphical user interface, a web interface, a command-line interface, an API, and/or any other type of interface or combination of interfaces known now or later discovered. 
     The memories of the memory appliances associated with the management server  120  may be part of a memory pool. Alternatively, or in addition, the memories of the memory appliances known to the allocation logic  412  of the management server  120  may be part of the memory pool. Alternatively, or in addition, the memories of the memory appliances associated with multiple management servers and/or known to multiple allocation logics may be part of the memory pool. The pool of memory, or the memory pool, may be a collection of allocatable memory that spans one or more memory appliances. 
     Alternatively, or in addition, associations between management servers and memory appliances may be determined automatically. Automatic associations between management servers and memory appliances may be determined based upon characteristics or configurations of the management servers, the memory appliances, or both. Characteristics or configurations of the management server  120 , the memory appliance  110 , and/or the client  130  may include hardware revisions, firmware revisions, software revisions, protocol revisions, physical location, logical location, network location, network topology, network bandwidth, network capacity, network utilization, logical grouping, labels, names, server/appliance health, server/appliance utilization, server/appliance overall performance rating, processor type, number of processors, processor speed, memory bandwidth, memory capacity, memory utilization, memory health, backing store presence, backing store bandwidth, backing store input/output operations per second (IOPS), backing store latency, backing store capacity, backing store utilization, backing store health, battery presence, battery type, battery chemistry, battery capacity, battery utilization, battery % charged, battery time remaining, battery health, or any other characteristic or combination of characteristics of the management server  120 , the memory appliance  110 , and/or the client  130 . In an example, the allocation logic  412  may automatically associate the management server  120  with memory appliances in the same physical rack. In another example, the allocation logic  412  may automatically associate the management server  120  with memory appliances sharing the same protocol version. In another example, the allocation logic  412  may automatically associate the management server  120  with memory appliances with appliance health, memory health, backing store health, and/or battery health above or below a threshold or set of thresholds. The thresholds may be configurable by the administrator via the user interface, or may be predetermined when the management server starts up. 
     The allocation logic  412  may address region access logic requests to the region access logic  212  included in one or more memory appliances. The region access logic requests may be requests handled via an interface of the region access logic  212 . In one example, the region access logic requests may be messages transmitted via the communication interface(s)  230 ,  330 ,  430 . In another example, the region access logic requests may be programmatic interfaces, such as in an API. Other examples may use any other interface known now or later discovered for conveying the region access logic requests. The memory appliances including the region access logic  212  to which the requests are sent may be associated with the management servers including the allocation logic  412  and/or known by the allocation logic  412 . For example, region access logic requests received by the region access logic  212  may include requests to create the region  214 , requests to resize the existing region  214 , requests to restore contents of the region  214  from the backing store  260 , requests to get the status of the memory  210  included in the memory appliance  110 , requests to get health status from the memory appliance  110 , requests to persist the region  214  to the backing store  260  and remove the region  214  from the memory  210 , requests to destroy the region  214 , requests to get a list of available regions, requests to get information for the region  214 , requests to modify settings for the region  214 , requests to configure the memory appliance  110 , requests to migrate the region  214 , and/or any other request related to the memory appliance  110  and/or the regions included in the memory  210  of the memory appliance  110 . 
     The region access logic requests may be communicated over any communications protocol and/or interface capable of carrying messages. For example, the region access logic requests may be carried over UDP datagrams, a TCP connection, an SSL connection, InfiniBand reliable connections, RoCE, iWARP, HTTP, or any other communications protocol known now or later discovered. Alternatively, or in addition, the region access logic requests may be carried over remote procedure calls, such as using XML-RPC, SOAP, CORBA, Java Remote Method Invocation (Java RMI), Representational State Transfer (REST), JavaScript Object Notation (JSON) over REST, and/or any other remote procedure call protocol. Alternatively, or in addition, the region access logic requests may be carried over a communication protocol based on client-side memory access, such as by writing messages into a buffer on the memory appliance  110  via client-side-memory access. Alternatively, or in addition, the region access logic requests may be carried via invoking methods and/or interfaces in an API. For example, if the allocation logic  412  and region access logic  212  are co-located or combined, the region access logic requests may be methods and/or interfaces in an API. The allocation logic  412  and region access logic  212  may be co-located in examples where the memory appliance  110  also functions as the management server  120 , or, alternatively, the management server  120  also functions as the memory appliance  110 . 
     Upon receiving a request to create the region  214 , the region access logic  212  may allocate a portion of the memory  210  included in the memory appliance  110  for the region  214 . Allocating a portion of the memory  210  may include initializing the contents of the allocated memory. Alternatively, or in addition, the memory being allocated may be pre-initialized, such as by an operating system. Alternatively, or in addition, the memory being allocated may be partially pre-allocated, such as a free list including one or more portions of pre-allocated and/or pre-initialized memory. Alternatively, or in addition, the region access logic  212  may configure the communication interface  230  for the region  214  without allocating the entire portion of the memory for the region  214  and/or without initializing the contents of the memory. The region access logic  212  may configure the communication interface  230  to treat un-allocated and/or un-initialized portions as not present. Attempting to access data that is not present using client-side memory access may fail. Alternatively, or in addition, attempting to access data that is not present using client-side memory access may cause the processor  240  to be notified. Upon being notified, the processor  240  may take some action related to the attempt to access data that is not present, such as allocating a portion of the memory  210  to satisfy the attempt to access data that is not present and/or initializing the portion of the memory. The region access logic  212  may also associate an identifier with the region  214 . The identifier may be chosen by the region access logic  212  or it may be included in the request to create the region  214 . Additionally, the region access logic  212  may create, update, and/or associate metadata, such as the region metadata  215 , with the region  214 . The region access logic  212  may respond to the request to create the region  214  with a response message. The response message may include the identifier associated with the region  214  and/or a status, indicating whether the operation was successful. 
     Upon receiving a request to resize the existing region  214 , if the region  214  is being expanded, the region access logic  212  may allocate a portion of the memory  210  of the memory appliance  110 . Allocating a portion of the memory  210  may include initializing the contents of the allocated memory. Alternatively, or in addition, the memory being allocated may be pre-initialized, such as by an operating system. Alternatively, or in addition, the memory being allocated may be partially pre-allocated, such as a free list including one or more portions of pre-allocated and/or pre-initialized memory. The size of the portion of the memory may be related to the size of the expanded region minus the size of the existing region  214 . For example, the size of the portion of the memory may be the size of the expanded region minus the size of the existing region  214  rounded up to the nearest unit of allocation, such as a page, a huge page, a slab, and/or any other unit of allocation. The units of allocation may have respective predetermined sizes. The predetermined sizes may vary, such as based on an operating system being used by the client  130 , the memory appliance  110 , and/or the management server  120 . Alternatively, or in addition, the size of the portion of the memory may be specified by the request to resize the existing region  214 . Alternatively, or in addition, the request to resize the existing region  214  may specify an offset within the region  214  where the allocated portion may appear. For example, the request to resize the existing region  214  may be re-allocating a portion of the region  214  that was previously de-allocated by a different request to resize the same region  214 . The region access logic  212  may assign the allocated portion to the region  214 . The region access logic  212  may update the region metadata  215  to include references to the allocated portion of the memory  210 . 
     Upon receiving the request to resize the existing region  214 , if the region  214  is being contracted, the region access logic  212  may update the region metadata  215  to remove references to a portion of the memory allocated to the region  214  and/or may de-allocate the portion of the memory. De-allocating may include listing the de-allocated portion on the free list. The portion of the memory may be the portion at the logical end of the region  214 , at the logical beginning of the region  214 , and/or at any other logical location within the region  214 . The portion of the region to be removed may be specified by the request to resize the existing region  214 . For example, the request to resize the existing region  214  may specify that the data at the end of the region  214  may be discarded. Alternatively, or in addition, the request to resize the existing region  214  may specify that the data at the beginning of the region  214  may be discarded. Discarding data at a location other than the end of the region  214  may cause the offsets of data after the discarded data to change. For example, removing 100 MiB (mebibyte) from the region starting at an offset of 200 MiB may cause data that previously occurred at an offset of 300 MiB to instead occur at an offset of 100 MiB. Alternatively, discarding data at a location other than the end of the region  214  may cause the offsets of data after the discarded data to stay the same. The region access logic  212  may configure the communication interface  230  to treat the discarded data as not-present. Attempting to access data that is not present using client-side memory access may fail. Alternatively, attempting to access data that is not present using client-side memory access may cause the processor  240  to be notified. Upon being notified, the processor  240  may take some action related to the attempt to access data that is not present, such as allocating a portion of the memory and/or initializing the portion to satisfy the attempt to access the data that is not present. 
     Upon receiving a request to restore the contents of the region  214  from the backing store  260 , the region access logic  212  may allocate a portion of the memory  210  included in the memory appliance  110  for the region  214 . The region access logic  212  may copy the contents of persisted data related to the region  214  into the allocated memory. Alternatively, or in addition, the region access logic  212  may associate portions of the backing store  260  with portions of the region  214  without immediately allocating the portion of the memory and/or without immediately copying the contents by configuring the communication interface  230  to treat the portions of the region  214  as not present. Attempting to access data that is not present using client-side memory access may fail. Alternatively, attempting to access data that is not present using client-side memory access may cause the processor  240  to be notified. Upon being notified, the processor  240  may take some action related to the attempt to access data that is not present, such as allocating a portion of the memory to satisfy the attempt to access data that is not present, initializing the portion of the memory, and/or copying the contents of persisted data related to the portion into the portion. The region access logic  212  may respond to the request to restore the contents of the region  214  from the backing store  260  with a response message. The response message may include the identifier associated with the region  214  and/or a status, indicating whether the operation was successful. 
     Upon receiving a request to get the status of the memory  210  included in the memory appliance  110 , the region access logic  212  may respond to the request with a response message. The response message may include one or more attributes of the memory  210  included in the memory appliance  110 . The attributes may include the total memory available to hold regions, the amount of memory currently in use by existing regions, the amount of memory available to hold additional regions, and/or any other attributes of the memory  210  included in the memory appliance  110 . 
     Upon receiving a request to get health status from the memory appliance  110 , the region access logic  212  may respond to the request with a response message. The response message may include one or more attributes describing the health of the memory appliance  110 , of components included in the memory appliance  110 , and/or of components connected to the memory appliance  110 . Examples of health that may be described by the attributes include temperatures, voltages, cooling system health, backing store status, memory health, CPU health, battery capacity, projected battery run time, or any other hardware, firmware, or software status and/or health relevant to the health of the memory appliance  110 . Temperatures may include CPU temperatures, integrated circuit temperatures, circuit board temperatures, chassis intake temperatures, chassis output temperatures, power supply temperatures, uninterruptible power supply (UPS) temperatures, battery temperatures, backing store temperatures, and/or any other temperatures relevant to the health of the memory appliance  110 . Voltages may include CPU voltages, integrated circuit voltages, circuit board voltages, power supply input voltages, power supply output voltages, UPS input voltages, UPS output voltages, battery voltages, and/or any other voltages relevant to the health of the memory appliance  110 . Cooling system health may include fan speeds, liquid coolant temperatures, liquid coolant flow rates, facility cooling health, and/or any other status related to the ability to maintain stable temperatures. Backing store status may include status of individual storage devices included in the backing store, status of hardware and/or software storage controllers, status of logical volumes included in the storage devices, status of logical associations between storage devices, status of logical associations between logical volumes, and/or any other status related to the ability to reliably store data in the backing store  260 . Logical associations may include software or hardware mechanisms used to aggregate storage devices or logical volumes and/or to provide redundancy and/or resiliency. For example, a logical association may be a redundant array of independent disks (RAID). Memory health may include number and/or source of memory errors, memory redundancy status, and/or any other status related to the ability to reliably store data in the memory. Memory errors may include recoverable error correcting code (ECC) errors, unrecoverable ECC errors, rank spared indications, mirror broken indications, and/or any other errors and/or indications reported by the memory and/or memory controller. CPU health may include CPU errors, CPU voltage/frequency scaling mode, and/or any other status related to the CPU&#39;s ability to reliably execute instructions. CPU errors may include hardware watchdog expirations, machine checks, cache ECC errors, processor faults, and/or any other error or indication reported by the CPU and/or CPU monitoring hardware. 
     Upon receiving a request to persist the region  214  to the backing store  260  and remove it from memory  210 , the region access logic  212  may copy the contents of the region  214  to a portion of the backing store  260 . The portion of the backing store  260  may be determined at the time the request to persist the region  214  to the backing store  260  and remove it from the memory  210  is received. Alternatively, the portion may have been determined in advance, such as when the region  214  was created, and/or when the region  214  was configured. After the contents of the region  214  are copied to the portion of the backing store  260 , the region access logic  212  may de-allocate the portion of the memory included in the memory appliance  110  for the region  214 . De-allocating may include listing the de-allocated portions on a free list, which indicates portions of the memory  210  that have not been allocated to any particular region. Alternatively, or in addition, as the contents of portions of the region  214  are copied to the backing store  260 , the region access logic  212  may selectively de-allocate the portions of the region  214  that have already been copied to the backing store  260 . Alternatively or in addition, if portions of the region  214  have already been copied to the backing store  260  prior to receiving the request to persist the region  214  to the backing store  260  and remove it from memory  210 , the region access logic  212  may not copy the portions of the region  214  that have already been copied to the backing store  260 . The region access logic  212  may respond to the request to persist the region to the backing store  260  and remove it from memory  210  with a response message. The response message may include the identifier associated with the region  210  and/or a status, indicating whether the operation was successful. 
     Upon receiving a request to destroy the region  214 , the region access logic  212  may de-allocate the portion of the memory included in the memory appliance  110  for the region  214 . De-allocating may include listing the de-allocated portions on the free list. Alternatively, or in addition, the region access logic  212  may delete any persisted data related to the region  214  included in the backing store  260 . The region access logic  212  may respond to the request to destroy the region  214  with a response message. The response message may include the identifier associated with the region  214  and/or a status, indicating whether the operation was successful. 
     Upon receiving a request to get a list of available regions, the region access logic  212  may respond to the request with a response message. The response message may include a number of available regions and/or attributes related to the available regions. Available regions may include regions that are included in the memory  210 , regions that are included in the backing store  260 , and/or regions that are partially included in the memory  210  and/or backing store  260 . The attributes related to an available region, such as the region  214 , may include the identifier associated with the region, the size of the region, the status of the region, information about external memory allocations that reference the region, and/or any other information related to the region. The status of the region  214  may include one or more indications of whether the region is included in the memory  210 , whether the region  214  is included in the backing store  260 , whether the contents of the region  214  are being persisted to the backing store  260 , whether the contents of the region  214  are being restored from the backing store  260 , and/or whether the contents of the region  214  are being transferred to or from another memory appliance or storage device. Information about an external memory allocation may include an identifier for the external memory allocation, information about the external memory allocation, and/or information about the region&#39;s role in the external memory allocation. For example, an external memory allocation may include metadata describing a logical relationship between the regions referenced by the external memory allocation. In one example, the logical relationship could be a concatenation of regions located in different memory appliances, the metadata describing the logical relationship may include an indication that concatenation is in use and/or a logical address within the external memory allocation at which the region&#39;s data logically exists. Alternatively, or in addition, the logical relationship could be a striping relationship, such as RAID-0; a mirroring relationship, such as RAID-1; a parity relationship, such as RAID-2, RAID-3, RAID-4, RAID-5, or RAID-6; a partial data redundancy relationship, a combination of relationships, such as striping with mirroring; or any other relationship between regions known now or later discovered. 
     Upon receipt of a request to get information for the region  214 , the region access logic  212  may respond to the request with a response message. The response message may include information related to the region  214 . The information related to the region  214  may include all of or a portion of the region metadata  215 . Alternatively, or in addition, the information related to the region  214  may be derived from the region metadata  215 . Alternatively, or in addition, the information related to the region  214  may be different depending upon parameters included in the request to get information for the region  214 . For example, the parameters may include an indication of which portion of the metadata  215  may be included in the information related to the region  214 . 
     Upon receiving a request to modify settings for the region  214 , the region access logic  212  may update the region metadata  215  to reflect the modified settings. For example, the request to modify settings for the region  214  may set a human readable label to be associated with the region  214 , modify access parameters, and/or modify other portions of the region metadata  215 . The region access logic  212  may restrict which parts of the region metadata  215  may be modified. For example, the region access logic  212  may use the access parameters to control which parts of the region metadata  215  may be modified. If the region access logic  212  determines that the requested changes are not allowed, the request to modify settings of the region  214  may fail. In another example, the request to modify settings of the region  214  may include a request to enable replication and/or may include identifiers for regions and/or memory appliances to which the data of the region  214  of the memory appliance  110  may be replicated. When replication is enabled, the observer logic  218  may replicate affected portions of the region  214  to the regions and/or memory appliances indicated in the request to modify settings for the region  214  using the methods described throughout. The region access logic  212  may respond to the request to modify settings for the region  214  with a response message. The response message may include the identifier associated with the region  214 , information related to the modified region metadata  215 , and/or a status, indicating whether the operation was successful. 
     Upon receiving a request to configure the memory appliance  110 , the region access logic  212  may cause one or more characteristics and/or configurations of the memory appliance to be updated to reflect the requested configuration. Alternatively, the characteristics and/or configurations may not be updated, such as if the request to configure the memory appliance  110  indicates that one or more characteristics and/or configurations are to be retrieved but not updated. 
     In addition to those listed elsewhere in this disclosure, characteristics and/or configurations of the memory appliance may include: memory appliance name, system time, time zone, time synchronization settings, time server(s), network configuration, hostname, power configuration, battery policy, Uninterruptible Power Supply (UPS) configuration, disk policy, backing store configuration, persistence configuration, persistence mode, customer-support configuration(s), service configuration(s), user interface configuration, user configuration, user-group configuration, health monitoring configuration(s), network monitoring configuration(s), SNMP configuration, logic version, software version, firmware version, microcode version, and/or any other aspect of the memory appliance which may be configured and/or changed. For example, the region access logic  212  and/or another logic may update the system time, time zone, time synchronization settings, and/or time servers(s) in response to the request to configure the memory appliance  110 . In another example, the region access logic  212  and/or another logic may update the firmware version, such as by modifying one or more logics of the memory appliance  110 . Alternatively or in addition, characteristics and/or configurations may include associations between memory appliances, management servers, and/or clients. For example, the region access logic may cause the memory appliance  110  to be associated with a management server  120 , such as a backup management server. 
     In one example, the region access logic  212  may cause one or more logic, software, firmware, and/or microcode versions of the memory appliance  110  to be changed in response to the request to configure the memory appliance  110 . The request to configure the memory appliance  110  may indicate the desired version(s), may provide the desired logic, hardware description language, software, firmware, and/or microcode, and/or may provide a network location where the desired logic, hardware description language, software, firmware, and/or microcode may be downloaded by the region access logic  212  (and/or another logic, such as a logic-updating logic). Upon retrieving the desired logic, hardware description language, software, firmware, and/or microcode, the region access logic  212  and/or another logic may execute and/or trigger one or more logic, software, firmware, and/or microcode update procedures. For example, the region access logic  212  and/or another logic may update one or more of the logics of the memory appliance  110  with the logic, hardware description language, software, firmware, and/or microcode. Upon updating the one or more of the logics, the region access logic  212  and/or another logic may cause the corresponding logic(s) to be restarted and/or reloaded. Alternatively or in addition, the region access logic and/or another logic may restart all logics of the memory appliance, such as with a system reboot. 
     The region access logic  212  and/or another logic may check the integrity of the software, firmware, and/or microcode one or more times. For example, the region access logic  212  may check the integrity of the software, firmware, and/or microcode upon retrieving the software, firmware, and/or microcode; may check the integrity of one or more updated logics upon updating; and/or may perform one or more diagnostics tests upon updating the logic(s) and/or upon starting (and/or restarting) the updated logic(s). For example, each updated logic may perform a power-on self-test upon being restarted and/or rebooted. If one or more integrity tests fail, if one or more diagnostics fail, if one or more logics fail to initialize upon being restarted and/or rebooted, and/or if any other error occurs while updating one or more logics, the logics may be returned to their original state and/or the request to configure the memory appliance  110  may fail. For example, the memory appliance may maintain a backup-copy of the logic(s) being updated and may restore the logic(s) from the backup-copy to return to the original state. Alternatively or in addition, the memory appliance  110  may include a recovery logic which may include similar logic for causing the one or more software, firmware, and/or microcode versions of the memory appliance  110  to be changed in response to the request to configure the memory appliance  110 . The recovery logic may be write-protected, such that it may not be modified in response to the request to configure the memory appliance  110 . 
     The region access logic  212  may respond to the request to configure the memory appliance  110  with a response message. The response message may include the values or one or more characteristics and/or configurations and/or a status, indicating whether the operation was successful. 
       FIGS.  5 A and  5 B  illustrate an example scenario of a request to migrate a region. A request to migrate a region may indicate a request  502   a  to migrate data from a first region  214   a  included in a memory of a first memory appliance,  110   a  to a second region  214   b  included in a memory of a second memory appliance  110   b , as illustrated in  FIG.  5 A . The request to migrate the first region  214   a  to the second region  214   b  may be a request  502   a  received by the region access logic  212   a  of the first memory appliance  110   a  and/or a request  502   b  received by the region access logic  212   b  of the second memory appliance  110   b.    
       FIG.  5 B  illustrates an exemplary flowchart of steps taken upon receipt of a request  502   a  to migrate the first region  214   a  by the region access logic  212   a  of the first memory appliance  110   a  ( 502 ). The region access logic  212   a  may determine if the second region has to be newly created within the second memory appliance ( 504 ). The request  502   a  to migrate the first region  214   a  may include an identifier for the second memory appliance  110   b . Alternatively or in addition, the request  502   a  to migrate the first region  214   a  may include an identifier for the second region  214   b . Alternatively, or in addition, the request  502   a  to migrate the first region  214   a  may indicate creating the second region  214   b  at the second memory appliance  110   b , in case the second region  214   b  does not exist ( 530 ). If creation of the second region  214   b  fails, the migration request  502   a  may fail ( 530 ,  550 ). Alternatively, if the second region  214   b  is successfully created, the contents of the first region  214   a  may be transferred to the second region  214   b  as part of a successful migration ( 540 ,  545 ). 
     If the request  502   a  to migrate the first region  214   a  includes the identifier for the second region  214   b , the region access logic  212   a  may send a request to get a list of available regions and/or a request to get information of the second region  214   b  to the region access logic  212   b  of the second memory appliance  110   b  ( 510 ). The received information of the second region  214   b  may be analyzed to determine if the second region  214   b  is compatible to complete the migration request  502   a  ( 512 ). For example, if the response to the request to get the list of available regions indicates failure and/or does not include the identifier for the second region  214   b , then the request  502   a  to migrate the first region  214   a  may fail ( 550 ). Alternatively, or in addition, if the response to the request to get information for the second region  214   b  indicates failure and/or includes information indicating that the second region  214   b  cannot be used to hold all of the data of the first region  214   a , then the request  502   a  to migrate the first region  214   a  may fail ( 550 ). 
     Information indicating that the second region  214   b  is incompatible for completion of the migration request  502   a , such as cannot be used to hold all of the data of the first region  214   a , may include a size of the second region  214   b  that is smaller than the size of the first region  214   a , access permissions for the second region  214   b  that indicate that the first memory appliance  110   a  may not write to the second region  214   b , or any other limiting information. In such cases of incompatibility, the first memory appliance  110   a  may request reconfiguring the second region  214   b  so as to conform the second region  214   b  for the migration ( 516 ). For example, if the size of the second region  214   b  is smaller than the size of the first region  214   a , the region access logic  212   a  of the first memory appliance  110   a  may send a request to resize the second region  214   b  to the region access logic  212   b  of the second memory appliance  110   b . If the response to the reconfiguration request, such as the request to resize the second region  214   b , indicates failure ( 520 ), the request  502   a  to migrate the first region  214   a  may fail ( 550 ). If the reconfiguration is successful, the first memory appliance  110   a  may confirm that the second region  214   b  is now compatible for the migration ( 522 ). Once compatibility is confirmed, the region access logic  212   a  may attempt to perform a first client-side memory access to write data from the first region  214   a  to the second region  214   b  ( 540 ) and on successful completion, mark the migration as successful ( 545 ). 
     Copying data from the first region  214   a  to the second region  214   b  may further involve transferring any changes that may be made to the first region  214   a  which are not captured by the first client-side memory access ( 540 ). For example, one or more clients  130  may continue to access the region  214   a  during the client-side memory access, including writing to the region  214   b . Concurrently with the client-side memory access, the observer logic  218   a  of the first memory appliance  110   a  may observe which portions of the first region  214   a  are modified during the client-side memory access. Upon completing the first client-side memory access, the region access logic  212   a  may attempt to perform client-side memory access to write the modified portions from the first region  214   a  to the second region  214   b  ( 540 ). The region access logic  212   a  and observer logic  218   a  may repeat this procedure until no portions are left to be migrated. Alternatively, or in addition, the region access logic  212   a  of the first memory appliance  110   a  may modify the access permissions of the first region  214   a  to prohibit writing additional changes to the first region  214   a . For example, the access permissions may be changed after the number or total data size of the portions left to be migrated is above or below a configured threshold. The migration may complete when there are no portions left to be migrated. If the one or more attempts to perform client-side memory access fails, the request to migrate a region may fail. 
     Upon receipt of the request  502   b  to migrate the first region  214   a  by the region access logic  212   b  of the second memory appliance  110   b , similar steps may be performed by the second memory appliance  110   b . The region access logic  212   b  may determine if a new region needs to be created ( 504 ). The request to migrate the first region  214   a  may include an identifier for the first memory appliance  110   a  and/or the first region  214   a . Alternatively or in addition, the request to migrate the region may include an identifier for the second region  214   b . Alternatively, or in addition, the request  502   b  to migrate the first region  214   a  may indicate that a new region may be created at the second memory appliance  110   b . The region access logic  212   b  may send a request to get a list of available regions and/or a request to get information for the first region to the region access logic  212   a  of the first memory appliance  110   a . If the response to the request to get a list of available regions indicates failure and/or does not include the identifier for the first region  214   a , then the request  502   b  to migrate the first region may fail ( 550 ). Alternatively, or in addition, if the response to the request to get information for the first region  214   a  indicates failure and/or includes information indicating that the second region  214   b  cannot be used to hold all of the data of the first region  214   a , then the request  502   b  to migrate the first region  214   a  may fail ( 550 ). Information indicating that the second region  214   b  is incompatible for completion of the migration request  502   b , such as, cannot be used to hold all of the data of the first region  214   a , may include a size of the first region  214   a  that is larger than the size of the second region  214   b , access permissions for the first region  214   a  that indicate that the second memory appliance  110   b  may not read from the first region  214   a , or any other incompatibility criteria ( 512 ). In such cases of incompatibility, the second memory appliance  110   b  may reconfigure the second region  214   b  so as to conform the second region  214   b  for the migration  516 ). For example, if the size of the second region  214   b  is smaller than the size of the first region  214   a , the region access logic  212   b  of the second memory appliance  110   b  may attempt to resize the second region  214   b  using the methods and/or logic of the request to resize an existing region. If the attempt to resize the second region  214   b  fails, the request  502   b  to migrate a region may fail. In case, the reconfiguration is successful, the second memory appliance  110   b  may confirm that the second region  214   b  is now compatible for the migration ( 520 ,  522 ). Upon ensuring compatibility of the regions, the region access logic  212   b  may attempt to perform client-side memory access to read data from the first region  214   a  into the second region  214   b  and on successful completion, mark the migration as successful ( 522 ,  540 ,  545 ). If the attempt to perform client-side memory access fails, the request  502   b  to migrate a region may fail ( 550 ). 
     Prior to starting the client-side memory access, the region access logic  212   b  of the second memory appliance  110   b  may send a request to modify settings for the first region  214   a  to the region access logic  212   a  of the first memory appliance  110   a . The request to modify settings for the first region  214   a  indicating that replication may be enabled from the first region  214   a  to the second region  214   b . If the response to the request to modify settings for the first region  214   a  indicates failure, the request  502   b  to migrate the first region  214   a  may fail. 
     Upon completing the client-side memory access, the region access logic  212   b  of the second memory appliance  110   b  may send one or more requests to get information for the first region  214   a  to the region access logic  212   a  of the first memory appliance  110   a , the requests to get information for the first region  214   a  requesting information about how much data has been written to the first region  214   a  but not yet replicated to the second region  214   b . If the response to one or more of the requests to get information for the first region indicates failure, the request  502   b  to migrate the first region  214   a  may fail. Alternatively, one or more of the requests to get information for the first region  214   a  may be retried. The region access logic  212   b  of the second memory appliance  110   b  may send the requests to get information for the first region  214   a  until the response to one or more of the requests to get information for the first region  214   a  indicates that no data has been written to the first region  214   a  that is not replicated to the second region  214   b . Alternatively, or in addition, the region access logic  212   b  of the second memory appliance  110   b  may send a second request to modify settings for the first region  214   a  to the region access logic  212   a  of the first memory appliance  110   a , the second request to modify settings for the first region  214   a  requesting that the access permissions of the first region  214   a  be changed to prohibit additional changes to the first region  214   a . For example, the access permissions may be changed after the number or total data size of the portions left to be migrated is above or below a configured threshold. If the response to the second request to modify settings for the first region  214   a  indicates failure, the request  502   b  to migrate the first region  214   a  may fail. The migration  502   b  may complete when there are no portions left to be migrated. 
     The region access logic  212   a  and/or  212   b , that receives a request to migrate a region, may respond to the request  502   a  and/or  502   b  with a response message. The response message may include the identifier associated with the first region  214   a , the identifier associated with the second region  214   b , and/or a status, indicating whether the operation was successful. 
     The client logic  312  of the client  130  may transmit allocation logic requests to the allocation logic  412  included in the management server  120 . Allocation logic requests may include requests to find available memory appliances, requests to query available space on a memory appliance, requests to create an external memory allocation, requests to resize an existing external memory allocation, requests to renew an external memory allocation, requests to destroy an external memory allocation, requests to persist and free an existing external memory allocation, requests to list existing external memory allocations, requests to get information regarding an external memory allocation, requests to restructure an external memory allocation, or any other request related to the management servers, the memory appliances, the external memory allocations, and/or the regions on the memory appliances. The allocation logic requests may be carried over any communications protocol and/or interface capable of carrying messages. For example, the allocation logic requests may be carried over UDP datagrams, a TCP connection, an SSL connection, InfiniBand reliable connections, RoCE, iWARP, HTTP, or any other communications protocol known now or later discovered. Alternatively, or in addition, allocation logic requests may be carried over remote procedure calls, such as using XML-RPC, SOAP, CORBA, Java Remote Method Invocation (Java RMI), Representational State Transfer (REST), JavaScript Object Notation (JSON) over REST, and/or any other remote procedure call protocol. Alternatively, or in addition, the allocation logic requests may be carried over a communication protocol based on client-side memory access, such as by writing messages into a buffer on the management server  120  via client-side-memory access. Alternatively, or in addition, the allocation logic requests may be carried via invoking methods and/or interfaces in an API. For example, if the client logic  312  and the allocation logic  412  are co-located or combined, the allocation logic requests may be methods and/or interfaces in an API. 
       FIG.  6    illustrates an example external memory allocation system  600 . The system  600  illustrates the client  130 , the management server  120 , and a memory pool  610 . The memory pool  610  includes multiple memory appliances  110   a - 110   c . While  FIG.  6    illustrates only three memory appliances as being part of the memory pool  610 , in other examples, the memory pool  610  may include fewer or more number of memory appliances. The client  130  includes the client logic  312  and local memory  602 . The management server  120  includes the allocation logic  412  and the external memory allocation metadata  414 . Each of the memory appliances  110   a - 110   c  includes respective region allocation logic  212   a - 212   c  and memories  210   a - 210   c . The client  130 , management server  120 , and the memory appliances  110   a - 110   c  may include other components that are not illustrated in  FIG.  6   . The client  130  may request an external memory allocation, such as one of X 1 -X 3 , from the memory pool  610  via the management server  120  to complement the local memory  602 . For example, the local memory  602  may not be sufficient to handle the tasks operating on the client  130 , and therefore the client  130  may seek the external memory allocations X 1 -X 3 . Alternatively, or in addition, the client  130  may seek to use the external memory allocations X 1 -X 3  as the primary memory with the local memory  602  as a cache. 
     The external memory allocations may reference one or more regions. The one or more regions referenced by an external memory allocation may be included in a single memory appliance, or the regions may be distributed between multiple memory appliances. 
     The management server  120  may include external memory allocation metadata  414 . External memory allocation metadata  414  may include information describing the external memory allocations, such as indication of the regions referenced by the external memory allocation. For example, the external memory allocation X 1  may reference regions R 1 -R 3  as illustrated in  FIG.  6   , where R 1  is within memory appliance  110   a , R 2  is within memory appliance  110   b , and R 3  is within memory appliance  110   c . The external memory allocation X 2  may reference a single region R 5  from the memory appliance  110   b , while the external memory allocation X 3  may reference regions R 4  and R 6  on the memory appliances  110   a  and  110   c  respectively. It is understood that the described distributions of the regions are exemplary and that various other distributions of the regions referenced by an external memory allocation are possible. Further, while the example illustrates three external memory allocations X 1 -X 3 , other examples may involve fewer or more number of external memory allocations being present in the external memory allocation metadata  414 . The memory appliances  110   a - 110   c  including the regions R 1 -R 6  may be known to the allocation logic  412  of a management server  120  or associated with the management server  120  that is associated with the external memory allocation. 
     The external memory allocation metadata  414 , the region metadata  215 , and/or any other metadata may each be co-located with one or more logics which may access the corresponding metadata, such as in examples where the region metadata  215  is co-located with the region access logic  212  and/or where the external memory allocation metadata  318  is co-located with the allocation logic  412 . Alternatively or in addition, the metadata may be stored remotely, such as with a metadata server and/or in any other location. In other examples, the metadata may be replicated and/or dispersed to multiple locations, such as in examples where the region metadata  215  is included in one or more of the memory  210  and/or backing store  260  of the memory appliance  110 , the memory  310  and/or backing store  360  of the client  130 , and/or the memory  410  and/or backing store  460  of the management server  120 . 
     Further metadata may also be recorded in the external memory allocation metadata  414 . For example, information describing the external memory allocation X 1  may include the size of the external memory allocation X 1 , a lease expiration date and/or time for the external memory allocation X 1 , information about the regions R 1 -R 3  referenced by the external memory allocation X 1 , and/or any other information relevant to the external memory allocation X 1 . Alternatively, or in addition, the external memory allocation X 1  may include metadata describing one or more logical relationships between the regions R 1 -R 3  referenced by the external memory allocation X 1 . The various entries in the external memory allocation metadata  414  may contain the same fields of information, or different fields of information. The fields of information described are exemplary and other types of information may be recorded in other examples. The external memory allocation metadata  414  may be included in the memory  410  included in the management server  120 . Alternatively, or in addition, external memory allocation metadata  414  may be included in the backing store  460 , if included in the management server  120 . 
     The external memory allocation metadata  414  may be recoverable from the region metadata  215  included in one or more memory appliances  110   a - 110   c . In an example, the external memory allocation metadata  414  may be included in the region metadata  215  of the memory appliances  110   a - 110   c  including one or more of the regions R 1 -R 3  referenced by the external memory allocation. Accordingly, if the management server  120  fails, a backup management server may take its place by retrieving the external allocation metadata  414  from the region metadata  215  included in one of the memory appliances  110   a - 110   c . In a second example, the external memory allocation metadata  414  may be distributed amongst the region metadata  215   a - 215   c  of the memory appliances  110   a - 110   c  including the regions R 1 -R 3  referenced by the external memory allocation. Accordingly, if the management server  120  fails, a backup management server may take its place by retrieving the external allocation metadata  414  from the distributed portions included in the region metadata  215   a - 215   c  included in the memory appliances  110   a - 110   c . In a third example, the external memory allocation metadata  414  may be derived from the region metadata  215   a - 215   c  of the memory appliances  110   a - 110   c  including one or more of the regions R 1 -R 3  referenced by the external memory allocation. For example, the region metadata  215   a  may include information about other regions R 2 -R 3  referenced by the same external memory allocation as the region R 1 . Alternatively, or in addition, the region metadata  215   a  may include information about the logical relationships between the regions R 1 -R 3 . Accordingly, if the management server  120  fails, a backup management server may take its place by retrieving the region metadata  215   a - 215   c  included in one or more of the memory appliances  110   a - 110   c  and deriving the external memory allocation metadata  414  from the retrieved region metadata  215   a - 215   c . The allocation logic  412  may retrieve region metadata  215   a - 215   c  from the respective memory appliance  110   a - 110   c  by sending a request to get information for a region to the respective region access logic  212   a - 212   c  included in the memory appliances  110   a - 110   c.    
     The region metadata  215   a - 215   c  may include one or more flags, identifiers, semaphores and/or other data structures that may be used to identify the most up-to-date information that may be used to recover the external memory allocation metadata  414 . For example, the region metadata  215   a - 215   c  may include an identifier of a primary region and/or a secondary region, of which the corresponding metadata contains a primary copy of the information and/or a secondary copy of the information. Alternatively, or in addition, all copies of the information and/or the corresponding regions may be ranked in order from primary, through last. Updates to the copies of the information may be performed in order from primary through last. Recovery of external memory allocation data  414  may be performed by attempting to recover from the copies of the information in order from primary through last. For example, if an attempt to recover external memory allocation metadata  414  from a primary copy of the information fails, a second attempt may be made using the secondary copy, and so on. 
     An external memory allocation may be associated with one or more management servers. An external memory allocation may be associated with the management server that was used to create the external memory allocation. Alternatively, or in addition, an external memory allocation may be associated with other management servers, such as a backup management server, a centralized management server, a localized management server, a task-specific management server, and/or any other management server. Alternatively, or in addition, the external memory allocation may be associated with one or more memory appliances, one or more clients, one or more metadata servers, and/or any other entity. In other examples, the external memory allocation may be associated with one or more logics and/or non-physical entities, such as with one or more application logics, containers, jails, and/or zones. An external memory allocation may become associated with a management server by replicating information about the external memory allocation from one or more management servers associated with the external memory allocation or from one or more memory appliances including the regions referenced by the external memory allocation. In other examples, the external memory allocation may become associated with one or more other entities by replicating information about the external memory allocation to the entity/entities. For example, the external memory allocation metadata  318  may be stored within a client  130  and/or in the metadata for a container, a jail, and/or a zone. 
     The external memory allocation metadata  414  may be recoverable from information about the external memory allocation replicated onto other management servers and/or any other entity. For example, a copy of the external memory allocation metadata  414  may exist on one or more management servers. The external memory allocation metadata  414  may include one or more flags, identifiers, semaphores and/or other data structures that may be used to identify the most up-to-date copy of the external memory allocation metadata  414 . For example, the external memory allocation metadata  414  may include an identifier of a primary management server and/or a secondary management server which contains a corresponding primary copy and/or a secondary copy. Alternatively, or in addition, all copies and/or the corresponding management servers may be ranked in order from primary, through last. Updates to the copies of the information may be performed in order from primary through last. Recovery of external memory allocation metadata may be performed by attempting to recover from the copies of the information in order from primary through last. For example, if the primary management server fails, an attempt may be made to use a new management server in place of the primary management server and to recover the external memory allocation metadata  414  from the secondary management server. If the attempt to recover the external memory allocation metadata  414  from the secondary management server fails, a second attempt may be made using the tertiary management server, and so on. Alternatively, or in addition, recovery of external memory allocation metadata  414  may be performed by attempting to assign a new primary management server for the external memory allocation in order from primary through last. For example, if the primary management server fails, an attempt may be made to assign a new primary management server to be the secondary management server. Furthermore, if the attempt to assign the new primary management server for the external memory allocation to be the secondary management server fails, a second attempt may be made using the tertiary management server, and so on. If all management servers associated with an external memory allocation have failed, recovery may proceed using the region metadata, as described. 
     Upon receiving a request to find available memory appliances, the allocation logic  412  may reply with a list of memory appliances associated with the management server  120  including the allocation logic  412  receiving the request. Alternatively or in addition, the allocation logic  412  may reply with a list of memory appliances known to the allocation logic  412 . Alternatively or in addition, the allocation logic  412  may reply with a list of memory appliances meeting a set of characteristics and/or configurations specified by the request. 
     Upon receiving a request to query available space on the memory appliances, the allocation logic  412  may determine the available space on the memory appliances, such as  110   a - 110   c , associated with the management server  120  including the allocation logic  412  and/or known to the allocation logic  412 . The allocation logic  412  may determine the available space on a memory appliance, such as  110   a , by sending a request to get the status of the memory  210   a  included in the memory appliance  110   a  to the region access logic  212   a  of the memory appliance  110   a . Upon receiving the response message in response to the request to get the status of the memory  210   a  included in the memory appliance  110   a , the allocation logic  412  may inspect the response message to determine the available space on the memory appliance  110   a . Alternatively, or in addition, the allocation logic  412  may determine the available space on the memory appliance  110   a  by reading region metadata  215   a , if included in the memory  410  of the management server  120 . The region metadata  215   a  may be updated upon startup of the allocation logic  412  and/or management server  120 , upon detection of memory appliances  110   a - 110   c , upon association with memory appliances  110   a - 110   c , periodically, upon receiving a management request, when requested by the user interface, upon creating a region with a memory appliance, upon occurrence of any other event, and/or at any other time. The region metadata  215   a  may be updated by sending requests to get the status of the memory  210   a  included in the memory appliance  110   a . Alternatively, or in addition, the region metadata  215   a  may be updated by receiving an asynchronous notification of the status of the memory  210   a , such as the response message to the request to get the status of the memory  210   a  included in the memory appliance  110   a . The response messages to the request to get the status of the memory  210   a  included in the memory appliance  110   a  may be sent by the region access logic  212   a  in response to any event, such as a change in the region metadata  215   a  included in the memory  210   a  of the memory appliance  110   a . Upon receiving the response messages to the requests to get the status of the memory included in the memory appliances  110   a - 110   c , the allocation logic  412  may inspect the response messages to determine the available space on the memory appliances  110   a - 110   c  and/or update the region metadata  215   a - 215   c . The allocation logic  412  may respond to the request to query available space on memory appliances  110   a - 110   c  with a response message. The response message may include a summary of the status of the memory on the memory appliances  110   a - 110   c  to which the allocation logic  412  sent the requests to get the status of the memory included in the memory appliance  110   a  to the region access logic  212   a  of the memory appliance  110   a  and/or of the memory  210   a - 210   c  on the memory appliances  110   a - 110   c  which are described in the region metadata  215   a - 215   c . For example, the response message may include an aggregate of the total memory available to hold regions, the amount of memory currently in use by existing regions, and/or the amount of memory available to hold additional regions. Alternatively, or in addition, the response message may include the status of the memory on each of the memory appliances  110   a - 110   c  to which the allocation logic  412  sent the requests to get the status of the memory included in the memory appliance to the region access logic of the memory appliance and/or of the memory on the memory appliances  110   a - 110   c  which are described in the region metadata. Alternatively, or in addition, the response message may include a status, indicating whether the operation was successful. 
     Upon receiving a request to create an external memory allocation, the allocation logic  412  may determine how to structure the external memory allocation. The request may be received from the client  130 . Alternatively, or in addition, the request may be sent by a user via the user interface to provision external memory for the client  130 . In an example, the allocation logic  412  may determine to structure the external memory allocation as a single region on a single memory appliance (such as external memory allocation X 2  of  FIG.  6   ). In a second example, the allocation logic  412  may determine to structure the external memory allocation as multiple regions on multiple memory appliances (such as external memory allocations X 1  and X 3  of  FIG.  6   ). In a third example, the allocation logic  412  may determine to structure the external memory allocation using one or more logical relationships, such as striping with parity, between multiple regions on multiple memory appliances, etc. Other examples of logical relationships are described elsewhere in this disclosure. The allocation logic  412  may determine how to structure the external memory allocation, using an algorithm or heuristic. 
     The dynamic allocation of external memory may include a provisioning of a predetermined amount of memory for the client and/or for a user account on the client. One or more subsequent requests to allocate external memory for the client and/or the user account may be allocated from the predetermined amount of external memory that was provisioned for the client and/or for the user account. The request to allocate external memory and/or a subsequent request to allocate external memory may result in allocation of a subset or all of the provisioned external memory. The provisioning may be part of the dynamic allocation of the external memory. Alternatively or in addition, the provisioning may be separate from the allocation of the external memory. Thus, allocation may or may not include the provisioning depending on, for example, whether sufficient external memory has already been provisioned. The provisioning of the memory may reserve the memory such that after the memory is reserved for the client, the reserved memory may not be accessed by other clients unless the reserved memory is freed. Alternatively or in addition, if provisioned to a user account, the reserved memory may not be accessed by other user accounts until the reserved memory is freed. 
     In some examples, the dynamic allocation of external memory may include over-subscribing and/or under-provisioning of memory. Over-subscribing and/or under-provisioning of memory may be assigning, allocating, and/or provisioning more memory to one or more clients, users, and/or other entities than is actually available to be used. In a first example, a user, client  130 , application logic  314 , and/or other entity may be assigned, allocated, and/or provisioned 300 GB of memory on a client  130 , and/or the user, client  130 , application logic  314 , and/or other entity may be allowed to subsequently request up to 500 GB of memory, despite only 300 GB of memory being physically present in the client  130 . As the user, client  130 , application logic  314 , and/or other entity uses memory, the memory  310  of the client  130  may be utilized first, such as with anonymous memory and/or local primary memory. As memory use increases, the memory  210  of one or more memory appliances may be used to hold some data for the user, client  130 , and/or other entity, such as with file-backed memory and/or external primary memory. The data may be data that was initially placed in the memory  310  of the client  130 , but then subsequently infrequently used, and/or the data may be data that is recently generated by the user, client  130 , application logic  314 , and/or other entity. 
     In a second example, multiple users, clients  130 , application logics  314 , and/or other entities may be assigned, allocated, and/or provisioned a total of 100 TB of memory (such as by assigning, allocating, and/or provisioning smaller amounts to each user, client  130 , application logic  314 , and/or other entity, such that the total amount assigned, allocated, and/or provisioned equals 100 TB), and/or the multiple users, clients  130 , application logics  314 , and/or other entities may be allowed to subsequently request up to a total 200 TB (such as by allowing each user client  130 , application logic  314 , and/or other entity to request additional amounts of memory, which may be the same or different for each, such that the total amount allowed to be requested equals the remaining 100 TB), despite only 100 TB of memory being present in the memory pool. This approach may be advantageous when the statistical expected value for the amount of memory to use at any given point in time is well below the maximum allowed to be requested. During operation, the allocation logic  412  and/or another logic may monitor how much memory is actually used, assigned, allocated, and/or provisioned, and/or may determine whether and/or how much additional memory should be added to the pool to meet the actual demand. In some examples, the additional amount of memory that should be added to the pool may be reported to a user and/or system administrator, such as through a user interface. The additional amount of memory that should be added to the pool may be used by the user and/or system administrator to determine how much additional memory, how many additional memory appliances  110 , and/or what size of memory appliances  110  should be added to the memory pool. In other examples, the allocation logic  412  and/or another logic may activate one or more additional memory appliances  110  that may be powered off, in a standby state, in a low-power mode, deactivated, and/or configured not to be used. Activating the one or more additional memory appliances  110  may affect how the one or more users, clients  130 , application logics  314 , and/or other entities are billed for memory usage, such as in examples, where utilizing the additional memory appliances  110  represents a disproportionately increased cost of operation. For example, one or more users, clients  130 , application logics  314 , and/or other entities that requested memory beyond the initially assigned, allocated, and/or provisioned amount may be charged a higher rate for the additionally requested memory than for the initially assigned, allocated, and/or provisioned memory. 
     One or more user accounts may be logically grouped together and/or may be operated upon as a group. A group of one or more user accounts may be considered a user group. Accordingly, actions described throughout this disclosure as being performed upon and/or by one or more user accounts may alternatively or in addition be performed upon and/or by one or more user groups. User groups may be any logical grouping of user accounts, such as Lightweight Directory Access Protocol (LDAP) groups, Active Directory security groups, and/or any other logical grouping of user accounts known now or later discovered. 
     Provisioning may be the reservation of memory, but alternatively or in addition, provisioning the external memory may include providing an indication of how to allocate memory, in other words, provisioning may include providing or creating an indication of an allocation strategy. The allocation logic, for example, may use the indication of the allocation strategy to determine the allocation strategy used in allocating memory. The indication of the allocation strategy may be created by a user logged into a user account, such as an administrator account. Alternatively or in addition, the indication of the allocation strategy may be created by a configuration unit  415  or any other module. The configuration unit may be a component that creates the indication of the allocation strategy based on information received through a third user interface and/or API. The third user interface may be included, in some examples, in the user interface and/or the second user interface described above. The third user interface may be a graphical user interface, a web interface, a command-line interface, an API, and/or any other type of interface or combination of interfaces known now or later discovered through which data may be received. 
     The configuration unit  415  may be included in the management server  120  as illustrated in  FIG.  6   . Alternatively or in addition, the configuration unit  415  may be included in any other device, such as the client  130  or the memory appliance  110 . 
     The indication of the allocation strategy may include one or more policies, passed functions, steps, and/or rules that the allocation logic follows to determine how to allocate external memory. Determining how to allocate external memory, for example, may include identifying the memory appliances on which to allocate requested memory. Alternatively, or in addition, the indication of the allocation strategy may include profiles for memory appliances, clients, and/or user accounts. The profiles may indicate to the allocation logic how to allocate the memory. 
     Passed functions may be any logic which may be provided by a first logic as a parameter when interacting with a second logic. For example, the passed function may be computer executable instructions or computer code. The passed function may be embodied in a computer readable storage medium, such as the memory  210 ,  310 ,  410  and/or the backing store  260 ,  360 ,  460  and/or may be transmitted via an interconnect, such as the interconnects  140 , for operation with another entity. For example, the passed function may be transmitted via the interconnects  140  from the client logic  312  or any other logic to the management server  120 , for execution with the processor  440  of the management server. Passed functions may be used to define custom, adaptive, and/or flexible allocation strategies that may be cumbersome to express in other ways. 
     Creating or providing the indication of the allocation strategy may be provisioning external memory for one or more application logics  314 , for one or more of the clients  130 , for one or more client groups, for one or more user accounts, for one or more user groups, and/or for predetermined purposes. In a first example, creating the indication of the allocation strategy may include associating a user account with a high priority setting. Creating such an association may provision external memory for use by the user account from a set of the memory appliances that are configured to be used by any high priority user accounts. In a second example, creating the indication of the allocation strategy may include setting a time limit for a user account, such as a time-of-day limit or a duration-of-use limit. Setting the time limit may provision external memory for use by the user account only during predetermined times, such as during a predetermined time of day or during predetermined days of a week, or only for a predetermined length of time. In a third example, creating the indication of the allocation strategy may include setting a maximum external memory usage limit for a user account, thus limiting the amount of external memory that may be allocated to the third user account. In a fourth example, creating the indication of the allocation strategy may include creating one or more policies, passed functions, steps, and/or rules that indicate the allocation logic is to prefer to allocate memory on the memory appliances having low network bandwidth when satisfying requests from the clients that have low network bandwidth. In other words, low network bandwidth clients may be provisioned with low network bandwidth external memory and/or lower speed external memory. The client profile, for example, may indicate that the client is a low network bandwidth client. In a fifth example, creating the indication of the allocation strategy may include identifying one or more policies, passed functions, steps, and/or rules that indicate the allocation logic is to prefer to allocate memory on the memory appliances that have a network locality near to the clients. In other words, external memory may be provisioned to the clients with a network locality within a threshold distance of the memory appliances that contain the provisioned memory. Other examples of provisioning may include configuring the allocation logic to execute any of the other policies, passed functions, steps, and/or rules described elsewhere in this document. Provisioning external memory may include configuring any of the characteristics and/or configurations of the memory appliance, the client, and/or the user account described elsewhere in this document on which the allocation logic determines how to allocate memory. The policies, passed functions, steps, and/or rules to use may be configured for each memory appliance, for each client, and/or for each user account. Alternatively or in addition, the policies, passed functions, steps, and/or rules to use may be configured globally for all memory appliances, clients, and/or user accounts. The policies, passed functions, steps, and/or rules to use may be configured with relative priorities to each other, such as by ranking the policies, passed functions, steps, and/or rules in order of precedence. All or part of the profiles may be determined and/or identified by the allocation logic. For example, the allocation logic may auto-detect that the client and/or the memory appliance has low network bandwidth by measuring performance and/or by retrieving information indicating performance. 
     The allocation logic  412  may include one or more policies, passed functions, steps, and/or rules to determine which memory appliances to use or select for a particular external memory allocation. The allocation logic  412  may determine the memory appliances based on factors such as, how much memory to use on each memory appliance, which one or more logical relationship types to use if any, which restrictions to place upon the external memory allocation if any, and/or whether to reject the request. For example, the allocation logic  412  may use memory appliances that are associated with the management server  120  and/or known to the allocation logic  412  of the management server  120 . Alternatively, or in addition, the allocation logic  412  may determine the memory appliances to use based on a profile that includes one or more of the characteristics and/or configurations of the memory appliances. Alternatively, or in addition, one or more clients  130 , one or more client groups, one or more user accounts, and/or one or more user groups may be provisioned to use and/or select from one or memory appliances  110  specified by the provisioning. For example, a user group containing users in an accounting department may be provisioned to use one or more memory appliances  110  of a group of memory appliances  110  owned by the accounting department. In another example, a user group containing users in an engineering department may be provisioned to use one or more memory appliances  110  of a group of memory appliances owned by the engineering department. In other examples, the interconnects  140  may be partitioned into two or more interconnect partitions, and a first client group containing clients  130  in a first interconnect partition may be provisioned to use and/or select one or more memory appliances  110  of a group of memory appliances in the first interconnect partition, while a second client group containing clients  130  in a second interconnect partition may be provisioned to use and/or select one or more memory appliances  110  of a group of memory appliances in the second interconnect partition. 
     In a first example, the allocation logic  412  may use or select memory appliances that have the least amount of available memory while still having enough to hold the entire external memory allocation in a single region. In a second example, the allocation logic  412  may use or select memory appliances that have network locality near to the client  130 . In a third example, the allocation logic  412  may use or select memory appliances that have a backing store. In a fourth example, the allocation logic  412  may use or select memory appliances that have low network utilization. In a fifth example, the allocation logic  412  may use or select memory appliances that have low latency for client-side memory access. In a further example, the allocation logic  412  may use or select memory appliances that have high bandwidth for client-side memory access. In other examples, the allocation logic  412  may use or select memory appliances  110  based upon current and/or recent usage of the memory appliances  110 , such as the lowest, highest, average, mean, median, mode, etc. value of one or more parameters. Examples of such parameters may include: utilization, amount of capacity allocated, bandwidth consumed, processor load, number of regions allocated, operations-per-second handled, number of clients  130  accessing the region(s)  214  of the memory appliance  110 , number of client logics accessing the region(s)  214 , etc. 
     Alternatively, or in addition, the allocation logic  412  may utilize a profile that includes one or more characteristics and/or configurations of the client  130  and/or of a user account. In addition to those listed elsewhere in this disclosure, characteristics and/or configurations of the client  130  and/or of the user account may include, for example: relative priority, absolute priority, quotas, maximum external memory usage limits, current external memory usage, maximum persistent external memory usage limits, current persistent external memory usage, maximum volatile external memory usage limits, current volatile external memory usage, time-of-day limits, duration-of-use limits, last access time, maximum allowed not-in-use threshold, and/or any other properties describing the capabilities of, actions of, and/or privileges assigned to, the client  130  and/or the user account. In a first example, the allocation logic  412  may use or select memory appliances with older hardware revisions for user accounts with low relative priority. In a second example, the allocation logic  412  may use or select memory appliances with low latency for client-side memory access for clients with high absolute priority. In a third example, the allocation logic  412  may reject a request to create an external memory allocation outside a time-of-day limit for the user account. In a further example, the allocation logic  412  may prefer to use or select memory appliances with low network bandwidth for clients with low network bandwidth. In a fifth example, the allocation logic  412  may assign a short lease time for user accounts with a short duration-of-use limit. 
     Alternatively, or in addition, a separate module, other than the allocation logic  412  may be included in the management server  120  to determine the distribution of the external memory across the memory appliances. Alternatively, or in addition, the distribution may be determined by the client logic  312  and/or the region access logic  212 . Alternatively, or in addition, the determination of the distribution of the regions of the external memory allocation may be distributed between multiple logics, such as the client logic  312  and the allocation logic  412 . All of, or a portion of, the steps performed for the determination may be included in the request to create an external memory allocation or in any other message or data sent from the client logic  312  to the allocation logic  412 . Alternatively or in addition, the request to create an external memory allocation may include an indication of which factors to use to determine how to structure the external memory allocation. Alternatively or in addition, the request to create an external memory allocation may include parameters to be used when determining the structure of the external memory allocation. For example, the parameters may include one or more physical locations to be used when choosing memory appliances based upon physical locations. Alternatively, or in addition, the parameters may include information describing the user account and/or access parameters to be used when choosing memory appliances based upon user accounts. Alternatively, or in addition, the user account and/or access parameters may be specified at the time a connection, such as an SSL connection, is established between the client logic  312  and the allocation logic  412 . 
     The allocation logic  412  or another logic may adapt the allocation strategy, policies, passed functions, steps, rules, and/or any other factor(s) affecting the provisioning and/or allocation of external memory to accommodate changing circumstances. Alternatively, or in addition, the allocation strategy, policies, passed functions, steps, and/or rules, and/or other factor(s) may be defined to be adaptive. For example, an adaptive allocation strategy may specify that when network congestion is high, the allocation logic  412  is to prefer to allocate memory on the memory appliances  110  that have a network locality near to the client  130  and/or may specify that when network congestion is low, the allocation logic  412  is to prefer to allocate memory on the memory appliances  110  with low bandwidth utilization. 
       FIG.  7    illustrates an example flowchart for creation of an external memory allocation. The request to create an external memory allocation may include additional parameters that the allocation logic  412  may use to affect the external memory allocation, the regions, the logical relationships between the regions, and/or a combination of these ( 702 ,  704 ). In a first example, the request to create an external memory allocation may include a size to use for the external memory allocation. In a second example, the request to create the external memory allocation may include a preferred external memory allocation identifier. The preferred external memory allocation identifier may refer to an existing external memory allocation. If an existing external memory allocation referenced by the preferred external memory allocation identifier exists, the allocation logic  412  may select the existing external memory allocation instead of creating a new external memory allocation ( 706 ,  710 ). The allocation logic  412  may perform additional checks before selecting an existing external memory allocation, such as confirming that the other parameters specified in the request to create an external memory allocation match the existing external memory allocation ( 710 ). Upon selecting an existing external memory allocation, if the regions referenced by the existing external memory allocation are not included in the memories of the corresponding memory appliances, the allocation logic  412  may optionally send a request to restore the contents of a region from a backing store for each region referenced by the existing external memory allocation ( 712 ,  714 ) and/or may cause the contents of the region(s)  214  to be initialized, such as by writing zeros to the region(s)  214  and/or by requesting the region access logic  212  to initialize the region(s)  214 . Alternatively, or in addition, the request to create an external memory allocation may include an indicator that only existing external memory allocations may be considered when attempting to satisfy the request. If no existing external memory allocations match the parameters indicated in the request to create an external memory allocation, then the request to create an external memory allocation may fail ( 780 ). Alternatively, the allocation logic  412  may reconfigure existing regions by sending region reconfiguration requests to the region access logic  212  of the memory appliances including the existing regions ( 716 ). For example, the allocation logic  412  may request the region access logic  212  to resize the existing regions, or revise other parameters such as the access parameters associated with the existing region. In case reconfiguration of the existing region fails, the external memory allocation may fail ( 780 ). In another example, if the request for external memory allocation indicates, such as using parameters, a preference not to use reconfigured regions, the request may fail ( 716 ,  780 ). Alternatively, the allocation logic  412  may proceed to request creation of memory regions instead of the request failing ( 720 ). In a third example, the request to create an external memory allocation may include an allocation type. The allocation type may indicate how the allocation logic  412  and/or the region access logic  212  may treat the external memory allocation after it is created. 
     In a first example, the allocation type may indicate that the external memory allocation may be treated as volatile. A volatile external memory allocation may be destroyed if the allocation logic  412  determines that the external memory allocation is no longer in use. As such, a client logic  312  may avoid memory leaks by specifying that its external memory allocation may be treated as volatile, since a failure of the client  130 , the client logic  312 , and/or the application logic  314  may eventually lead to the allocation logic  412  determining that the external memory allocation is no longer in use, then destroying it. 
     The allocation logic  412  may determine whether or not an external memory allocation is in use by requesting and/or inspecting the status of the external memory allocation and/or of the regions referenced by the external memory allocation. The status may include an indication of how many clients are accessing the external memory allocation and/or the regions referenced by the external memory allocation. Alternatively, or in addition, the status may include an indication of the last time at which a client accessed the external memory allocation and/or the regions referenced by the external memory allocation. The allocation logic  412  may determine that the external memory allocation is not in use if the status indicates the last time at which a client accessed the external memory allocation and/or the regions referenced by the external memory allocation was older than a not-in-use threshold. The not-in-use threshold may be a parameter of the request to create an external memory allocation. Alternatively, or in addition, the not-in-use threshold may be specified by an administrator using the user interface. Alternatively, or in addition, the not-in-use threshold may be determined automatically, such as based-upon an algorithm, a heuristic, and/or a formula. An example algorithm, heuristic, and/or formula may adjust the not-in-use threshold based upon the characteristics and/or configuration of the management server  120  and/or of the memory appliances  110   a - 110   c , such as the amount of available memory. Alternatively, or in addition, the allocation logic  412  may determine whether or not an external memory allocation is in use by inspecting region metadata  215  and/or external memory allocation metadata  414 , if either or both are included in the management server  120 . Region metadata may include an indication of the last time at which a client accessed the region. External memory allocation metadata  414  may include an indication of the last time at which a client accessed the external memory allocation. 
     In a second example, the allocation type may indicate that the external memory allocation may be treated as persistent. A persistent external memory allocation may be preserved if the allocation logic  412  determines that the external memory allocation is no longer in use. A persistent external memory allocation may be preserved by enabling the external memory allocation and/or the regions referenced by the external memory allocation to continue to exist as created. Alternatively, or in addition, if the allocation logic  412  determines that the external memory allocation is no longer in use, the allocation logic  412  and/or the region access logic  212  may automatically persist the external memory allocation and/or the regions referenced by the external memory allocation to the backing stores and remove one or both from memory. As such, the client logic  312  may avoid memory leaks by specifying that its external memory allocation may be treated as persistent with a not-in-use threshold, since a failure of the client  130 , the client logic  312 , and/or the application logic  314  may eventually lead to the allocation logic  412  determining that the external memory allocation is no longer in use, then persisting it and removing it from memory. The allocation logic  412  updates the external memory allocation metadata according to the allocation type specified ( 730 ). 
     The allocation logic  412  may also associate an external memory identifier with the external memory allocation metadata ( 730 ). The external memory identifier may be chosen by the allocation logic  412  or it may be included in the request to create an external memory allocation. For example, the external memory identifier may be the preferred external memory identifier included in the request to create an external memory allocation. 
     The allocation logic  412  may send a request to create a region to the region access logic  212  of each memory appliance selected to hold memory regions referenced by the external memory allocation ( 720 ,  722 ,  724 ). The memory appliances are selected as described earlier. Upon receiving the response message in response to each request to create a region, the allocation logic  412  may inspect the response message and/or record any relevant metadata ( 728 ). In case any of the region allocation requests fail, the creation of the external memory allocation may fail ( 780 ). 
     The allocation logic  412  may respond to the request to create an external memory allocation with a response message. The response message may include the external memory identifier. Alternatively, or in addition, the response message may include identifiers and/or information describing the regions referenced by the external memory allocation and/or the logical relationships between the regions. Alternatively, or in addition, the response message may include a status, indicating whether the operation was successful ( 790 ,  740 ,  780 ). 
     Upon receiving a request to resize an existing external memory allocation, the allocation logic  412  may determine how to structure the resized external memory allocation. In a first example, the allocation logic  412  may determine to structure the resized external memory allocation as a single region on a single memory appliance. In a second example, the allocation logic  412  may determine to structure the external memory allocation as multiple regions on multiple memory appliances. In a third example, the allocation logic  412  may determine to structure the external memory allocation using one or more logical relationships, such as striping with parity, between multiple regions on multiple memory appliances. Determining how to structure the resized external memory allocation may include determining whether to resize the existing regions referenced by the external memory allocation, whether to create additional regions, and/or whether to replace existing regions with new regions. 
     The allocation logic  412  may determine how to structure the resized external memory allocation. As additional examples to those listed elsewhere in this disclosure, the allocation logic  412  may resize existing regions when expanding or contracting an external memory allocation that only references one region, if sufficient space is available in the memory of the memory appliance including the region. In another example, the allocation logic  412  may create additional regions when expanding an external memory allocation that uses a logical relationship involving a concatenation of the regions referenced by the external memory allocation. In another example, the allocation logic  412  may resize existing regions when expanding or contracting an external memory allocation that uses a logical relationship involving striping and/or mirroring, if sufficient space is available in the memory of the memory appliances including the regions referenced by the external memory allocation. 
     The allocation logic  412  may create additional regions by sending a request to create a region to the region access logic  212  of the memory appliances selected to include the additional regions. Alternatively, or in addition, the allocation logic  412  may resize existing regions by sending a request to resize an existing region to the region access logic  212  of the memory appliances including the existing regions. Alternatively, or in addition, the allocation logic  412  may destroy existing regions by sending a request to destroy a region to the region access logic  212  of the memory appliances including the existing regions. 
     The allocation logic  412  may replace an existing region with a new region by creating the new region, then replicating the data from the existing region onto the new region. For example, the allocation logic  412  may perform client-side memory access to read the data from the existing region and write to the new region. Alternatively, or in addition, the allocation logic  412  may send a request to migrate a region to the region access logic  212  of the memory appliance including the existing region and/or to the region access logic  212  of the memory appliance selected to include the new region. 
     The allocation logic  412  may respond to the request to resize an existing external memory allocation with a response message. The response message may include the external memory identifier. Alternatively, or in addition, the response message may include identifiers and/or information describing the regions referenced by the external memory allocation and/or the logical relationships between the regions. Alternatively, or in addition, the response message may include a status, indicating whether the operation was successful. 
     Upon receiving a request to renew an external memory allocation, the allocation logic  412  may update the external memory allocation metadata  414  associated with the external memory allocation identified by the request. The request to renew an external memory allocation may include an external memory allocation identifier. Alternatively, or in addition, the request to renew an external memory allocation may include a value to be used for the not-in-use threshold of the external memory allocation. The allocation logic  412  may respond to the request to renew an external memory allocation with a response message. The response message may include a status, indicating whether the operation was successful. Alternatively, or in addition, the response message may include the not-in-use threshold of the external memory allocation and/or the time left until the external memory allocation may be considered not-in-use. 
     Upon receiving a request to destroy an external memory allocation, the allocation logic  412  may send to the region access logic  212  of each corresponding memory appliance, a request to destroy a region for each region referenced by the external memory allocation. Upon receiving the response message in response to each request to destroy a region, the allocation logic  412  may inspect the response message and/or record any relevant metadata. The allocation logic  412  may respond to the request to destroy an external memory allocation with a response message. The response message may include a status, indicating whether the operation was successful. 
     Upon receiving a request to persist and free an existing external memory allocation, the allocation logic  412  may, for each region referenced by the external memory allocation, send to the region access logic  212  of each corresponding memory appliance, a request to persist a region to a backing store and remove it from memory. Upon receiving the response message in response to each request to persist a region to a backing store and remove it from memory, the allocation logic  412  may inspect the response message and/or record any relevant metadata  414 . The allocation logic  412  may respond to the request to persist and free an existing external memory allocation with a response message. The response message may include a status, indicating whether the operation was successful. 
     Upon receiving a request to list existing external memory allocations, the allocation logic  412  may respond with a response message. The response message may include a list of external memory allocation identifiers for the external memory allocations associated with the management server  120 . For example, in case of the system as illustrated in  FIG.  6   , the management server  120  may provide a list containing information of the external memory allocations X 1 -X 3 . Alternatively, or in addition, the response message may include a status, indicating whether the operation was successful. 
     Upon receiving a request to get information regarding an external memory allocation, the allocation logic  412  may respond with a response message. The response message may include the external memory identifier. Alternatively, or in addition, the response message may include identifiers and/or information describing the regions referenced by the external memory allocation and/or the logical relationships between the regions. 
     For example, in case the allocation logic  412  of the management server  120  of  FIG.  6    receives a request to get information regarding the external memory allocation X 1 , the allocation logic  412  may retrieve region metadata  215   a - c  by sending a request to get information for the regions R 1 -R 3  referenced by X 1 . The requests may be sent to the region access logic  212   a - 212   c  included in the memory appliances  110   a - 110   c  including the regions R 1 -R 3 . The identifiers and/or information may include access parameters for the regions, such as a password and/or an encryption key. Alternatively, or in addition, the response message may include metadata describing the external memory allocation X 1 , such as the not-in-use threshold for the external memory allocation X 1  and/or for the regions R 1 -R 3 . Alternatively, or in addition, the response message may include a status, indicating whether the operation was successful. 
     Upon receiving a request to restructure an external memory allocation, the allocation logic  412  may determine how to structure the external memory allocation. The allocation logic  412  may determine how to structure the external memory allocation as described elsewhere in this disclosure. Alternatively, or in addition, the request to restructure an external memory allocation may include parameters specifying a desired structure for the external memory allocation and/or specifying parameters to be used to determine a revised structure. For example, the parameters may cause the allocation logic  412  to use a logical relationship involving striping between the regions referenced by the external memory allocation. Alternatively, or in addition, the parameters may cause the allocation logic  412  to use memory appliances with higher or lower bandwidth than the memory appliances including the regions currently referenced by the external memory allocation. Alternatively, or in addition, the parameters may cause the allocation logic  412  to use memory appliances that are closer to or farther from a specified physical and/or network location than the memory appliances including the regions currently referenced by the external memory allocation. As such, a request to restructure an external memory allocation may be used to migrate data closer to or further away from the client  130 , for increasing or decreasing locality and/or for increasing or decreasing performance. This migrating of data may be part of a broader approach for balancing the cost/performance of memory included in the client  130 , memory included in the memory appliances, and/or other media capable of holding the data, such as the backing store or other backing stores. Determining how to structure the external memory allocation may include determining whether to resize the existing regions referenced by the external memory allocation, whether to create additional regions, and/or whether to replace existing regions with new regions. The allocation logic  412  may resize the existing regions referenced by the external memory allocation, create additional regions, and/or replace existing regions with new regions using the methods described throughout this disclosure. 
     Using information provided by the allocation logic  412 , by the region access logic  212 , or both, the client logic may access one or more regions using client-side memory access. The client  130  may present a data interface to the application logic  314 . The data interface may take many forms and/or may depend upon the preferences of the application logic  314  and/or of the users. Some examples of data interfaces may include: an API, block-level interface, a character-level interface, a memory-mapped interface, a memory allocation interface, a memory swapping interface, a memory caching interface, a hardware-accessible interface, a graphics processing unit (GPU) accessible interface and/or any other interface used to access the data and/or metadata of the memory appliance  110 , the management server  120 , the region  214 , the external memory allocation, and/or the regions referenced by the external memory allocation. Alternatively or in addition, the data interface may include multiple interfaces. The data interface may be a data interface unit. The functionality of any of the data interfaces may be provided using all of or a portion of the functionality of any one or more of the other data interfaces. For example, a block-level interface may use methods and/or interfaces of an API in order to retrieve and/or manipulate external memory allocations and/or the regions referenced by an external memory allocation. Alternatively, or in addition, an API may include methods and/or interfaces to manipulate a block device interface. 
     Multiple data interfaces may be provided by the client logic. Alternatively, or in addition, multiple data interfaces may be accessed simultaneously. For example, a first application logic operating in a first client may access the region  214  via an API while a second application logic operating in a second client may access the same region  214  via a block-level interface. The first application logic and the second application logic may coordinate access to ensure data integrity and/or the application logics may not coordinate, such as when only performing read operations on the region  214 . Alternatively, or in addition, a first client logic operating in the first client may coordinate with a second client logic operating in the second client to ensure data integrity. Coordinating to ensure data integrity may be performed using any known or later discovered logic and/or method for coordinating access to data, including but not limited to atomic operations, lock primitives, read-copy-update, etc. 
     In a first example, the data interface may include an API. An API may provide one or more interfaces for the application logic  314  to invoke that manipulate a region. The interface(s) for the application logic  314  to invoke that manipulate a region may include interface(s) that manipulate data included in the region, interface(s) that manipulate the metadata associated with the region, interface(s) that manipulate the access controls for the region, and/or any other interface(s) related to the region. For example, an interface may enable the application logic  314  to read or write data to a specific location within the region. Alternatively, or in addition, an API may provide one or more interface(s) for the application logic  314  to invoke that manipulate an external memory allocation. The interface(s) for the application logic  314  to invoke that manipulate an external memory allocation may include interface(s) that manipulate data included in the regions referenced by the external memory allocation, interface(s) that manipulate the metadata associated with the regions, interface(s) that manipulate the metadata associated with the logical relationships between the regions, interface(s) that manipulate the metadata associated with the external memory allocation, interface(s) that manipulate the access controls for the regions, interface(s) that manipulate the access controls for the external memory allocation, and/or any other interface(s) related to the external memory allocation, the logical relationships between the regions, and/or the regions referenced by the external memory allocation. In an example, an interface may enable the application logic  314  to read or write data to a specific location within the external memory allocation. Reading data from a first location within an external memory allocation may cause data to be read from one or more second locations within one or more regions referenced by the external memory allocation. Writing data to a first location within an external memory allocation may cause data to be written to one or more second locations within one or more regions referenced by the external memory allocation. The second locations and the regions may be determined based upon the logical relationships between the regions. In a second example, an interface may enable the application logic  314  to run a consistency check upon an external memory allocation that uses a parity-based logical relationship. In a third example, an interface may facilitate the application logic  314  to register the memory of the client and/or a portion of the memory with one or more communication interfaces. Registering memory may cause subsequent client-side memory access operations using the registered memory to proceed more quickly and/or more efficiently than operations not using the registered memory. 
     In another example, an API may allow the application logic  314 , the client logic  312 , and/or another logic to provide one or more passed functions to manipulate a region  214  and/or an external memory allocation. The passed function(s) may be used to perform custom and/or flexible manipulations upon the region  214  and/or the external memory allocation that may be cumbersome to express in other ways. For example, the passed function(s) may perform a matrix multiply operation upon the data of the region  214  and/or the external memory allocation. The logic of the passed function(s) may be operated at one or more of: the client  130 , the memory appliance  110 , the management server  120  and/or any other entity capable of operating the logic of the passed function, such as a metadata server. In one example, the logic of the passed function(s) may be operated at the client  130 , such as by the client logic  312  performing a sequence of read and/or write operations specified by the passed function. In another example, the logic of the passed function(s) may be operated at the memory appliance  110 , such as by the region access logic  212  performing a matrix add operation upon the data of the region  214 . In another example, the logic of the passed function(s) may be operated at the management server  120 , such as by the allocation logic  412  performing one or more operations upon the external memory allocation metadata  414 . In other examples, the logic of the passed function(s) may be operated at multiple locations, such as if one passed function is operated at the management server  120  and another is operated at the memory appliance  110  or if a single passed function is operated partially at the client  130  and continues operation at the memory appliance  110 . 
     Alternatively, or in addition, an API may provide one or more interfaces for the application logic  314  to invoke that retrieve, present, and/or manipulate information related to the management servers, the memory appliance, the external memory allocations, the regions referenced by the external memory allocations, and/or the logical relationships between the regions. The interface(s) may provide functionality similar to the allocation logic requests and/or region access logic requests. Alternatively, or in addition, the interface(s) may provide functionality similar to a combination of one or more of the allocation logic requests and/or region access logic requests. In a first example, an API may provide interface(s) for the application logic  314  to retrieve a list of management servers. In a second example, an API may provide interface(s) for the application logic  314  to retrieve a list of memory appliances, such as the memory appliances associated with a management server and/or known by the allocation logic of a management server. In a third example, an API may provide interface(s) for the application logic  314  to retrieve a list of external memory allocations, such as the external memory allocations associated with a management server. In a fourth example, an API may provide interface(s) for the application logic  314  to retrieve a list of regions, such as the regions included in the memory of a memory appliance or the regions associated with an external memory allocation. In a fifth example, an API may provide interface(s) for the application logic  314  to retrieve information related to an external memory allocation, such as the size of the external memory allocation, the regions referenced by the external memory allocation, and/or the logical relationships between the regions. In a fifth example, an API may provide interface(s) for the application logic  314  to manipulate an external memory allocation. An API may manipulate the external memory allocation using the allocation logic requests and/or the region access logic requests. In a sixth example, an API may provide interface(s) for the application logic  314  to manipulate a region. An API may manipulate a region using the region access logic requests. 
     In a second example, the data interface may include a block-level interface. The block-level interface may provide block-level access to data of a region. Alternatively or in addition, the block-level interface may provide block-level access to data of one or more of the regions referenced by an external memory allocation. Alternatively or in addition, the block-level interface may provide block-level access to data of the external memory allocation. Block-level access to data may include reading data from or writing data to a consistently-sized and/or aligned portion of a region or an external memory allocation. The client logic may provide block-level access using a block device interface. Alternatively, or in addition, the client logic may provide block-level access using any storage protocol now known or later discovered. A storage protocol may be any communications protocol used to transfer data between a block storage device, interface, or system, such as the block-level interface or any other data interface, and a device or system, such as the client or another client, that stores data in, and/or retrieves data from, the block storage device, interface, or system. A storage protocol may be implemented, for example, using one or more software and/or hardware storage controllers. The storage protocol and electrical characteristics of the hardware storage controller may be part of a common standard. In one example, the storage protocol may be the universal serial bus mass storage device class (USB MSC or UMS), which is a set of computing communications protocols defined by the USB Implementers Forum that runs on a hardware bus, such as the one or more interconnects, that conforms to the USB standard. In a second example, the storage protocol may be the Small Computer System Interface (SCSI) command protocol. In a third example, the storage protocol may be the Serial Advanced Technology Attachment (SATA) protocol. Additional examples of the storage protocol include Serial Attached SCSI (SAS) and Internet Small Computer System Interface (iSCSI). Alternatively or in addition, the block-level interface may provide block-level access using any storage protocol that transfers data with a data transfer protocol, such as SCSI over Fibre Channel, SCSI RDMA Protocol (SRP) over Remote Direct Memory Access (RDMA), iSCSI over TCP/IP, NVMe over Fabrics, NVMe over Fibre Channel, or any other combination of storage protocol and data transfer protocol known now or discovered in the future. Alternatively, or in addition, the block-level interface may provide block-level access by emulating the storage protocol and/or data transfer protocol. In one example, the block-level interface may provide block-level access by providing a SCSI command interface to the application logic. In a second example, the block-level interface may provide block-level access using a storage protocol with an emulated data transfer protocol, such as with a virtualized communication interface. 
     In a third example, the data interface may include a character-level interface. The character-level interface may provide character-level and/or byte-level access to data of a region. Alternatively or in addition, the character-level interface may provide character-level and/or byte-level access to data of one or more of the regions referenced by an external memory allocation. Alternatively or in addition, the character-level interface may provide character-level and/or byte-level access to data of the external memory allocation. The client logic may provide character-level access using a character device interface. Character-level access may enable the application logic  314  to read and/or write to character-aligned portions of the external memory allocation or of the regions referenced by the external memory allocation. Byte-level access may enable the application logic  314  to read and/or write to byte-aligned portions of the external memory allocation or of the regions referenced by the external memory allocation. Alternatively or in addition, the character-level interface may enable the application logic  314  to seek to a specified location within the external memory allocation or the regions referenced by the external memory allocation. Seeking to a specified location may cause subsequent attempts to read and/or write to the external memory allocation or the regions referenced by the external memory allocation to start at the most recently seeked-to location. Alternatively, or in addition, attempts to read and/or write to the external memory allocation or the regions referenced by the external memory allocation may start at a location after the most recently read and/or written portion. 
     In a fourth example, the data interface may include a memory-mapped interface. The memory mapped interface may enable the application logic  314  to map all of or a portion of a region, an external memory allocation and/or of one or more regions referenced by the external memory allocation into a virtual address space, such as the virtual address space of the application logic. The memory-mapped interface may include an API. Alternatively, or in addition, the memory-mapped interface may include and/or utilize a block-level interface and/or a character-level interface. In one example, the memory-mapped interface may enable the application logic  314  to map all of or a portion of a block device interface into a virtual address space, such as the virtual address space of the application logic. 
     The virtual address space may be one or more ranges of virtual addresses that are made available for one or more application logics to access memory, hardware devices, input-output interfaces, and/or any other resource known now or later discovered that the application may be configured to access. The virtual addresses of the virtual address space may be translated to other addresses, such as physical addresses, guest physical addresses, guest virtual addresses, host virtual addresses, host physical addresses, and/or any other addresses, locations, and/or identifiers known now or later discovered which reference the resource that may be mapped to the virtual addresses. The virtual addresses may be referred to as virtual memory addresses. Guest physical addresses and/or guest virtual addresses may be addresses used by a virtual machine as physical addresses and/or virtual addresses, respectively, such as by an operating system running within the virtual machine. Guest physical addresses may be treated as virtual addresses by a hypervisor operating the virtual machine. Host virtual addresses and/or host physical addresses may be addresses used by the hypervisor as virtual and/or physical addresses, respectively. 
     The virtual address space of the application logic may be the virtual address space made available to the application logic  314 . The client  130  may include multiple virtual address spaces and/or multiple application logics  314 . Each application logic  314  may be associated with a corresponding virtual address space. Alternatively or in addition, multiple application logics  314  may be associated with the same virtual address space, such as with a multi-threaded application and/or with an application that utilizes additional logic, such as from a shared library. For example, if all of or a portion of the client logic  312  is implemented as a shared library, that portion of the client logic  312  may be associated with the virtual address space of the application logic when invoked by the application logic  314 . 
     The memory mapped interface may include a page fault handler interface. The page fault handler interface may be executed when the application logic attempts to access a first portion of the virtual address space. The first portion may be configured to trigger the page fault handler when accessed. The first portion may be a page of the virtual address space. Alternatively, or in addition, the first portion may be included in the mapped portion of the virtual address space. The page fault handler may perform client-side memory access to read a second portion of the external memory allocation and/or of one or more regions referenced by the external memory allocation into a third portion of the memory of the client. The third portion may be a page of the memory of the client. Alternatively, or in addition, the page fault handler may allocate the third portion of the memory of the client  130 . The page fault handler may map the first portion of the virtual address space to the third portion of the memory. The first portion may correspond to the second portion. For example, the offset of the first portion within the mapped portion of the virtual address space may equal the offset of the second portion within the external memory allocation or the regions referenced by the external memory allocation. Alternatively, or in addition, the second portion may include a fourth portion corresponding to the third portion. The portion of the second portion not included in the fourth portion may be considered a fifth portion. For example, the page fault handler interface may determine based upon a pattern of calls to the page fault handler interface that the fifth portion of the external memory allocation and/or of the one or more regions may be needed soon and therefore, may be read into the memory in anticipation, such as with a read-ahead predicting algorithm. 
     Alternatively, or in addition, the memory mapped interface may include a background process and/or logic. The background process and/or logic may periodically flush dirty pages. Flushing dirty pages may include performing client-side memory access to write the data from the dirty pages to the corresponding locations within the external memory allocation and/or the one or more regions referenced by the external memory allocation. Dirty pages may be pages included in the memory of the client which have been written to by the application logic  314  and/or the client logic  312  since they were last read from or written to the external memory allocation and/or the one or more regions referenced by the external memory allocation. 
     Alternatively, or in addition, the memory mapped interface may include one or more page-reading interfaces. For example, the memory mapped interface may include an interface to read data for a page, and/or to read data for multiple pages. The interface to read data for a page may perform client-side memory access to read a corresponding portion of the external memory allocation and/or of one or more regions referenced by the external memory allocation (such as the second portion) into the page. The interface to read data for multiple pages may perform client-side memory access to read one or more corresponding portions of the external memory allocation and/or of one or more regions referenced by the external memory allocation into the pages. The corresponding portion(s) may be fully or partially contiguous portion(s) of the external memory allocation and/or of the one or more regions, such that a single client-side memory access operation may be performed to read the data for all of the pages. Alternatively, or in addition, the interface to read data for multiple pages may utilize multiple client-side memory access operations and/or a scatter-gather operation to access multiple and/or non-contiguous portions. The page-reading interface(s) may be invoked by the page fault handler interface. Alternatively, or in addition, the page-reading interface(s) may invoked by another logic, such as a prefetch and/or read-ahead logic. 
     The prefetch and/or read-ahead logic may invoke the page-reading interface(s) and/or may otherwise cause one or more portions of the external memory allocation and/or of one or more regions referenced by the external memory allocation to be read into the memory  310  of the client  130 . By reading the portion(s) into the memory  310 , the prefetch and/or read-ahead logic may avoid one of more future invocations of the page fault handler interface for one or more of the portion(s) and/or may allow the page fault handler interface to complete without needing to read the corresponding portion(s) into the memory, such as by adding a page-table entry for a page that has already been read into the memory  310  of the client  130  by the prefetch and/or read-ahead logic. The prefetch and/or read-ahead logic may determine the portion(s) to read using any mechanism and/or combination of mechanisms for selecting pre-fetch portion(s) and/or for predicting future page faults known now or later discovered. Examples of mechanisms for selecting pre-fetch portion(s) and/or for predicting future page faults may include fixed synchronous prefetching, adaptive synchronous prefetching, fixed asynchronous prefetching, adaptive asynchronous prefetching, perfect prefetching, etc. Alternatively or in addition, the prefetch and/or read-ahead logic may incorporate additional information to help determine the portion(s), such as prefetch hints from a compiler and/or profiling logic. Alternatively or in addition, the prefetch and/or read-ahead logic or another logic, such as the client logic  312 , may include an interface that may enable the application logic  314  to request one or more portions of the memory to be read into the memory  310  of the client  130 . The interface may be utilized by the application logic  314  explicitly, such as by adding pre-fetch requests to the application logic&#39;s source code, and/or implicitly, such as if pre-fetch requests are added by a compiler, a profiler, and/or other logic. 
     Alternatively, or in addition, the memory mapped interface may include one or more page-writing interfaces. For example, the memory mapped interface may include interface(s) to write data for a page, and/or to write data for multiple pages. The interface(s) to write data for a page may perform client-side memory access to write data from the page to a corresponding portion of the external memory allocation and/or of one or more regions referenced by the external memory allocation (such as the second portion). The interface(s) to write data for multiple pages may perform client-side memory access to write data from the pages to one or more corresponding portions of the external memory allocation and/or of one or more regions referenced by the external memory allocation into the pages. The corresponding portion(s) may be fully or partially contiguous portion(s) of the external memory allocation and/or of the one or more regions, such that a single client-side memory access operation may be performed to write the data for all of the pages. Alternatively, or in addition, the interface(s) to write data for multiple pages may utilize multiple client-side memory access operations and/or a scatter-gather operation to access multiple and/or non-contiguous portions. The page-writing interface(s) may be invoked by the background process and/or logic, a page evicting interface, and/or another logic, such as a writeback worker of an operating system. 
     Alternatively, or in addition, the memory mapped interface may include a page evicting interface. Pages to be evicted may include the one or more pages of the memory used to hold the third portion of the memory of the client. The page evicting interface may be executed when the memory-mapped interface and/or an operating system determines that the pages to be evicted are unlikely to be accessed again soon, when the memory-mapped interface determines that the pages to be evicted are needed to hold data for other executions of the page fault handler interface, and/or when the pages to be evicted are needed to hold data for any other purpose. If one or more of the pages to be evicted are dirty pages, the page evicting interface may perform client-side memory access to write data from the dirty pages to a corresponding portion of the external memory allocation and/or the regions referenced by the external memory allocation. The page evicting interface may update metadata to indicate that the pages to be evicted may be re-used for other purposes, such as by the page fault handler interface. 
     In a fifth example, the data interface may include a memory allocation interface. The memory allocation interface may include an API. The memory allocation interface may include one or more interfaces that enable the application logic  314  to allocate one or more buffers. For example, an application may allocate a buffer to hold an integer, an array of integers, a character, a string, and/or any other data. Alternatively, or in addition, the memory allocation interface may include one or more interfaces that enable an application-level memory allocator to allocate one or more slabs of memory. A slab of memory may include one or more pages. The one or more pages included in the slab may be contiguous in a physical address space and/or in a virtual address space. A slab of memory may be further sub-divided by the application-level memory allocator. For example, the application-level memory allocator may enable the application logic  314  to allocate buffer(s) from one or more portions of the slab of memory. The memory allocation interface may utilize a memory-mapped interface. For example, allocating the buffer(s) and/or allocating the slabs of memory may include mapping all of or a portion of an external memory allocation and/or of one or more regions referenced by the external memory allocation into a virtual address space, such as the virtual address space of the application. The virtual address of a buffer and/or of a slab may be included in a portion of the virtual address space corresponding to a portion of the external memory allocation and/or of the regions. Alternatively, or in addition, allocating the buffer(s) and/or allocating the slabs of memory may include creating one or more external memory allocations and/or regions. The memory allocation interface may be made available selectively to one or more application logics. Alternatively, or in addition, the memory allocation interface may be made available to all application logics. 
     The memory allocation interface may initialize the buffer(s), the slab(s), and/or the portion(s) of memory prior to mapping. Initializing the buffer(s), slab(s), and/or the portion(s) of memory may have security advantages, such as by avoiding exposing data in freed memory of one application logic  314  to another application logic. In one example, the memory allocation interface may fill the buffer(s), the slab(s), and/or the portion(s) with zeros, such as by writing to the external memory allocation and/or the region(s). In another example, the memory allocation interface may allocate one or more cache portions, such as pages of page cache, initialize the cache portion(s), and/or associate the cache portion(s) with the buffer(s), the slab(s), and/or the portion(s) of memory. 
     In other examples, the memory allocation interface may not initialize some or all of the buffer(s), the slab(s), and/or the portion(s) of memory prior to mapping. For example, the memory allocation interface may defer initialization of some or all of the buffer(s), the slab(s), and/or the portion(s) of memory until the memory is accessed and/or written to, such as during a page fault. In examples such as these, at least a second portion may be initialized after it is mapped. When handling page faults for reading a mapped portion of the buffer(s), the slab(s), and/or the portion(s) of memory, the memory allocation interface may register a read-only page table entry for a pre-initialized page, such as a page full of zeros. The page full of zeros may be provided for multiple page faults and/or may be referenced by multiple page table entries. When handling page faults for reading a mapped portion of the buffer(s), the slab(s), and/or the portion(s) of memory, the memory allocation interface may allocate one or more cache portion(s), such as pages of page cache, initialize the cache portion(s), and/or associate the cache portion(s) with the buffer(s), the slab(s), and/or the portion(s) of memory. Alternatively or in addition, the memory allocation interface may allocate the one or more cache portion(s) when handling page faults for reading. 
     Deferring initialization of some or all of the buffer(s), the slab(s), and/or the portion(s) of memory until accessed and/or written to may be advantageous, because it may enable the memory allocation interface to handle requests to allocate memory more quickly and/or because it may enable the memory allocation interface to avoid accessing the external memory allocation and/or the region(s) for initialization of the buffer(s), the slab(s), and/or the portion(s) of memory. For example, the external memory allocation and/or the region(s) may only be written to if and/or when the cache portion(s) are to be written-back to the external memory allocation and/or the region(s). For buffer(s), slab(s), and or portion(s) or memory which are freed shortly after being allocated, the memory allocation interface may avoid accessing the external memory allocation and/or the region(s) entirely, such as by initializing the cache portion(s) when the memory is accessed and discarding the cache portion(s) when the buffer(s), the slab(s), and/or the portion(s) of memory are freed. 
     In some embodiments, the client logic  312  may select a portion of the region  214  and/or of the external memory allocation to be the slab in coordination with one or more other logics. For example, the client logic  312  may select a portion of the region  214  and/or of the external memory allocation to be the slab in coordination with at least one of a different client logic, the region access logic  212 , the observer logic  218 , the allocation logic  412 , the application logic  314 , and/or any other logic. Collectively, the client logic  312  and/or the one or more other logics may be considered allocation-coordinating logics. 
     The client logic  312  and/or other logic(s) may coordinate access using one or more memory-allocation data structures. The memory-allocation data structure(s) may be shared by the client logic  312  and/or other logic(s). For example, the memory-allocation data structure(s) may be included in a shared memory and/or shared storage medium. In some embodiments, the memory-allocation data structure(s) may be accessible via client-side memory access. Alternatively, or in addition, one or more of the client logic  312  and/or other logic(s) may maintain a copy of the memory-allocation data structure(s). 
     The memory-allocation data structure(s) may include one or more indicators which convey whether one or more portions of the region  214  and/or of the external memory allocation have been allocated. For example, the indicator(s) may include one or more flags corresponding to one or more fixed-size portions of the region and/or of the external memory allocation. In some examples, the fixed-size portion(s) may be the size of a page of memory. In other examples, the fixed-size portion(s) may be smaller or larger than a page of memory, such as a chunk, a huge page, and/or an arena of memory. In other examples, the indicator(s) may correspond to one or more variable-sized portions of the region and/or of the external memory allocation. For example, the indicator(s) may be associated with and/or included in one or more data structures which identify one or more portions of the region  214  and/or of the external memory allocation. The one or more data structures may be organized to facilitate efficient memory allocation and/or deallocation. For example, the one or more data structures may include one or more collections of portions which may be allocated to a corresponding application logic and/or which may be unallocated. In some examples, the one or more data structures may convey whether portion(s) of the region  214  and/or of the external memory allocation have been allocated, without including explicit indicator(s), such as if the one or more data structures include one or more collections of portions which may be allocated to a corresponding application logic  314  and/or which may be unallocated. Collection(s) of portions may be organized, such as with an array, a list, a tree, a red-black tree, a B-tree, a B+ tree, a hash table, a distributed hash table, and/or with any other collection data structure. 
     In at least one example implementation, the memory-allocation data structure(s) may include an array of indicators, such as a bit-array, wherein the nth indicator corresponds to the nth fixed-sized portion of the region  214  and/or of the external memory allocation. 
     The memory-allocation data structure(s) may include other information related to the portion(s). For example, the memory-allocation data structure(s) may include addresses, offsets, and or sizes which describe the position and/or size of the portion(s) within the region  214  and/or the external memory allocation. Alternatively, or in addition, the memory-allocation data structure(s) may include client identifier(s), process identifier(s), and/or virtual address(es) to which the portion(s) are mapped. Alternatively, or in addition, the memory-allocation data structure(s) may be associated with one or more other data structures which include the other information, such as by including a pointer to the other data structure(s). 
     The memory-allocation data structure(s) and/or the other data structure(s) may be stored in the region  214 , in the external memory allocation, in the region metadata  215 , in the external memory allocation metadata  318   414 , and/or in another area of memory and/or storage. The memory-allocation data structure(s) and/or the other data structure(s) may be stored at one or more well-known locations, such as at the beginning and/or the end of the region  214 , at a specific offset within the region metadata  215  and/or external memory allocation metadata  318   414 , etc. 
       FIG.  8    illustrates an example of the memory-allocation data structure(s). In an example implementation, the memory allocation data structures may be stored in the region  214 . The memory allocation data structures may include a collection of free-memory arenas  802  and/or a collection of memory users  804 . 
     In some examples, there may be only one arena, and/or the collection of free-memory arenas  802  may be replaced by a free-memory arena data structure  806 . Alternatively or in addition, there may be only one memory user, and/or the collection of memory users  804  may be replaced by a memory user data structure  816 . 
     Collection(s), such as the collection of free-memory arenas  802 , the collection of memory users  804 , and/or any other collections described herein, may be organized, such as with an array, a list, a tree, a red-black tree, a B-tree, a B+ tree, a hash table, a distributed hash table, and/or with any other collection data structure. Collection(s) may include and/or may reference zero or more other data structures. For example, the collection of free-memory arenas  802  may include and/or may reference zero or more free-memory arena data structures  806 , such as if the collection of free-memory arenas  802  includes an array of free-memory arena data structures  806 . In another example, the collection of free-memory arenas  802  may reference zero or more free-memory arena data structures  806 , such as with an array of references to free-memory arena data structures, a linked list of free-memory arena data structures, a tree of free-memory arena data structures, and/or any other collection data structure. In other examples, the collection of memory user(s)  804  may include and/or may reference zero or more memory user data structures  816 . 
     The free-memory arena data structure(s)  806  may include an arena definition  808 , statistics  810 , one or more collections of free portions  812 , and/or collection-related data  814 . The free-memory arena data structure(s)  806  may include more, fewer, and/or different elements. For example, the free-memory arena data structure(s)  806  may not include the arena definition  808 . As another example, the free-memory arena data structure(s)  806  may not include the statistics  810 . In some examples, the free-memory arena data structure(s)  806  may only include the collection(s) of free portions  812 . The arena definition  828  may describe the corresponding arena, such as by including an offset within the region and/or a size of the arena. Alternatively, or in addition, the arena definition  828  may include a start and/or an end offset within the region for the corresponding arena. As such, the arena definition may enable the client logic  312  and/or another logic to identify the appropriate arena(s) for portions being freed. The statistics  810  may include a total of already-allocated memory associated with the free-memory arena, a total of available memory associated with the free-memory arena, a total of un-allocated memory associated with the free-memory arena, high watermarks, low watermarks, and/or any other information and/or statistics which may be useful for allocating, freeing, and/or tracking portions of the region/arena, and/or for any other purpose. 
     The one or more collections of free portions  812  may include and/or may reference zero or more free portion data structures  826 . For example, the one or more collections of free portions  812  may include a red-black tree of zero or more free portion data structures  826 . Other examples may involve using any other collection data structure to include and/or reference zero or more free portion data structures  826 . 
     The collection-related data  814  may be data which enables the free-memory arena data structure(s)  806  to be considered part of the collection of free-memory arenas  802 . For example, the collection-related data  814  may include one or more references to other data structures, such as other free-memory arena data structure(s)  806  and/or the collection of free memory arenas  802 . The references may be organized in such a way as to suit the particular collection of which it is considered a part. For example, for a list collection, the references may point to a previous and/or next element in the list. For a tree collection, the references may point to a left, right, and/or upward element in the tree. In some examples, the free-memory arena data structure  806  may not contain any collection-related data  814 , such as if there is only one free-memory arena and/or if the collection of free-memory arenas  802  is based upon an array collection. 
     The free portion data structure(s)  826  may each include a portion definition  828  and/or collection-related data  830 . The free portion data structure(s)  826  may include more, fewer, and/or different elements. For example, the free portion data structure(s)  826  may not include the collection-related data  830 . In some examples, the free portion data structure(s)  826  may only include the portion definition  828 . The portion definition  828  may describe the corresponding portion, such as by including an offset within the region and/or a size of the portion. Alternatively, or in addition, the portion definition  828  may include a start and/or an end offset within the region for the corresponding portion. 
     The collection-related data  830  may be data which enables the free portion data structure  826  to be considered part of a collection of free portions  812 . For example, the collection-related data  830  may include one or more references to other data structures, such as other free-portion data structure(s)  826  and/or the collection of free portions  812 . The references may be organized in such a way as to suit the particular collection of which it is considered a part. For example, for a list collection, the references may point to a previous and/or next element in the list. For a tree collection, the references may point to a left, right, and/or upward element in the tree. In some examples, the free portion data structure  826  may not contain any collection-related data  830 , such as if there is only one free portion and/or if the collection of free portions  812  is based upon an array collection. 
     The memory user data structure(s)  816  may include a collection of allocated portions  818 , statistics  820 , an owner identifier and/or reference  822 , and/or collection-related data  824 . The memory user data structure(s)  816  may include more, fewer, and/or different elements. For example, the memory user data structure(s)  816  may not include the owner identifier and/or reference  822 . In some examples, the memory user data structure(s)  816  may only include the collection of allocated portions  818 . The statistics  820  may include a total of allocated memory associated with the memory user, a total of available memory associated with the memory user, high watermarks, low watermarks, and/or any other information and/or statistics which may be useful for allocating, freeing, and/or tracking portions of the region/arena, and/or for any other purpose. 
     The owner identifier and/or reference  822  may refer to one or more application logics  314  and/or virtual address spaces of the application logic(s)  314  and/or may associate the memory user data structure  816  with the one or more application logics  314  and/or virtual address spaces of the application logic(s)  314 . The one or more application logics  314  and/or virtual address spaces referenced and/or associated by the owner identifier and/or reference  822  may be considered the memory user for the memory user data structure  816 . 
     The collection of allocated portions  818  may include and/or may reference zero or more allocated portion data structures  832 . For example, the collection of allocated portions  818  may include a red-black tree of zero or more allocated portion data structures  832 . Other examples may involve using any other collection data structure to include and/or reference zero or more allocated portion data structures  832 . 
     The collection-related data  824  may be data which enables the memory user data structure(s)  816  to be considered part of the collection of memory users  804 . For example, the collection-related data  824  may include one or more references to other data structures, such as other memory user data structure(s)  816  and/or the collection of memory users  804 . The references may be organized in such a way as to suit the particular collection of which it is considered a part. For example, for a list collection, the references may point to a previous and/or next element in the list. For a tree collection, the references may point to a left, right, and/or upward element in the tree. In some examples, the memory user data structure  816  may not contain any collection-related data  824 , such as if there is only one memory user and/or if the collection of memory users  804  is based upon an array collection. 
     The allocated portion data structure(s)  832  may each include a portion definition  834 , collection-related data  836 , and/or a virtual address  838 . The allocated portion data structure(s)  832  may include more, fewer, and/or different elements. For example, the allocated portion data structure(s)  832  may not include the virtual address  838 . In some examples, the allocated portion data structure(s)  832  may only include the portion definition  834 . The portion definition  834  may describe the corresponding portion, such as by including an offset within the region and/or a size of the portion. Alternatively, or in addition, the portion definition  834  may include a start and/or an end offset within the region for the corresponding portion. 
     The virtual address  838  may be an address within the virtual address space of one or more application logics  314  to which the corresponding portion is mapped. The one or more application logics  314  associated with the virtual address space may be associated with the memory user data structure  816 . For example, the collection of allocated portions  818  may be a collection of portions allocated for the application logic(s)  314  and/or mapped to the virtual address space of the application logic(s)  314 . 
     The collection-related  836  data may be data which enables the allocated portion data structure  832  to be considered part of a collection of allocated portions  818 . For example, the collection-related data  836  may include one or more references to other data structures, such as other allocated-portion data structure(s)  832  and/or the collection of allocated portions  818 . The references may be organized in such a way as to suit the particular collection of which it is considered a part. For example, for a list collection, the references may point to a previous and/or next element in the list. For a tree collection, the references may point to a left, right, and/or upward element in the tree. In some examples, the allocated portion data structure  832  may not contain any collection-related data  836 , such as if there is only one allocated portion and/or if the collection of allocated portions  818  is based upon an array collection. 
     The collection(s) may be ordered and/or unordered. For example, the free portion data structure(s)  826  may be ordered by starting offset of the corresponding portion(s) within the region  214 . Ordering may be advantageous in that it may improve the computational efficiency of freeing memory and/or of identifying adjacent portions while freeing and/or allocating memory. Alternatively and/or in addition, the collection(s) may be binned and/or unbinned. Binning may be advantageous in that it may improve the computational efficiency of allocating memory and/or may help minimize memory fragmentation. For example, the collection(s) may be binned based on the size of the portion(s) described by the corresponding free portion data structure(s)  826 . In such examples, one bin may include a collection of free portion data structure(s)  826  with sizes in a range corresponding to small portions, another bin may include a collection of free portion data structures  826  with sizes in a range corresponding to medium-sized portions, while other bins may include collection(s) of free portion data structures  826  with sizes in a range corresponding to larger portions. Consequently, in handling an allocation request for a specified buffer size, the client logic  312  and/or the memory allocation interface may ignore bins containing portions smaller than the specified portion size, and may begin searching in the smallest bin with portions larger than the specified portion size. The bins may be designed to suit any preferred allocation strategy. For example, the minimum and/or maximum sizes of portions included in and/or referenced by a bin may be powers of two, multiples of a page-size, and/or, power-of-two multiples of page-sizes. Alternatively or in addition, the minimum and/or maximum sizes may be based upon some other one or more criteria and/or may be arbitrarily chosen. 
     In some examples, a free portion data structure  826  may be considered part of multiple collections of free portions  812 , such as if a free memory arena  806  includes an overall collection of free portions containing and/or referencing all free portion data structures  826  for the arena plus a set of binned collections of free portions containing and/or referencing free portion data structures  826  associated with corresponding bins. In these examples, the collection-related data  830  may include data which enables the free portion data structure  826  to be considered part of multiple collections of free portions  812 , such as the overall collection of free portions and/or one or more binned collections of free portions. For example, the collection-related data  830  may include multiple sets of references to other data structures. In one example, if the free portion data structure  826  is considered to be part of multiple list collections, the collection-related data  830  may include references pointing to multiple previous and/or next elements for the multiple list collections. If the free portion data structure  826  is considered to be part of multiple tree collections, the collection-related data  830  may include references pointing to multiple left, right, and/or upward elements in the tree. Other examples are possible, such as if the free portion data structure  826  is considered to be part of a single list collection and a single tree collection, multiple list and/or tree collections, and/or any other combination of multiple collection data structures. 
     The collection of free-memory arenas  802 , the collection of memory users  804 , and/or any other data and/or data structures may be stored at one or more well-known locations. For example, the collection of free-memory arenas  802  may be stored at the beginning of the region  214 , as illustrated in this example. Alternatively or in addition, the collection of memory users  804  may be stored immediately after the collection of free-memory arenas  802 , such as illustrated in this example. Examples of other well-known locations include: at the end of the region  214 , at a particular offset within the region  214 , immediately preceding other data and/or data structure(s) within the region, and/or at any other absolute and/or relative position that can be relied upon to locate and/or identify the data structure(s). Alternatively or in addition, the collection(s), data, and/or data structure(s) may be stored elsewhere, such as with the region metadata  215 , the memory  210 , the backing store  260 , and/or any other location capable of holding information. In some examples, the collection(s), data, and/or data structure(s) may be stored in multiple locations, such as by replicating and/or distributing them. 
     A reference to a data structure included in the region may enable the client logic and/or any other logic to access the data structure included in the region, such as by providing its location within the region. The location may be provided directly, such as with a reference value equal to an offset in the region at which the data structure is stored, and/or indirectly, such as with a virtual address which may be translatable to a position within the region. In cases where references are virtual addresses, coordinating client logics  312  may use a similarly-configured address space to map one or more portions of the region, such that virtual addresses referencing a given data structure would be the same at each corresponding client  130 , and therefore may be dereferenced directly by the client logic  312  and/or other logic to access the data structure. 
     Access to the memory-allocation data structure(s) may be coordinated using one or more synchronization primitives and/or methods. For example, the memory-allocation data structure(s) may be accessed using atomic operations. Atomic operations may be any operation which appears to the rest of the system to occur at once without being interrupted. Examples of atomic operations may include read, write, exchange, compare-and-exchange, compare-and-swap, test-and-set, test-and-clear, add-and-return, subtract-and-return, fetch-and-add, and/or fetch-and-subtract. In a first example, the client logic  312  and/or other logic(s) may perform a test-and-set upon a flag corresponding to a portion of the region  214  and/or external memory allocation to mark the portion as allocated. In second example, the client logic  312  and/or other logic(s) may perform a test-and-clear upon the flag to mark the corresponding portion as unallocated. In a third example, the client logic  312  and/or other logic(s) may perform atomic operations to access and/or manipulate a more complex data structure. For example, the client logic  312  and/or other logic(s) may perform compare-and-swap operations to manipulate a red-black tree, as described in “Lock-Free Red-Black Trees Using CAS” by Jong Ho Kim, et al. 
     Alternatively or in addition, one or more lock primitives may be used to coordinate access to the memory-allocation data structure(s). Lock primitives may include logic and/or data structures which enforces a concurrency control policy. Examples of lock primitives include binary semaphores, counting semaphores, spinlocks, readers-writers spinlocks, mutexes, recursive mutexes, and/or readers-writers mutexes. Lock primitives may include distributed locks and/or may be provided by a distributed lock manager. Examples of distributed locks and/or distributed lock managers include VMSCluster, Chubby, ZooKeeper, Etcd, Redis, Consul, Taooka, OpenSSl, and/or Corosync and/or any other distributed lock(s) and/or distributed lock manager(s) known now or later discovered. 
     Alternatively, or in addition, access to and/or updates to the memory-allocation data structures may be coordinated via message-passing. For example, a first client logic and/or another logic may serve as a primary owner of the memory-allocation data structure(s) and/or a second client logic may request the first client logic and/or other logic (such as by sending a message to the first client logic and/or other logic) to check and/or update the memory-allocation data structure(s), to perform atomic operations upon the memory-allocation data structure(s), to allocate the portion and/or the slab, and/or to free the portion and/or the slab. Upon completing the requested operation, the first client logic and/or other logic may respond to the request, such as by sending a response message to the second client logic. 
     The primary owner may be elected using any leader election logic and/or coordinator election logic now known or later discovered. Examples of leader election logics and/or coordinator election logics are Shout, Mega-Merge, YO-YO, Bully, Chang and Roberts, Hirschberg-Sinclair, Korach-Kutten-Moran, Paxos, Virtual Synchrony, and/or Raft. The leader election logic and/or coordinator election logic may use any one or more characteristics and/or configurations of the client  130  to differentiate the corresponding client logic(s) and/or other logic(s), such as serial numbers of the client  130  and/or any component parts (such as the communication interface(s)  330 ), configured priority value(s), uptime, resource availability, etc. In examples where one or more preferred differentiating characteristics and/or configurations fail to differentiate between two or more logics, the leader election logic and/or coordinator election logic may use other characteristics and/or configurations as secondary, tertiary, etc. differentiators until each logic is differentiated from all others. 
     In other examples, the allocation-coordinating logics may coordinate allocation via message-passing without a primary owner. For example, the second client logic may select a portion to allocate and/or may announce the selected portion to all other allocation-coordinating logics, such as by sending a message to the first client logic and/or other logic(s). Upon receiving the message, each allocation-coordinating logic may check if the portion is already allocated and may respond (such as by sending a response message) acknowledging the message announcing the selected portion. If all other allocation-coordinating logics acknowledge the message announcing the selected portion, the second client logic may conclude that the selected portion is allocated to it. Message-passing may include a transmission of a message using User Datagram Protocol (UDP) or any other connectionless and/or unreliable communication protocol. Alternatively or in addition, Message-passing may include a transmission of a message using a TCP connection, an SSL connection, InfiniBand reliable connections, RoCE, iWARP, HTTP, or any other communications protocol known now or later discovered. 
       FIG.  9 A ,  FIG.  9 B ,  FIG.  9 C , and  FIG.  9 D  illustrate an example scenario of coordinated allocation of external memory. A client logic  312  of a first client  130  (a first client logic  312   a ) may receive a first request for allocation of memory ( 902 ). In response to the first request, the first client logic  312   a  may select one or more portions  914  of the external memory allocation X 1  and/or of the region(s)  214  (such as at least one of the constituent regions R 1  R 2  R 3  of the external memory allocation X 1 ) ( 904 ). The selected portion(s) may be and/or may constitute the slab. Alternatively or in addition, the selected portion(s) may be and/or may constitute a buffer.  FIG.  9 B  illustrates the state of the eternal memory allocation X 1  prior to selecting ( 904 ) the one or more portions  914 .  FIG.  9 C  illustrates the state of the external memory allocation X 1  after selecting ( 904 ) the one or more portions  914 . 
     Upon selecting the portion(s) ( 904 ), the first client logic  312   a  may optionally map the portion(s) to a first virtual address space of the first client  130  ( 906 ). In other examples, the portion(s) may already be mapped, such as if the first client logic  312   a  and/or the application logic  314  maps all of and/or a portion of the external memory allocation X 1  and/or of one or more regions  214  to the first virtual address space prior to receiving the request ( 902 ) and/or prior to selecting the portion(s) ( 904 ). In examples where the portion(s) are already mapped, mapping the portion(s) to the first virtual address space ( 906 ) may be skipped. 
     Upon completing and/or skipping mapping the portion(s) to the first virtual address space ( 906 ), the first client logic  312   a  may complete the first request for allocation of memory, such as by returning the virtual address and/or size of the selected portion to the first application logic  314 . 
     Concurrent to the first client logic  312   a  handling the first request for allocation of memory ( 902 ), a client logic  312  of a second client  130  (a second client logic  312   b ) may receive a second request for allocation of memory ( 908 ). In response to the second request, the second client logic  312   b  may select one or more portions  916  of the external memory allocation and/or of the region  214  ( 910 ). The external memory allocation and/or the region  214  may be the same external memory allocation X 1  and/or the same region(s) R 1  R 2  and/or R 3  as used by the first client logic  312   a . Alternatively, the external memory allocation and/or the region  214  may be different. The selected portion(s) may be and/or may constitute the slab. Alternatively or in addition, the selected portion(s) may be and/or may constitute a buffer.  FIG.  9 D  illustrates the state of the external memory allocation X 1  after selecting ( 910 ) the one or more portions  916  by the second client logic  312   b.    
     Selecting the portion ( 904 ,  910 ) may include accessing and/or updating the memory-allocation data structure(s). For example, the first client logic  312   a  and/or the second client logic  312   b  may perform one or more atomic operations to check if the portion(s) are allocated and/or to mark the portion(s) as allocated, as described elsewhere in this disclosure. In other examples, the first client logic  312   a  and/or the second client logic  312   b  may use one or more lock primitives to coordinate access to the memory-allocation data structure(s). Therefore, selecting the portion ( 904 ,  910 ) and/or accessing and/or updating the memory-allocation data structure(s) may be performed such that multiple client logics  312   a    312   b  can handle multiple concurrent requests for allocation of memory without any client logics  312   a    312   b  selecting the same portion(s) for different requests. 
     Upon selecting the portion(s) ( 910 ), the second client logic  312   b  may optionally map the portion(s) to a second virtual address space of the second client  130  ( 912 ). In other examples, the portion(s) may already be mapped, such as if the second client logic  312   b  and/or the application logic  314  maps all of and/or a portion of the external memory allocation and/or of one or more regions  214  to the second virtual address space prior to receiving the request ( 908 ) and/or prior to selecting the portion(s) ( 910 ). In examples where the portion(s) are already mapped, mapping the portion(s) to the second virtual address space ( 912 ) may be skipped. 
     Upon completing and/or skipping mapping the portion(s) to the second virtual address space ( 912 ), the second client logic  312   b  may complete the second request for allocation of memory, such as by returning the virtual address and/or size of the selected portion to the second application logic  314 . 
     In response to one or more application logic(s)  314  being destroyed, the client logic  312  may free one or more slabs, such as all slabs previously allocated to the application logic(s)  314 . For example, the application logic(s) may be destroyed upon completing a sequence of logic operations, executing an exit operation, and/or being terminated by a user and/or a supervisor logic. To facilitate freeing the one or more slabs, the client logic  312  may track which slab(s) are allocated to each application logic  314 , such as with the memory-allocation data structure(s) and/or other data structure(s). 
     Alternatively or in addition, one or more application logic(s)  314  may be considered destroyed if the corresponding client  130  stops operation and/or becomes unresponsive and/or unreachable, such as if the client  130  has lost power, crashed, and/or has become disconnected from its interconnect  140 . Upon one or more application logic(s)  314  being considered destroyed, a different client logic  312  and/or another logic may free the one or more slabs, such as all slabs allocated to the application logic(s)  314 . The one or more application logic(s)  314  may be considered destroyed if other logic(s), such as other client logic(s)  312 , region access logic(s), and/or the allocation logic  412  determines that the application logic(s)  314  are no longer operational. The other logic(s) may determine that the application logic(s)  314  are no longer operational in a distributed manner, such as by checking connectivity to the corresponding client  130  from one or more other client(s)  130 , memory appliance(s)  110 , and/or management server(s)  120 . In another example, the other logic(s) may determine that the application logic(s)  314  are no longer operational by checking the last time at which the application logic(s)  314 , the corresponding client logic(s)  312 , and/or client(s)  130  accessed the external memory allocation and/or the region(s) referenced by the external memory allocation. 
     In some embodiments, the client logic  312  and/or another logic may select a portion of local primary memory to be the slab in response to some memory allocation requests and/or may select a portion of external primary memory to be the slab in response to other (and/or the same) memory allocation requests. In some examples, in response to a memory allocation request for a slab of memory having a size A, the client logic  312  and/or another logic may select a portion of local primary memory having a size A or a portion of external primary memory having a size A to be the slab. In some other examples, in response to a memory allocation request for a slab of memory having a size A, the client logic  312  and/or another logic may select a portion of local primary memory having a size B and a portion of external primary memory having a size C to be the slab, where A=B+C. Selecting a portion of local primary memory to be the slab may be useful, for example, where the amount of available local primary memory exceeds the amount of available external primary memory and/or when the amount of available local primary memory is nearly sufficient to cover the needs of one or more application logics  314 . For example, an application logic  314  that allocates 1.4 TiB of memory may be operated with a client  130  containing 1 TiB of available local primary memory in communication with a memory appliance  110  containing 512 GiB of available local primary memory. In examples such as this, the client logic  312  and/or another logic may utilize some portion of the memory  310  of the client  130  as a cache for the data stored in external primary memory. In one such example, 0.9 TiB of local primary memory may be allocated for direct use by the application logic  314  plus 512 GiB of external primary memory. The remaining 0.1 TiB of local primary memory may be operated as a cache for the data in the 512 GiB of external primary memory. In some examples, this approach may enable over-subscribing and/or under-provisioning of memory, such as discussed elsewhere in this disclosure. 
       FIG.  10    illustrates an example flowchart for selecting between multiple types of memory in response to a request to allocate memory. The client logic  312  and/or another logic may perform the operations shown in  FIG.  10    and/or other figures herein. The client logic  312  may include, for example, a swap implementation of an operating system. In addition to the operations shown in  FIG.  10   , the swap implementation of the operating system may manage, for example, operations related to swapping data into or out of local primary memory. Although external primary memory, in some examples, may be implemented at least in part in the swap implementation of the operating system, may be accessible via a memory swapping interface, and/or may be accessible in the client logic  312  via a storage protocol, the term “swap space” as used herein refers to memory stored in a conventional swap file, swap partition, and/or swap device, which may be remotely accessed via a storage protocol and/or may be accessed over a local bus via a storage protocol, such as with the backing store  360 . 
     The client logic  312  and/or another logic may receive a request for allocation of memory ( 1002 ). The request for allocation of memory ( 1002 ) may be any one or more mechanisms that may trigger the operations described for  FIG.  10   . Examples of possible requests include: a message, a programmatic signal, an electrical signal, an optical signal, a wireless signal, a method invocation, a function call, a TCP and/or UDP message, an HTTP request, etc. The request ( 1002 ) may be received from another logic, such as the application logic  314 , and/or the request ( 1002 ) may be received from the client logic  312 , such as by the client logic  312  making a determination to allocate memory. The request for allocation of memory may include an invocation of any operating system programmatic procedure configured to allocate memory for a caller of the operating system programmatic procedure. For example, the request for allocation of memory may include a method invocation of malloc( ) mmap( ), brk( ), and/or sbrk( ) in C programs. Alternatively, the request for allocation of memory may include an invocation of a programmatic procedure that is not part of an operating system. The client logic  312 , the application logic  314 , and/or any other one or more logics may make the determination independently and/or in coordination with each other and/or with any other one or more logics based on any one or more conditions, parameters, configurations, and/or other properties of the client  130  and/or of the one or more memory appliances  110  and/or for any other reason. In some examples, there may be no request ( 1002 ), such as if the determination is made by the client logic  312 . 
     In response to the request and/or determination, the client logic  312  and/or another logic may perform a determination of available memory ( 1004 ). The determination of available memory ( 1004 ) may include evaluating between whether to provide anonymous memory and/or local primary memory and/or whether to provide file-backed memory (for example, from a memory-mapped file) and/or external primary memory in response to the request to allocate memory. The determination ( 1004 ) may be based on an evaluation of one or more conditions, parameters, configurations, and/or other properties of the client  130 , of one or more memory appliances  110 , and/or of the request to allocate memory. Conditions, parameters, configurations, and/or other properties of the client  130  may include: the total amount of memory  310  installed in the client  130 ; the total amount of memory  210  installed in one or more memory appliances  110 ; the amount of available local primary memory; the amount of available external primary memory; the amount of local primary memory configured for use as a cache of data in external primary memory; the amount of memory requested by the request to allocate memory; configurable thresholds, margins, and/or hysteresis settings; performance characteristics of local primary memory; performance characteristics of one or more memory appliances  110 ; configured constraints, such as constraints configured by a user and/or administrator; and/or any other information, configuration(s), policy/policies, assignment(s), and/or other one or more criteria that may be useful for determining one or more types of memory to select in response to the request to allocate memory. Alternatively or in addition, the determination of available memory ( 1004 ) may be based on an evaluation of one or more of: duration of use (e.g. not above 80% of time), time, rate of change, margin, frequency of use, periodicity, load, load balancing, number of times reused, and/or one or more computations, algorithm(s), function(s), heuristic(s), passed function(s), event(s), trigger(s), event trigger(s), applied heuristic(s), computational benefit(s), policy/policies, data pattern(s), read/write relationship(s), modify/update relationship(s), defined trigger(s), etc. 
     In one example, the determination of available memory ( 1004 ) may include comparing the amount of memory requested to the amount of available local primary memory. The client logic  312  and/or another logic may choose to use anonymous memory and/or local primary memory for the request to allocate memory if the amount of memory requested is available as local primary memory. In comparing the amount of memory requested to the amount of available local primary memory, the client logic  312  and/or another logic may optionally include an amount of margin in the comparison, and/or may choose to use anonymous memory and/or local primary memory for the request if the amount of memory requested plus a margin is available as local primary memory. 
     Margin may be an amount of memory to attempt to keep available such as to accommodate operating system use, allocation overhead, memory-conversion overhead, and/or for any other purpose. The margin may be specified in absolute and/or relative terms in one or more ways, such as by one or more numbers of bytes, kilobytes, megabytes, gigabytes, kibibytes, mebibytes, gibibytes, pages, chunks, buffers, percentages, ratios, and/or any other way of quantifying an amount of memory and/or data. In some examples, the margin may be specified in relation to some other quantity, such as a percentage of total and/or available local primary memory and/or external primary memory, a percentage and/or amount of data above and/or below the total and/or available local primary memory and/or external primary memory, and/or in any other way derived from one or more of these and/or any other useful quantity related to the memory allocation request(s). In other examples, the margin may be specified in dynamic terms, such as via one or more policies, passed functions, steps, and/or rules. Memory-conversion may include activities for converting between anonymous memory mappings and file-backed memory mappings, such as described for  FIG.  11    and/or  FIG.  12   . 
     In another example, the determination of available memory ( 1004 ) may include comparing the amount of memory requested to the amount of available external primary memory and/or may choose to use anonymous memory and/or local primary memory for the request to allocate memory if the amount of memory requested is not available as external primary memory. In comparing the amount of memory requested to the amount of available external primary memory, the client logic  312  and/or another logic may optionally include an amount of margin in the comparison, and/or may choose to use anonymous memory and/or local primary memory for the request if the amount of memory requested plus or minus a margin is not available as external primary memory. An advantage of choosing to use anonymous memory and/or local primary memory for the request to allocate memory if the amount of memory requested is not available as external primary memory may be that systems using this approach may continue to operate, even after exhausting all available local primary memory and all available external primary memory such as by utilizing swap space to hold data for some of the allocated anonymous memory and/or local primary memory. 
     Alternatively or in addition, the determination of available memory ( 1004 ) may include an evaluation of the rate of change of available local primary memory and/or external primary memory and/or the margin may be adjusted based in part and/or in whole upon the rate of change of available local primary memory and/or external primary memory. For example, if the amount of available local primary memory is decreasing rapidly (in other words, decreasing at a rate greater than a threshold rate), the client logic  312  and/or another logic may determine ( 1004 ) to use file-backed memory and/or external primary memory in examples where the currently-available local primary memory may otherwise be adequate to accommodate the request to allocate memory. Alternatively or in addition, if the amount of available local primary memory is decreasing slowly (in other words, decreasing at a rate less than a threshold rate), the client logic  312  and/or another logic may determine ( 1004 ) to use anonymous memory and/or local primary memory. In another example, if the amount of available external primary memory is decreasing rapidly (in other words, decreasing at a rate greater than a threshold rate), the client logic  312  and/or another logic may determine ( 1004 ) to use anonymous memory and/or local primary memory in examples where the currently-available external primary memory may otherwise be adequate to accommodate the request to allocate memory. Alternatively or in addition, if the amount of available external primary memory is decreasing slowly (in other words, decreasing at a rate less than a threshold rate), the client logic  312  and/or another logic may determine ( 1004 ) to use file-backed memory and/or external primary memory. Other examples may include any one or more combinations of parameters increasing and/or decreasing rapidly (e.g. greater than a threshold) and/or slowly (e.g. less than a threshold), such as: the amount of available local primary memory; amount of external primary memory; amount of used anonymous memory; amount of used local primary memory; amount of used file-backed memory; amount of used external primary memory; amount of remaining margin(s) for any one or more parameters; percent and/or ratio of remaining margin(s) for any one or more parameters; any characteristic(s) and/or configuration(s) of one or more clients  130 , memory appliances  120 , and/or management servers  120 ; etc. Examples of the available memory may include unmapped memory in address space, unallocated local primary memory, unallocated local primary memory within a division of the local primary memory, and/or unmapped file-backed and/or external primary memory. 
     In some examples, there may be no determination of available memory ( 1004 ), and/or file-backed memory and/or external primary memory may always be chosen for the request to allocate memory. In other examples, anonymous memory and/or local primary memory may always be chosen for the request to allocate memory. In other examples, multiple types of memory may be chosen for the request to allocate memory, such as by choosing anonymous memory and/or local primary memory for one or more portions of the request to allocate memory and/or choosing file-backed memory and/or external primary memory for one or more other portions of the request to allocate memory. In some examples, such as where multiple types of memory are chosen for the request to allocate memory, the resulting operations, ( 1006 ) ( 1008 ) and/or ( 1010 ) ( 1012 ) ( 1014 ), may be performed in sequence, such as ( 1006 ) ( 1008 ) followed by ( 1010 ) ( 1012 ) ( 1014 ) or ( 1010 ) ( 1012 ) ( 1014 ) followed by ( 1006 ) ( 1008 ); the operations may be interleaved, such as ( 1010 ) ( 1006 ) ( 1012 ) ( 1008 ) ( 1014 ), or any other interleaved ordering; and/or the operations may be performed concurrently, such as ( 1006 ) ( 1008 ) concurrent with ( 1010 ) ( 1012 ) ( 1014 ). Each set of operations may be performed upon portions of the request corresponding to anonymous memory, local primary memory, file-backed memory, and/or external primary memory. 
     Upon completing or skipping the determination of available memory ( 1004 ), if anonymous memory and/or local primary memory is to be used for all of and/or for one or more portions of the request, the client logic  312  and/or another logic may map anonymous memory and/or local primary memory to a virtual address space ( 1006 ), such as a virtual address space of the application logic  314 . The anonymous memory and/or local primary memory to be mapped may be selected from a pool of available portions of anonymous memory and/or local primary memory. Mapping anonymous memory and/or local primary memory to the virtual address space ( 1006 ) may include updating, modifying, and/or replacing one or more data structures indicating that one or more portions of the virtual address space are mapped to anonymous memory and/or local primary memory. Alternatively or in addition, mapping anonymous memory and/or local primary memory to the virtual address ( 1006 ) space may include updating, modifying, and/or replacing one or more page table entries to reference portions of local primary memory and/or swap space. Upon completing mapping anonymous memory and/or local primary memory to the virtual address space ( 1006 ), the client logic  312  and/or another logic may complete handling of the request to allocate memory ( 1008 ), such as by returning a result code to the application logic  314  and/or other requesting logic. 
     Upon completing or skipping the determination of available memory ( 1004 ), if file-backed memory and/or external primary memory is to be used for all of and/or for one or more portions of the request, the client logic  312  and/or another logic may select one or more portions of the file and/or of external primary memory ( 1010 ). The one or more portions of the file and/or of external primary memory may be sufficient to cover the amount of memory requested and/or determined to be covered by file-backed and/or external primary memory. For example, a single portion may be selected that is equal to or larger than the amount of memory requested and/or determined to be covered by file-backed and/or external primary memory. In another example, multiple portions may be selected, such that the total of the sizes of the multiple portions is equal to or larger than the amount of memory requested and/or determined to be covered by file-backed and/or external primary memory. Alternatively or in addition, the one or more portions of the file and/or of external primary memory may be insufficient to cover the amount of memory requested and/or determined to be covered by file-backed and/or external primary memory. For example, one or more portions may be selected that have a total size less than the amount of memory requested and/or determined to be covered by file-backed and/or external primary memory. In examples such as these, the client logic  312  and/or another logic may cover a subset of the amount of the memory requested and/or previously determined to be covered by file-backed and/or external primary memory. For example, more of the memory requested may be covered by anonymous memory and/or local primary memory. Alternatively or in addition, the request may fail. 
     In some examples, the file portion(s) and/or external memory portion(s) may be included in one or more regions  214  and/or external memory allocations. The file portion(s) and/or external memory portion(s) may be selected and/or the region(s)  214  and/or external memory allocation(s) in which the portion(s) are selected may be selected using any of the methods and/or logic described herein, such as for provisioning, allocation, selection, slab allocation, allocation strategies, policies, passed functions, steps, and/or rules, etc. Alternatively or in addition, the portion(s) may be selected and/or the region(s)  214  and/or external memory allocation(s) in which the portion(s) are selected may be selected using any selection strategy known now or later discovered. In examples where more than one portion of external primary memory is to be selected, the same region  214  and/or external memory allocation may be selected for multiple portions to be selected and/or one or more different regions and/or external memory allocations may be selected for one or more portions to be selected. 
     In some examples, such as when there is an implicit mapping between file offsets and corresponding offsets in an underlying region  214  and/or external memory allocation, selecting a portion of the file may be equivalent to selecting a corresponding portion of the underlying region  214  and/or external memory allocation associated with the file. An implicit mapping may be a logical relationship between sets of numbers in which a mathematical or otherwise derivable relationship exists between corresponding pairs of numbers in each set. For example, corresponding numbers may be the same in both sets, and/or there may be a linear relationship between the two sets of numbers. 
     In alternative examples, there may not be an implicit mapping between file offsets and corresponding offsets in an underlying region  214  and/or external memory allocation. Such a scenario is possible, at least initially, if the file is a pseudo file. When there is no implicit mapping between file offsets and offsets in the region  214  and/or external memory allocation, selecting a portion of the file may serve to allocate metadata space, such as parts of a data structure that tracks page-cache pages in an operating system, without selecting a corresponding portion of the underlying region  214  and/or external memory allocation associated with the file. In these examples, the client logic  312  and/or another logic may proceed to select one or more portions of the underlying region  214  and/or external memory allocation to associate with the file portion(s), and/or the client logic  312  and/or another logic may delay selecting one or more portions of the underlying region  214  and/or external memory allocation to a later time, such as prior to or while writing data from local primary memory to the file. 
     In some examples when there is no implicit mapping between file offsets and offsets in the region  214  and/or external memory allocation, one or more regions  214  and/or external memory allocations may be created after selecting the portion of the file. For example, one or more regions and/or external memory allocations may not be created until there are insufficient available portions of previously-created region(s)  214  and/or external memory allocation(s) to satisfy current needs. In some examples, no regions  214  and/or external memory allocations may be created until after at least one portion of the underlying region  214  and/or external memory allocation is needed to map to a portion of the file. This approach may be advantageous in that it may reduce the amount of external primary memory reserved for created region(s)  214  and/or external memory allocations in example systems where creating a region  214  and/or external memory allocation reserves some amount of external primary memory for the region  214  and/or external memory allocation. 
     Upon selecting the file portion(s) and/or portions of external primary memory ( 1010 ), the client logic  312  and/or another logic may map the one or more file portions to the virtual address space ( 1012 ). Mapping the one or more file portions to the virtual address space ( 1012 ) may include updating, modifying, and/or replacing one or more data structures indicating that one or more portions of the virtual address space are mapped to the file portion(s) and/or external primary memory portion(s) and/or indicating that the one or more file portions and/or external primary memory portion(s) are mapped to one or more portions of the virtual address space. Alternatively or in addition, mapping the one or more file portions and/or external primary memory portions to the virtual address space ( 1012 ) may include updating, modifying, and/or replacing one or more page table entries to reference one or more portions of local primary memory, the file portion(s), and/or the portion(s) of external primary memory. 
     In other examples, the file portion(s) and/or portion(s) of external primary memory may already be mapped, such as if the client logic  312  and/or the application logic  314  maps all of and/or a portion of the external memory allocation and/or of one or more regions  214  to the virtual address space prior to receiving the request ( 1002 ) and/or prior to selecting the portion(s) ( 1010 ). In examples where the file portion(s) and/or external primary memory portion(s) are already mapped, mapping the portion(s) to the virtual address space ( 1012 ) may be skipped. 
     Upon completing or skipping the mapping of the one or more file portions and/or external primary memory portions to the virtual address space ( 1012 ), the client logic  312  and/or another logic may complete handling of the request to allocate memory ( 1014 ), such as by returning a result code to the application logic  314  and/or other requesting logic. 
     During operation of the system  100 , one or more of the conditions, parameters, configurations, and/or other properties of the client  130  and/or of one or more memory appliances  110  may change. Alternatively, or in addition, the application logic  314  may have one or more varying resource requirement(s) and/or access pattern(s) over time. To accommodate these changes, the client logic  312  and/or another logic may provide a capability to change previous allocations from anonymous memory to file-backed memory and/or from file-backed memory to anonymous memory. Alternatively or in addition, a background process may monitor memory usage and change previous allocations from anonymous memory to file-backed memory and/or from file-backed memory to anonymous memory. 
       FIG.  11    illustrates an example flowchart of operations for converting anonymous memory to file-backed memory. A client logic  312  and/or another logic may receive a request to convert anonymous memory to file-backed memory ( 1102 ). The request to convert anonymous memory to file-backed memory may indicate one or more virtual addresses and/or one or more ranges of virtual addresses as the anonymous memory to convert. 
     The request to convert anonymous memory to file-backed memory ( 1102 ) may be any one or more mechanisms that may trigger the operations described for  FIG.  11   . Examples of possible requests include: a message, a programmatic signal, an electrical signal, an optical signal, a wireless signal, a method invocation, a function call, a TCP and/or UDP message, an HTTP request, or any other programmatic or electrical signal. The request ( 1102 ) may be received from another logic, such as the application logic  314 , and/or the request ( 1102 ) may be received from the client logic  312 , such as by the client logic  312  making a determination to convert anonymous memory to file-backed memory. The client logic  312 , the application logic  314 , any other one or more logics may make the determination independently and/or in coordination with any other one or more logics based on any one or more of the conditions, parameters, configurations, and/or other properties of the client  130  and/or of the one or more memory appliances  110  and/or for any other reason. In some examples, there may be no request ( 1102 ), such as if the determination is made by the client logic  312 . 
     In response to the request and/or determination to convert anonymous memory to file-backed memory, the client logic  312  and/or another logic may acquire one or more lock primitives ( 1104 ). Acquiring one or more lock primitives may prevent a page fault logic and/or a segmentation fault logic from making progress until the lock primitive is released. As a result, acquiring the one or more lock primitives may prevent the application logic  314  from accessing and/or modifying the contents of the anonymous memory pages. For example, the page fault logic and/or the segmentation fault logic may attempt to acquire one or more of the same lock primitive(s), causing it to wait until the corresponding lock primitive(s) are released. 
     Upon acquiring one or more lock primitives ( 1104 ), the client logic  312  and/or another logic may unmap and/or restrict access to portion(s) of one or more virtual address spaces ( 1106 ). Unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) may prevent the application logic  314  from accessing and/or modifying the contents of the anonymous memory and/or may cause attempts to access and/or modify the contents of the anonymous memory to trigger the page fault logic and/or the segmentation fault logic. 
     Unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) may include invalidating, removing, replacing, updating, and/or modifying one or more page table entries and/or may include flushing and/or shooting-down one or more TLB (Translation Lookaside Buffer) entries for one or more virtual address spaces. For example, one or more page table entries may be marked not writable, invalid, clean, not dirty, not modified, and/or not present, and/or corresponding TLB entries may be flushed and/or shot down. A TLB may be a processor&#39;s cache of translations from virtual memory addresses to physical memory addresses. When the processor changes the virtual-to-physical mapping of an address, the processor may notify any other processors sharing that memory to invalidate that mapping in their TLBs. This process may be called “shooting-down” a TLB entry. Alternatively or in addition, unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) may include updating, modifying, and/or replacing one or more data structures related to the anonymous memory, such as data structure(s) describing which portions of a virtual address space are mapped to anonymous memory, data structure(s) describing access permissions, and/or any other data structure(s) related to the anonymous memory or related to the virtual address space(s). Alternatively or in addition, unmapping and/or restricting access to the portion(s) may include invalidating one or more page table entries and/or clearing one or more valid and/or present bits in one or more page table entries without removing one or more references to corresponding portion(s) of local primary memory from the one or more page table entries. In cases where the anonymous memory is mapped to multiple virtual address spaces, the client logic  312  and/or another logic may skip these portions. Alternatively or in addition, the client logic  312  and/or another logic may unmap and/or restrict access to portion(s) of the multiple virtual address spaces. 
     Upon unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ), the client logic  312  and/or another logic may acquire one or more references to one or more anonymous memory portions ( 1108 ). For example, the client logic  312  and/or another logic may identify the address(es) of one or more portions of local primary memory referenced by one or more page table entries associated with the anonymous memory to convert. Alternatively or in addition, the client logic  312  and/or another logic may identify the address(es) of one or more data structures related to the portion(s) of local primary memory referenced by the page table entries. Alternatively or in addition, the client logic  312  and/or another logic may increment, decrement, and/or change one or more reference counts associated with portion(s) of local primary memory. Incrementing, decrementing, and/or changing the reference count(s) associated with a portion may prevent the portion from being reclaimed for other purposes, such as with a reference-counting memory-freeing algorithm that reclaims portions of memory when a corresponding reference count reaches a specified number (such as 1, 0, −1, and/or any other number). As such, by incrementing, decrementing, and/or changing the reference count(s) of the one or more portions of local primary memory referenced by the one or more page table entries associated with the anonymous memory to convert, the client logic  312  and/or another logic may prevent the portions(s) of local primary memory from being reclaimed. 
     In cases where unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) would otherwise release one or more references to one or more anonymous memory portions, acquiring one or more references to the one or more anonymous memory portions ( 1108 ) may be performed in coordination with unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ), such that the corresponding page(s) of local primary memory would not be reclaimed. For example, the client logic  312  and/or another logic may avoid incrementing, decrementing, and/or changing the reference count(s) of the one or more pages of local primary memory when unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ). Alternatively or in addition, the client logic  312  and/or another logic may acquire the one or more references to the one or more anonymous memory portions ( 1108 ) prior to unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ). 
     Upon acquiring the one or more references to one or more anonymous memory portions ( 1108 ), the client logic  312  and/or another logic may select one or more file portions to serve as backing portions for the file-backed memory ( 1110 ). The one or more file portions may be sufficient to cover the amount of memory being converted from anonymous memory to file-backed memory. For example, a single portion may be selected that is equal to or larger than the amount of memory specified in the request. In another example, multiple portions may be selected, such that the total of the sizes of the multiple portions is equal to or larger than the amount of memory specified in the request. 
     Alternatively or in addition, the one or more file portions selected ( 1110 ) may have a total size less than the amount of memory specified in the request. For example, the one or more portions selected may have a total size equal to or less than the total size of the anonymous memory portions for which the client logic  312  has acquired references. In cases such as these, one or more portions of the virtual address(es) and/or the range(s) of virtual addresses indicated by the request may be unused—for example, may never have been accessed, may never have been written-to, and/or may reference a read-only portion of local primary memory containing zeroed memory. Instead of, or in addition to, selecting one or more file portions to correspond with these unused portions, the client logic  312  and/or another logic may update, modify, and/or replace one or more data structures related to the file indicating that the portions are to be zeroed. In other cases, one or more portions of the virtual address(es) and/or the range(s) of virtual addresses indicated by the request may reference corresponding swap portion(s). Instead of or in addition to selecting one or more file portions to correspond with the swap portions, the client logic  312  and/or another logic may select one or more portions of the region  214  and/or of the external memory allocation for the swap portions. 
     In other examples, the one or more file portions selected ( 1110 ) may be insufficient to cover the amount of memory being converted from anonymous memory to file-backed memory, such as if the total of the sizes of the one or more file portions is less than the amount of memory specified in the request and/or than the total size of the anonymous memory portions for which the client logic  312  has acquired references. In examples such as these, the client logic  312  and/or another logic may convert a subset of the amount of the memory specified in the request. Alternatively or in addition, the request may fail. 
     In some examples, the file portion(s) may be included in one or more regions  214  and/or external memory allocations. The file portion(s) may be selected and/or the region(s)  214  and/or external memory allocation(s) in which the portion(s) are selected may be selected using any of the methods and/or logic described herein, such as for provisioning, allocation, selection, slab allocation, allocation strategies, policies, passed functions, steps, and/or rules, etc. Alternatively or in addition, the portion(s) may be selected and/or the region(s)  214  and/or external memory allocation(s) in which the portion(s) are selected may be selected using any selection strategy known now or later discovered. In examples where more than one portion of external primary memory is to be selected, the same region  214  and/or external memory allocation may be selected for multiple portions to be selected and/or one or more different regions and/or external memory allocations may be selected for one or more portions to be selected. 
     Upon selecting one or more file portions to serve as backing portions for the file-backed memory ( 1110 ), the client logic may initialize and/or memory-map the file portion(s) ( 1112 ). Initializing the file portion(s) may include copying data from the one or more anonymous memory portions and/or one or more portions of local primary memory to the one or more file portions. For example, the client logic  312  and/or another logic may allocate one or more portions of local primary memory, copy data from one or more of the anonymous memory portions to the portion(s) of local primary memory, and/or associate the one or more portions of local primary memory with corresponding file portion(s) (such as by including an address associated with the portion(s) in a data structure related to the file portion(s)). Alternatively or in addition, initializing the file portion(s) may include copying data from the one or more swap portions to the one or more file portions. For example, the one or more anonymous memory portions and/or the one or more portions of local primary memory to be converted to file-backed memory may have been swapped out to a swap file, swap partition, and/or swap device, so data from these portions may be copied to the one or more file portions that are to serve as backing portions for the file-backed memory. Memory-mapping the file portion(s) may include associating the file portion(s) with corresponding addresses of the virtual address space(s) where the corresponding anonymous memory portions had been mapped. Alternatively or in addition, the client logic  312  and/or another logic may restore access to the portion(s) of the virtual address space(s) that had been previously restricted, if any. 
     Alternatively or in addition, the client logic  312  and/or another logic may re-associate one or more of the anonymous memory portions with one or more corresponding file portions. Re-associating one or more anonymous memory portions with corresponding file portion(s) may include dis-associating the portion(s) from one or more virtual address space(s) that reference the portion(s), associating the portion(s) with the corresponding file portion(s), and/or associating the corresponding file portion(s) with the one or more virtual address space(s). Dis-associating the portion(s) from one or more virtual address space(s) that reference the portion(s) may include removing, replacing, updating, and/or modifying page table entries, and/or may include flushing and/or shooting-down TLB entries, if not already done. Associating the corresponding file portion(s) with the one or more virtual address space(s) may include updating, modifying, and/or replacing one or more data structures related to the virtual address space(s), such as data structures describing which portions of the virtual address space(s) are mapped to which portions of which file(s). For example, the client logic  312  and/or another logic may add one or more entries to an interval tree and/or other data structure describing contiguous portions of a virtual address space that are mapped to corresponding contiguous portions of the file. Alternatively or in addition, associating the corresponding file portion(s) with the one or more virtual address space(s) may include adding, replacing, updating, and/or modifying page table entries, such as by causing the page table entries to reference the file portions and/or the anonymous memory portion(s) being re-associated with the file. In some examples where one or more portions of anonymous memory may already be associated with the one or more virtual address spaces, the portion(s) may be left associated with the virtual address space(s), and/or the portion(s) may be associated with the corresponding file portion(s). 
     Alternatively or in addition, the client logic  312  and/or another logic may initialize the contents of the file, the region  214 , and/or the external memory allocation. For example, the client logic  312  and/or another logic may copy data from the one or more anonymous memory portions to corresponding portions of the file, the region  214 , and/or the external memory allocation. Alternatively or in addition, the client logic  312  and/or another logic may copy data from the one or more swap portions to corresponding portions of the file, the region  214 , and/or the external memory allocation. In cases such as these, the client logic  312  and/or another logic may or may not allocate local primary memory to hold the data from one or more of the anonymous memory portion(s), may or may not copy the data to the local primary memory, and/or may or may not re-associate one or more of the anonymous memory portion(s) with corresponding file portion(s). Instead or in addition, the client logic  312  and/or another logic may associate the corresponding portions of the file, the region  214 , and/or the external memory allocation with the file portion(s), and/or may memory-map the file portions, the file, the region  214 , and/or the external memory allocation to the virtual address space(s) where the corresponding anonymous memory portions had been mapped. 
     Upon initializing and/or memory mapping the file portion(s) ( 1112 ), the client logic  312  and/or another logic may release the one or more references to the anonymous memory portion(s) and/or the one or more lock primitives ( 1114 ). 
     Upon releasing the one or more references to the anonymous memory portion(s) and/or the one or more lock primitives ( 1114 ), the client logic  312  and/or another logic may complete handling of the request to convert anonymous memory to file-backed memory ( 1116 ), such as by returning a result code to the application logic  314  and/or other requesting logic. 
     While the descriptions for  FIG.  11    illustrate a simple set of operations, it would be clear to one skilled in the art that the operations described here may be performed iteratively upon successive sub-portions of the request to convert anonymous memory to file-backed memory. Performing these operations iteratively may have the advantage of reducing the impact upon the application logic of acquiring the one or more lock primitives ( 1104 ) and/or of unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ). For example, the one or more lock primitives acquired may only prevent a page fault logic and/or a segmentation fault logic from making progress upon a corresponding subset of virtual address space addresses until the lock primitive is released. Similarly, the portion(s) of the virtual address space(s) that are unmapped and/or restricted access to may be a smaller subset than would be needed to convert the entirety of the portion(s) specified via the request to convert anonymous memory to file-backed memory. 
     In some examples, the one or more lock primitives may not be acquired ( 1104 ) and/or released ( 1114 ), the one or more lock primitives may be held for a reduced duration, the one or more lock primitives may affect corresponding subset(s) of virtual address space addresses and/or file portions, and/or the one or more lock primitives may correspond to individual portions. One or more of these approaches may be advantageous in that performance impacts of locking may be reduced. 
     In a first example, performance impacts of locking may be reduced by avoiding overhead associated with initializing the file portion(s) ( 1112 ), such as by re-associating one or more of the anonymous memory portions with one or more corresponding file portions. In examples where one or more of the anonymous memory portions are re-associated with one or more corresponding file portions, unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) may be skipped, such as by not invalidating, removing, replacing, updating, and/or modifying one or more page table entries and/or by not flushing and/or shooting down TLB entries for one or more virtual address spaces. Alternatively or in addition, the client logic  312  and/or another logic may update, modify, and/or replace one or more data structures related to the anonymous memory, such as data structure(s) describing which portions of a virtual address space are mapped to file-backed and/or anonymous memory, data structure(s) describing access permissions, and/or any other data structure(s) related to the anonymous memory or related to the virtual address space(s). For example, the client logic  312  and/or another logic may associate one or more of the anonymous memory portion(s) with the corresponding file portion(s) by including an address and/or reference associated with the portion(s) in a data structure related to the file portion(s) and/or by remapping the corresponding portion(s) of the virtual address space(s) to the file portion(s) (such as by adding one or more entries to the interval tree and/or other data structure describing contiguous portions of the virtual address space(s) that are mapped anonymous memory and/or to corresponding contiguous portions of the file). In examples such as these, where re-associating one or more of the anonymous memory portions with one or more corresponding file portions, the one or more lock primitives may not be acquired ( 1104 ) and/or released ( 1114 ), and/or one or more page faults for corresponding virtual address(es) may be allowed to make progress in parallel with converting anonymous memory portion(s) to file-backed. 
     In a second example, performance impacts of locking may be reduced by initializing the file portion(s) ( 1112 ) without preventing page faults from making progress, such as with the one or more lock primitives. For example, acquiring the one or more lock primitives ( 1104 ) may be skipped, one or more page table entries may be marked not writable, invalid, clean, not dirty, not modified, and/or not present, corresponding TLB entries may be shot down, and/or initializing the file portion(s) ( 1112 ) may be performed without preventing page faults from making progress in parallel. In lieu of preventing page faults from making progress in parallel, after initializing the file portion(s), the client logic  312  and/or another logic may detect whether one or more of the anonymous memory portion(s) may have been modified prior to completing the initialization. For example, the client logic  312  and/or another logic may determine whether one or more page faults have occurred for the anonymous memory portion(s) by checking one or more indicators and/or counters in one or more portion tracking data structures associated with the portion(s). The one or more indicators may be set, cleared, updated, modified, and/or replaced when page faults are handled, may indicate whether or not corresponding portion(s) are mapped into one or more page tables, and/or may indicate the number of page tables into which the portion(s) are mapped. Alternatively or in addition, the client logic  312  and/or another logic may determine whether one or more anonymous memory portions may have been modified by checking a dirty and/or modified bit in one or more page table entries and/or page-tracking data structures. If the client logic  312  and/or another logic determines that one or more of the anonymous memory portions may have been modified, then unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) and/or initializing the file portion(s) ( 1112 ) may be repeated. Alternatively or in addition, converting the corresponding anonymous memory portion(s) to file-backed portion(s) may be skipped, aborted, and/or cancelled. In cases where converting is skipped, aborted, and/or cancelled, selected file portion(s) may be deselected and/or deallocated, and/or any acquired lock primitives (if any) may be released. In some examples, unmapping and/or restricting access to portion(s) of the virtual address space(s) ( 1106 ) and/or initializing the file portion(s) ( 1112 ) may be repeated a limited number of times before skipping the conversion, aborting the conversion, cancelling the conversion, and/or executing any other applicable action and/or policy. If the client logic  312  and/or another logic determines that one or more page faults have not occurred for the file portion(s) and/or that the anonymous memory portion(s) have not been modified, then the client logic  312  and/or another logic may proceed to memory-map the file portion(s) ( 1112 ). In some examples, determining that one or more page faults have not occurred for the anonymous portion(s) and/or that the anonymous portion(s) have not been modified may be performed atomically with memory-mapping the file portion(s) ( 1112 ). For example, one or more compare-and-exchange operations may be performed on one or more corresponding page table entries to memory-map the file portion(s) ( 1112 ) if the value(s) being replaced do not have their accessed bit(s), modified bit(s), dirty bit(s), and/or any other relevant indicators set (and/or cleared, as appropriate). In other examples, one or more lock primitives and/or access bits may be used to perform these operations atomically, such as the one or more lock primitives described for step ( 1104 ). 
     In examples where one or more page faults, segmentation faults, and/or other accesses are allowed to make progress during the conversion, the client logic  312  and/or another logic may detect whether one or more additional portions need to be converted. For example, the client logic  312  and/or another logic may identify additional portion(s) of local primary memory referenced by one or more page table entries associated with the anonymous memory to convert that were not present while performing one or more of the operations described for  FIG.  11    and/or may repeat one or more of the operations to convert one or more of these portions. Alternatively or in addition, one or more of these portions may not be converted. 
     Furthermore, while some of the operations may have been described as an ordered set of operations, it would be clear to one skilled in the art that other sequences may be equally effective in response to the request to convert anonymous memory to file-backed memory. For example, other sequences may have additional operations, some operations may be skipped, and/or some operations may be performed in a different order than described. For example, restoring access to the portion(s) of the virtual address space(s) that had been previously restricted may be performed while releasing the one or more references to the anonymous memory portion(s) and/or the one or more lock primitives ( 1114 ). 
       FIG.  12    illustrates an example flowchart for converting file-backed memory to anonymous memory. A client logic  312  and/or another logic may receive a request to convert file-backed memory to anonymous memory ( 1202 ). The request to convert file-backed memory to anonymous memory may indicate one or more virtual addresses and/or one or more ranges of virtual addresses as the file-backed memory to convert. 
     The request to convert file-backed memory to anonymous memory ( 1202 ) may be any one or more mechanisms that may trigger the operations described for  FIG.  12   . Examples of possible requests include: a message, a programmatic signal, an electrical signal, an optical signal, a wireless signal, a method invocation, a function call, a TCP and/or UDP message, an HTTP request, etc. The request ( 1202 ) may be received from another logic, such as the application logic  314 , and/or the request ( 1202 ) may be received from the client logic  312 , such as by the client logic  312  making a determination to convert file-backed memory to anonymous memory. The client logic  312 , the application logic  314 , any other one or more logics may make the determination independently and/or in coordination with any other one or more logics based on any one or more of the conditions, parameters, configurations, and/or other properties of the client  130  and/or of the one or more memory appliances  110  and/or for any other reason. In some examples, there may be no request ( 1202 ), such as if the determination is made by the client logic  312 . 
     In response to the request and/or determination to convert to file-backed memory to anonymous memory, the client logic  312  and/or another logic may acquire one or more lock primitives ( 1204 ). Acquiring one or more lock primitives may prevent a page fault logic and/or a segmentation fault logic from making progress until the lock primitive is released, which may prevent the application logic  314  from accessing and/or modifying the contents of the file-backed memory pages. For example, the page fault logic and/or the segmentation fault logic may attempt to acquire one or more of the same lock primitive(s), causing it to wait until the corresponding lock primitive(s) are released. 
     Upon acquiring one or more lock primitives ( 1204 ), the client logic  312  and/or another logic may update, modify, and/or replace one or more memory mappings ( 1206 ). Updating, modifying, and/or replacing one or more memory mappings ( 1206 ) may prevent the application logic  314  from accessing and/or modifying the contents of the mapped memory and/or may cause attempts to access and/or modify the contents of the mapped memory to trigger the page fault logic and/or the segmentation fault logic. For example, the mapped memory may be specified with access permissions that disallow reading, writing, and/or executing the contents of the mapped memory. An example protection mode that disallows reading, writing, and/or executing the contents of the mapped memory is PROT_NONE on a Linux or a Microsoft Windows operating system. Alternatively or in addition, updating, modifying, and/or replacing one or more memory mappings ( 1206 ) may include invalidating, removing, replacing, and/or updating one or more page table entries and/or may include flushing and/or shooting-down one or more TLB entries for one or more virtual address spaces. For example, one or more page table entries may be marked not writable, invalid, clean, not dirty, not modified, and/or not present, and/or corresponding TLB entries may be flushed and/or shot down. Alternatively or in addition, updating, modifying, and/or replacing one or more memory mappings ( 1206 ) may include updating, modifying, and/or replacing one or more data structures related to the mapped memory, such as data structure(s) describing which portions of a virtual address space are mapped to file-backed and/or anonymous memory, data structure(s) describing access permissions, and/or any other data structure(s) related to the mapped memory or related to the virtual address space(s). In cases where the file-backed memory is mapped to multiple virtual address spaces, the client logic  312  and/or another logic may skip these portions. Alternatively or in addition, the client logic  312  and/or another logic may update, modify, and/or replace one or more memory mappings for the multiple virtual address spaces. 
     Upon updating, modifying, and/or replacing the one or more memory mappings ( 1206 ), the client logic may populate the memory mapping(s) with data ( 1208 ). Populating the memory mappings with data ( 1208 ) may include copying data from one or more portion(s) of file-backed memory included in local primary memory (such as portion(s) of file data) to one or more anonymous memory portions. For example, the client logic  312  and/or another logic may allocate one or more portions of local primary memory, copy data from one or more of the file-backed memory portions to the portion(s) of local primary memory, and/or associate the portion(s) of local primary memory with the memory mappings. Associating the portion(s) of local primary memory with the memory mappings may include adding, replacing, updating, and/or modifying one or more page table entries, such as by causing the page table entries to reference the portion(s) of local primary memory. Alternatively or in addition, data may be copied from the file, the region  214 , and/or the external memory allocation to the portion(s) of local primary memory, such as if the data is not present in one or more portions of file-backed memory included in local primary memory. Alternatively or in addition, populating the memory mappings with data ( 1208 ) may include re-associating one or more portions of file-backed memory included in local primary memory with the memory mappings. Re-associating one or more portions of file-backed memory may include dis-associating the portion(s) from the file and/or associating the portion(s) of file-backed memory with the corresponding memory mapping(s). In some examples, the client logic may populate the memory mappings with data ( 1208 ) using a combination of these approaches. For example, the client logic may allocate one or more portions of local primary memory, copying some data from one or more file-backed memory portions and other data from the file, the region  214 , and/or the external memory allocation. Other portions may be re-associated from file-backed memory to the memory mappings. 
     Upon completing populating the memory mapping(s) with data ( 1208 ), the client logic  312  and/or another logic may optionally update, modify, and/or replace access permissions ( 1210 ). Updating, modifying, and/or replacing access permissions may include restoring access permissions for the memory mappings to their original settings from before updating, modifying, and/or replacing the memory mapping(s) ( 1206 ). For example, if prior to updating, modifying, and/or replacing the memory mapping(s) ( 1206 ), the access permissions for one or more memory mappings were configured such that reading and writing was allowed, then the access permissions for the corresponding file-backed memory mapping(s) would be configured to allow reading and writing. 
     Upon optionally completing updating, modifying, and/or replacing access permissions ( 1210 ) and/or upon completing populating the memory mapping(s) with data ( 1208 ), the client logic may release the one or more lock primitives ( 1212 ). 
     Upon releasing the one or more lock primitives ( 1212 ), the client logic  312  and/or another logic may complete handling of the request to convert file-backed memory to anonymous memory ( 1214 ), such as by returning a result code to the application logic  314  and/or other requesting logic. 
     While the descriptions for  FIG.  12    illustrate a simple set of operations, it would be clear to one skilled in the art that the operations described here may be performed iteratively upon successive sub-portions of the request to convert file-backed memory to anonymous memory. Performing these operations iteratively may have the advantage of reducing the impact upon the application logic of acquiring the one or more lock primitives ( 1204 ) and/or of restricting access to memory mappings. For example, the one or more lock primitives acquired may only prevent a page fault logic and/or a segmentation fault logic from making progress upon a corresponding subset of virtual address space addresses until the lock primitive is released. Similarly, the memory mapping(s) to which access is restricted may be a smaller subset than would be needed to convert the entirety of the portion(s) specified via the request to convert file-backed memory to anonymous memory. 
     In some examples, the one or more lock primitives may not be acquired ( 1204 ) and/or released ( 1212 ), the one or more lock primitives may be held for a reduced duration, the one or more lock primitives may affect corresponding subset(s) of virtual address space addresses and/or file portions, and/or the one or more lock primitives may correspond to individual portions. One or more of these approaches may be advantageous in that performance impacts of locking may be reduced. 
     In a first example, performance impacts of locking may be reduced by avoiding overhead associated with populating the memory mappings with data ( 1208 ), such as by re-associating one or more portions of file-backed memory included in local primary memory with the memory mappings. In examples where one or more portions of file-backed memory are re-associated with the memory mappings, updating, modifying, and/or replacing the one or more memory mappings ( 1206 ) may be skipped, such as by not invalidating, removing, replacing, updating, and/or modifying one or more page table entries and/or by not flushing and/or shooting down TLB entries for one or more virtual address spaces. Alternatively or in addition, the client logic  312  and/or another logic may update, modify, and/or replace one or more data structures related to the mapped memory, such as data structure(s) describing which portions of a virtual address space are mapped to file-backed and/or anonymous memory, data structure(s) describing access permissions, and/or any other data structure(s) related to the mapped memory or related to the virtual address space(s). For example, the client logic  312  and/or another logic may re-associate one or more of the file-backed memory portions with the memory mappings by updating, modifying, replacing, and/or populating one or more page table entries to reference the corresponding portions of local primary memory and/or by re-mapping the corresponding portion(s) of the virtual address space(s) to anonymous memory (such as by adding one or more entries to the interval tree and/or other data structure describing contiguous portions of the virtual address space(s) that are mapped to anonymous memory and/or to corresponding contiguous portions of the file). In examples such as these, where re-associating one or more portions of file-backed memory included in local primary memory with the memory mappings, the one or more lock primitives may not be acquired ( 1204 ) and/or released ( 1212 ), and/or one or more page faults for corresponding virtual address(es) may be allowed to make progress in parallel with converting file backed portion(s) to anonymous memory. 
     In a second example, performance impacts of locking may be reduced by populating the memory mappings with data ( 1208 ) without preventing page faults from making progress, such as with the one or more lock primitives. For example, acquiring the one or more lock primitives ( 1204 ) may be skipped, one or more page table entries may be marked not writable, invalid, clean, not dirty, not modified, and/or not present, corresponding TLB entries may be shot down, and/or populating the memory mapping(s) with data ( 1208 ) may be performed without preventing page faults from making progress in parallel. In lieu of preventing page faults from making progress in parallel, after populating the memory mapping(s) with data ( 1208 ), the client logic  312  and/or another logic may detect whether one or more of the file-backed portion(s) may have been modified prior to completing the initialization. For example, the client logic  312  and/or another logic may determine whether one or more page faults have occurred for the file portion(s) by checking one or more indicators and/or counters in one or more portion tracking data structures associated with the portion(s). The one or more indicators may be set, updated, modified, and/or replaced when page faults are handled, may indicate whether or not corresponding portion(s) are mapped into one or more page tables, and/or may indicate the number of page tables into which the portion(s) are mapped. Alternatively or in addition, the client logic  312  and/or another logic may determine whether one or more file portions may have been modified by checking a dirty and/or modified bit in one or more page table entries and/or page-tracking data structures. If the client logic  312  and/or another logic detects that one or more of the file-backed portion(s) may have been modified prior to completing the initialization, then updating/modifying/replacing the memory mapping(s) ( 1206 ) and/or populating the memory mapping(s) with data ( 1208 ) may be repeated. Alternatively or in addition, converting the corresponding file-backed portion(s) to anonymous memory portion(s) may be skipped, aborted, and/or cancelled. In cases where converting is skipped, acquired lock primitives (if any) may be released. In some examples, updating/modifying/replacing the memory mapping(s) ( 1206 ) and/or populating the memory mapping(s) with data ( 1208 ) may be repeated a limited number of times before skipping the conversion, aborting the conversion, cancelling the conversion, and/or executing any other applicable action and/or policy. If the client logic  312  and/or another logic determines that one or more page faults have not occurred for the file portion(s) and/or that the portion(s) have not been modified, then the client logic  312  and/or another logic may proceed to memory-map the anonymous memory portion(s) and/or update/modify/replace the access permissions ( 1210 ), such as by updating, modifying, and/or replacing the one or more data structures related to the mapped memory (such as the data structure(s) describing which portions of a virtual address space are mapped to file-backed and/or anonymous memory). In some examples, determining that one or more page faults have not occurred for the file portion(s) and/or that the portion(s) have not been modified may be performed atomically with memory-mapping the anonymous memory portion(s) and/or by updating, modifying, and/or replacing the access permissions ( 1210 ). For example, one or more compare-and-exchange operations may be performed on one or more corresponding page table entries to memory-map the anonymous memory portion(s) and/or update/modify/replace the access permissions ( 1210 ) if the value(s) being replaced do not have their accessed bit(s), modified bit(s), dirty bit(s), and/or any other relevant indicators set (and/or cleared, as appropriate). In other examples, one or more lock primitives and/or access bits may be used to perform these operations atomically, such as the one or more lock primitives described for step ( 1204 ). 
     In examples where one or more page faults, segmentation faults, and/or other accesses are allowed to make progress during the conversion, the client logic  312  and/or another logic may detect whether one or more additional portions need to be converted. For example, the client logic  312  and/or another logic may identify additional portion(s) of file-backed memory included in local primary memory (such as portion(s) of file data) that were not present while populating the memory mapping(s) with data ( 1208 ) and/or may repeat one or more operations to convert one or more of these portions. Alternatively or in addition, one or more of these portions may not be converted. 
     Furthermore, while some of the operations may have been described as an ordered set of operations, it would be clear to one skilled in the art that other sequences may be equally effective in response to the request to convert file-backed memory to anonymous memory. For example, other sequences may have additional operations, some operations may be skipped, and/or some operations may be performed in a different order than described. For example, in some example implementations, populating the memory mappings with data ( 1208 ) may be performed in connection with updating, modifying, and/or replacing the one or more memory mappings ( 1206 ), such as with a MAP_POPULATE option to a memory mapping operation interface (such as mmap). 
     While  FIGS.  11  and  12    illustrate converting between anonymous memory and file-backed memory, converting between external primary memory and anonymous memory (local primary memory) is performed similarly even if the external primary memory is not implemented with a memory mapped file. For example, instead of selecting the portion(s) of the memory-mapped file ( 1110 ), portion(s) of the region(s) of external primary memory are selected. 
       FIG.  13 A  and  FIG.  13 B  illustrate example flow diagrams for handling a page fault for portions of an address space and/or for reclaiming portions of local primary memory using external primary memory. One or both flow diagrams may be useful for example systems in which there is no implicit mapping between file offsets and offsets in the region  214  and/or external memory allocation. 
     A page fault and/or other event, such as a request to access a portion of an address space and/or file, may occur ( 1302 ), triggering a flow such as the example illustrated in  FIG.  13 A . The request to access a portion of an address space and/or file may be any one or more mechanisms that may trigger the operations described for  FIG.  13 A . Examples of possible requests include: a message, a programmatic signal, an electrical signal, an optical signal, a wireless signal, a method invocation, a function call, a TCP and/or UDP message, an HTTP request, etc. The request may be received from another logic, such as the application logic  314 , and/or the request may be received from the client logic  312 , such as by the client logic  312  making a determination to access a portion of an address space and/or file. The client logic  312 , the application logic  314 , any other one or more logics may make the determination independently and/or in coordination with any other one or more logics based on any one or more of the conditions, parameters, configurations, and/or other properties of the client  130  and/or of the one or more memory appliances  110  and/or for any other reason. In some examples, there may be no request, such as if the determination is made by the client logic  312  and/or another logic. 
     In response to the page fault and/or other event, the client logic  312  and/or another logic (such as an operating system, a kernel, a device driver, a device firmware, a hypervisor service, a cloud computing interface, and/or any other hardware, software, and/or firmware entity which may perform the same functions as described) may allocate a portion of local primary memory ( 1304 ). The size of the portion may be selected to satisfy the triggering event ( 1302 ). For example, for a page fault, the size may be equal to the size of a page, a huge page, a chunk, and/or any other suitable size. In other examples, such as for the request to access the portion of the address space and/or file, the size may be specified by and/or implied by the request. 
     Upon allocating the portion of local primary memory ( 1304 ), the client logic  312  and/or another logic may determine ( 1306 ) whether data for the portion is in external primary memory. In an example implementation, the determination ( 1306 ) may be made by evaluating whether or not an address associated with the triggering event ( 1302 ) is associated with a portion of external primary memory, such as a portion of the region  214  and/or external memory allocation. If data for the portion is not in external primary memory, the portion of local primary memory may be initialized ( 1308 ). If data for the portion is in external primary memory, the data may be written ( 1310 ) to the portion of local primary memory, such as by copying the data from the portion of the region  214  and/or external memory allocation. In some examples, the data may be copied using client-side memory access, via one or more RDMA operations, and/or via any other one or more suitable data transfer operations. In these and/or other examples, the data may be optionally encrypted, decrypted, compressed, decompressed, and/or may undergo data translation before, during, and/or after being written ( 1310 ). Other examples of data translation may be described elsewhere in this disclosure. In some examples, the encryption, decryption, compression, decompression, and/or data translation may be performed in whole or in part by one or more communication interfaces  230 ,  330 ,  430 , one or more processors  240 ,  340 ,  440 , other devices, and/or other logic. 
     Upon initializing ( 1308 ) and/or writing data to ( 1310 ) the portion of local primary memory, the client logic  312  and/or another logic may map ( 1312 ) one or more addresses to the portion of local primary memory. The one or more addresses that are mapped may include the address associated with the triggering event ( 1302 ) used to determine ( 1306 ) whether the data for the portion is in external primary memory, and/or may include some other address(es), such as other address(es) within the address space and/or such as address(es) within one or more other address spaces. The address space and/or the other address space(s) may be virtual address spaces. Alternatively or in addition, the address space and/or the other address space(s) may be some other type of address space, such as a physical address space, an  10  virtual address space, and/or any other type of address space known now or later discovered and/or defined. In some examples, no addresses may be mapped, such as for some requests to access a portion of a file. Upon mapping ( 1312 ) the one or more addresses to the portion of local primary memory, handling of the page fault and/or other event may be complete ( 1314 ). 
     A low memory condition, exceeding configured memory limits, and/or other event(s), such as a determination that one or more portions of local primary memory have not been used recently and/or are unlikely to be used in the near future may occur ( 1316 ), triggering a portion reclaim flow such as the example illustrated in  FIG.  13 B . In response, the client logic  312  and/or another logic (such as an operating system, a kernel, a device driver, a device firmware, a hypervisor service, a cloud computing interface, and/or any other hardware, software, and/or firmware entity which may perform the same functions as described) may allocate one or more portions of external primary memory ( 1318 ). In allocating the one or more portion(s), the portion(s) of external primary memory may be selected at the client  130 , by the client logic  312 , by another logic, in coordination with other client logics  312 , in coordination with other logics, and/or independently of the memory appliance  110  and/or of any one or more logics, such as the region access logic  212  and/or the allocation logic  412 . The portion(s) of external primary memory may be portion(s) of one or more regions  214  and/or external memory allocations. One or more of the region(s)  214  and/or external memory allocation(s) including the selected portion(s) of external primary memory may be associated with one or more portions being reclaimed, with one or more address spaces associated with the portion(s) being reclaimed, with one or more application logics  214  associated with the portion(s) being reclaimed, and/or with one or more application logics associated with the address space(s). For example, the region(s)  214  and/or external memory allocation(s) may be associated with one or more virtualization instances, virtual machine(s), container(s), jail(s), and/or zone(s), that have previously accessed one or more of the portions being reclaimed. In other examples, one or more of the region(s)  214  and/or external memory allocation(s) may be provisioned to one or more entities associated with the address space(s), with one or more application logics  314  associated with the portion(s) being reclaimed, and/or with one or more application logics  314  associated with the address space(s). For example, the region(s)  214  may be provisioned to one or more virtualization instances, to one or more application logics  314 , to one or more clients  130 , and/or to any other one or more entities. 
     In some examples, one or more regions  214  and/or external memory allocations may be created in response to the low memory condition, exceeding configured memory limits, and/or other event(s). For example, one or more regions and/or external memory allocations may not be created until there are insufficient available portions of previously-created region(s)  214  and/or external memory allocation(s) to satisfy current needs. In some examples, no regions  214  and/or external memory allocations may be created until after at least one portion of external primary memory is needed to reclaim one or more portions of local primary memory. This approach may be advantageous in that it may reduce the amount of external primary memory reserved for created region(s)  214  and/or external memory allocations in example systems where creating a region  214  and/or external memory allocation reserves some amount of external primary memory for the region  214  and/or external memory allocation. 
     In some examples, the portion(s) of external primary memory may be, may include, and/or may constitute a slab, a page, a huge page, a chunk, a buffer, and/or any other suitable unit of allocation. In other examples, one or more of the portion(s) of external primary memory may have already been allocated and/or may already be associated with one or more portions of local primary memory to be reclaimed. In these examples, one or more previously-allocated portion(s) may not be allocated. 
     In some examples, one or more of the portion(s) of local primary memory to be reclaimed may be anonymous memory. Alternatively or in addition, one or more of the portion(s) of local primary memory to be reclaimed may be file-backed memory and/or a portion of a memory-mapped file. One or more of the region(s)  214  and/or external memory allocation(s) including the selected portion(s) of external primary memory may be associated with and/or provisioned to the memory-mapped file. 
     In some examples, such as if there is insufficient external primary memory to allocate the one or more portions of external primary memory, swap space may be selected. The swap space may be selected to accommodate data for all of, or a portion of, the portions to reclaim. In some examples, both swap space and external primary memory may be selected. An advantage of selecting swap space for all or a portion of the portions to reclaim may be that systems using this approach may continue to operate, even after exhausting all available local primary memory and external primary memory by utilizing swap space to hold data for portions to be reclaimed. 
     As explained above, the term “swap space” as used herein refers to memory stored in a conventional swap file, swap partition, and/or swap device, which may be remotely accessed via a storage protocol and/or may be accessed over a local bus via a storage protocol, such as with the backing store  360 . Swap space may be one or more areas of memory and/or storage that may hold data that would otherwise be held in local primary memory. For example, the data may have been previously stored in local primary memory and/or may have been removed from local primary memory in response to a low memory condition, in response to determining that the data is unlikely to be used soon, in order to make room in local primary memory for other data, and/or for any other reason. Once held in swap space, all or some of the data may be copied to local primary memory in response to and/or in anticipation of an attempt to access the data. The one or more areas of memory and/or storage of swap space may be any memory or combination of memories, such as a solid state memory, a random access memory (RAM), a dynamic random access memory (DRAM), a static random access memory (SRAM), a flash memory, a read-only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), a phase change memory, 3D XPoint memory, a memristor memory, any type of memory configured in an address space addressable by the processor, or any combination thereof. Alternatively or in addition, one or more areas of memory and/or storage of swap space may include one or more solid state memories, solid state storage devices, and/or any other one or more types of memory, storage, storage device, and/or storage system known now or later discovered. 
     Upon completing and/or skipping allocation of the one or more portions of external primary memory ( 1318 ), the client logic  312  and/or another logic may write ( 1320 ) data from the one or more portions of local primary memory to be reclaimed to the one or more portions of external primary memory, such as by copying the data to the one or more portions of one or more regions  214  and/or external memory allocations. In some examples, the data may be copied using client-side memory access, via one or more RDMA operations, and/or via any other one or more suitable data transfer operations. In these and/or other examples, the data may be optionally encrypted, decrypted, compressed, decompressed, and/or may undergo data translation before, during, and/or after being written ( 1320 ). Other examples of data translation may be described elsewhere in this disclosure. In some examples, the encryption, decryption, compression, decompression, and/or data translation may be performed in whole or in part by one or more communication interfaces  230 ,  330 ,  430 , one or more processors  240 ,  340 ,  440 , other devices, and/or other logic. In examples where the data is compressed, the compression may be performed prior to allocating ( 1318 ) the one or more portions of external primary memory. Compressing the data before allocating ( 1318 ) may be advantageous when using compression algorithms for which it may be difficult to predict the resulting size of compressed data. 
     Upon writing the data ( 1320 ), the client logic  312  and/or another logic may associate one or more addresses with the one or more portions of external primary memory ( 1322 ). The one or more addresses associated may be previously associated with and/or mapped to the one or more portions of local primary memory that had not been used recently and/or were determined to be unlikely to be used in the near future. In some examples, the one or more portions of local primary memory may be disassociated and/or unmapped from the one or more addresses before, after, or while associating the one or more addresses with the one or more portions of external primary memory ( 1322 ). 
     For example, such as may be the case with anonymous memory pages, one or more references to the one or more portions of local primary memory may be removed from one or more page table entries and/or other data structure(s), and/or references and/or identifiers to the one or more portions of external primary memory may be stored in place of the reference(s) to portion(s) of local primary memory. In the same and/or other examples, the one or more page table entries may be updated, modified, and/or replaced. While storing the references and/or identifiers to the one or more portions of external primary memory, the page table entries may be invalidated and/or may be marked as not present. This may be advantageous in cases where the portions of external primary memory are not addressable by the processor  340  of the client  130 . A portion may be considered not addressable by the processor if it cannot be accessed via a load and/or store instruction as part of the processor&#39;s instruction set without triggering a page fault. 
     In other examples, such as may be the case with file-backed memory, one or more references to the one or more portions of local primary memory may be removed from, updated in, modified in, and/or replaced in a data structure, such as a data structure associating portions of a corresponding file with portions of external primary memory and/or with some other backing store. Examples of data structures suitable for this purpose may include an array, an extensible array, a sparse array, a radix tree, a mapping, a hash table, and/or any other data structure that facilitates associating file portions and/or file offsets with references and/or identifiers. 
     The identifier(s) stored in, removed from, updated in, modified in, and/or replaced in the one or more page table entries and/or other data structure(s) may be usable to identify the corresponding portions of external primary memory. For example, the identifier(s) may indicate and/or include one or more references to one or more regions  214 , to one or more external memory allocations, and/or to one or more memory appliances  110 . Alternatively or in addition, the identifier(s) may be usable with some other data structure to identify the corresponding portions of external primary memory. For example, the identifier(s) may be usable with a distributed hash table and/or other data structure to identify one or more regions  214 , one or more external memory allocations, and/or one or more memory appliances  110  that include the corresponding portion(s) of external primary memory. Other examples of data structures suitable for this purpose may include an array, an extensible array, a sparse array, a radix tree, a mapping, a hash table, and/or any other data structure that may associate identifiers with: other identifiers, regions  214 , external memory allocations, and/or memory appliances  110 . 
     Upon associating the one or more addresses with the one or more portions of external primary memory ( 1322 ), handling of the low memory condition, exceeding configured memory limits, and/or other event(s) may be complete ( 1324 ). 
     The operations depicted in  FIG.  13 A  and  FIG.  13 B  may occur independently, concurrently, repeatedly, and/or may be interleaved. In some examples, after reclaiming a portion of local primary memory using the operations described for  FIG.  13 B , a page fault for the same address may occur, causing a second portion of local primary memory to be allocated ( 1304 ), data to be copied ( 1310 ) from the selected portion of external primary memory to the second portion of local primary memory, and causing the address to be mapped ( 1312 ) to the second portion of local primary memory. Similarly, the second portion of local primary memory may be subsequently selected to be reclaimed, triggering the operations described for  FIG.  13 B  for the second portion of local primary memory. 
     In some examples, the operations depicted in  FIG.  13 A  and  FIG.  13 B  may occur on the same and/or different clients  130 . For example, after one or more portions of local primary memory are reclaimed at a first client using the operations described for  FIG.  13 B , an application logic  314 , such as a virtualization instance, a virtual machine, a container, a jail, and/or a zone, may be migrated from the first client to a second client, and/or one or more page faults may occur while the application logic  314  is operating with the second client. In response to the page fault, the operations described for  FIG.  13 A  may be triggered at the second client. For example, one or more portions of local primary memory may be allocated ( 1304 ) at the second client, data from one or more portions of external primary memory that were selected ( 1318 ) during earlier reclaim operations may be copied ( 1310 ) to the one or more portions of local primary memory of the second client, and/or one or more addresses at the second client may be mapped ( 1312 ) to the one or more portions of local primary memory at the second client. As the application logic  314  continues to operate with the second client, the application logic  314  may then access the data in the portions of local primary memory of the second client, such as via the mapped address(es). 
     The portion(s) being reclaimed and/or the portion(s) of local primary memory that have not been used recently and/or are unlikely to be used in the near future may be selected using any one or more selection strategies, portion replacement strategies, and/or page replacement strategies known now or later discovered. For example, one or more portion(s) of local primary memory may be tracked using one or more sorted and/or unsorted collections, such as one or more least-recently used lists. As portion(s) of local primary memory are accessed, read, written, referenced, etc., the collection(s) may be updated, modified, and/or replaced to reflect the action that occurred. For example, the position of the portion of local primary memory may be moved to the back of a sorted list of least-recently used portions, and/or may be moved to a different collection, such as to a collection of active portions. Alternatively or in addition, the position of other portion(s) of local primary memory may be updated, modified, and/or replaced in response to the action. For example, one or more other portions may be moved from the collection of active portions to a collection of inactive portions. Other examples of portion replacement strategies may include not-recently-used, first-in-first-out, second-chance, clock, random replacement, not-frequently-used, aging, longest-distance-first, any other portion replacement strategy known now or later discovered, and/or any combination of two or more of these and/or any other strategies. 
     The one or more selection strategies, portion replacement strategies, and/or page replacement strategies used may be selected by a user and/or an administrator, may be configured, and/or may be selected based on any one or more policies, passed functions, steps, and/or rules that the client logic  312  and/or another logic follows to determine which one or more selection strategies, portion replacement strategies, and/or page replacement strategies to use and/or which parameter(s) and/or configuration(s) to use with the one or more selection strategies, portion replacement strategies, and/or page replacement strategies. The one or more policies, passed functions, steps, and/or rules may use any available information, such as any one or more of the characteristics and/or configurations of the client(s)  130 , the memory appliance(s) ( 110 ), and/or the management server(s), to select the one or more selection strategies, portion replacement strategies, and/or page replacement strategies. For example, a policy and/or passed function may specify to use a random replacement strategy unless the number of page faults that result in reading from external primary memory reaches a threshold, and/or to use a least-recently-used strategy after reaching the threshold. 
     Alternatively or in addition, the one or more selection strategies, portion replacement strategies, and/or page replacement strategies used may be affected by one or more policies, passed functions, steps, and/or rules that the client logic  312  and/or another logic follows to determine how the one or more selection strategies, portion replacement strategies, and/or page replacement strategies should operate. The one or more policies, passed functions, steps, and/or rules may use any available information, such as any one or more of the characteristics and/or configurations of the client(s)  130 , the memory appliance(s) ( 110 ), and/or the management server(s), to affect the one or more selection strategies, portion replacement strategies, and/or page replacement strategies. For example, a policy and/or passed function may specify an amount of memory to unmap from page table entries and/or to reclaim when activated. In another example, a policy and/or passed function may specify an amount of data to retain in local primary memory as frequently-used and/or “hot” data. 
     In some examples, a sorted collection may be approximated by storing one or more values with portion-tracking data structures. The one or more values stored may include one or more generation counters and/or timestamp values. For example, a generation counter may be maintained and/or may be incremented and/or decremented as portion(s) are accessed and/or as page fault(s) occur. As the portion(s) are accessed and/or as page fault(s) occur, the one or more values may be stored with one or more portion tracking data structures that correspond to the portion(s) being accessed and/or the portion(s) being page-faulted. Alternatively or in addition, the one or more values stored may include a count of the number of times corresponding portion(s) were accessed and/or page-faulted. 
     Periodically and/or in response to an event and/or condition, all of or a subset of all portion tracking data structures may be inspected and/or corresponding stored values may be collected in a data structure. For example, all portion tracking data structures for a file and/or allocation domain may be inspected and/or corresponding stored values may be collected. An allocation domain may be a logical partitioning of computing resources and/or may be controlled via one or more file data limits, such as described in U.S. Non-provisional patent application Ser. No. 15/424,395, filed Feb. 3, 2017, which is hereby incorporated by reference. In some examples, one or more virtualization instances, virtual machines, containers, jails, and/or zones may be and/or may be included in an allocation domain. The collected values may be stored in the data structure with one or more identifiers for the corresponding portions, such as an address, offset, and/or index of the corresponding portions and/or an address and/or reference to corresponding portion tracking data structures. The collected values may be sorted while being collected and/or after being collected. In some examples, all of or a subset of all portion tracking data structures may be updated, modified, and/or replaced periodically and/or in response to the event and/or condition and/or in response to another event and/or condition. For example, in examples where the one or more values stored include a count of the number of times corresponding portion(s) were accessed and/or page-faulted, the value(s) may be reset, cleared, zeroed, removed, updated, modified, replaced, and/or invalidated, such as by setting the value(s) to zero and/or setting and/or clearing one or more indicators indicating that the value(s) are invalid and/or not present. 
     The sorted collected values may be used to identify portion(s) of local primary memory that have not been used recently and/or are unlikely to be used in the near future by inspecting the portion-tracking data structures for the portions corresponding to the first and/or last value in the sorted collected values. For example, if the sorted collected values are sorted based upon a timestamp and/or generation counter for the most recent access and/or page fault of corresponding portions, then the lowest value may correspond to the portion which was least recently used. Alternatively or in addition, if the sorted collected values are sorted based upon the count of the number of times corresponding portions were accessed and/or page-faulted, then the lowest value may correspond to a portion which is relatively infrequently used and/or which has not been used frequently since the value(s) were last reset. As such, storing, collecting, and/or sorting the stored values may be used to approximate a least-recently-used strategy, a not-frequently-used strategy, and/or any other strategy that traditionally includes maintaining a sorted collection to identify one or more candidates to reclaim. 
     A timestamp value may be any value that corresponds to the current time. The timestamp value may be specified in any one or more units of time, such as seconds, milliseconds, microseconds, jiffies, etc., and or may be relative to some other defined time. For example, a timestamp may be the number of seconds since midnight on January 1st of 1970. In another example, a timestamp may be the number of seconds and microseconds since midnight on January 1st of 2000. The timestamp value may be specified in a defined time zone, such as UTC, GMT, and/or any other time zone. Alternatively or in addition, the timestamp value may be specified in the time zone where one or more clients  130 , one or more memory appliances  110 , one or more management servers  120 , and/or any other one or more entities are physically located. 
     In some examples, the actual values for one or more portions may change between when the values in the sorted collected values are collected and when portion(s) of local primary memory are to be identified for reclaim. Prior to selecting a portion of local primary memory for reclaim and/or prior to reclaiming the selected portion, the client logic  312  and/or another logic may inspect the corresponding portion tracking data structure and/or obtain the current value for the portion. If the current value for the portion is equal to the value in the sorted collected values data structure, the portion may be considered a good candidate to reclaim. Alternatively or in addition, the current value for the portion may be compared with a threshold value to determine if the portion is a good candidate to reclaim. For example, the threshold value may be the mean and/or median value from the sorted collected values, the 80th percentile value from the sorted collected values, any other value from the sorted collected values, a calculated value based upon one or more values from the collected values, a value based on the current timestamp and/or generation counter, and/or any other value which may be useful for determining suitability for reclaiming a portion. In some examples, the current value for the portion may be checked during other and/or additional steps of reclaiming portion(s) of local primary memory. For example, the current value may be compared to the stored and/or threshold value prior to associating the one or more addresses with the one or more portions of external primary memory ( 1322 ) and/or prior to removing the one or more references to the one or more portions of local primary memory from the one or more page table entries and/or other data structure(s). 
     In some examples, the collected values may be analyzed and/or used to determine the threshold value, but not used directly to identify candidate portions to reclaim. For example, the collected values may be sorted, analyzed, and/or partially sorted in order to determine the value that would be at the 80th percentile and/or any other position of the sorted collected values and choose this value as the threshold value. In another example, the threshold value may be calculated based on one or more of the collected values and/or sorted collected values. In lieu of using the collected values directly to identify candidate portions to reclaim, candidate portions may be chosen using any selection strategy, portion replacement strategy, and/or page replacement strategy known now or later discovered. For example, candidate portions to reclaim may be selected randomly and/or portion tracking data structures for candidate portions may be inspected to compare one or more stored values to one or more threshold values that were determined from the collected values. 
     An advantage of using the sorted collected values and/or threshold values as described herein over maintaining one or more sorted collections may be that lock contention may be reduced and/or eliminated. For example, if a lock primitive would be used to access, update, modify, and/or replace the sorted collection(s), the lock primitive could be avoided when not attempting to maintain the sorted collection(s), and/or the lock primitive may not need to be acquired when handling a page fault and/or other type of access operations which may otherwise cause the sorted collection(s) to be updated/modified/replaced and/or one or more portions to be moved within the sorted collection(s) and/or between multiple collections. 
     In some example systems the sorted collected values may be collected and/or sorted without acquiring any highly-contended lock primitives. For example, when using a lock-free page cache and/or other data structure, portions of a file and/or address space may be iterated by speculatively referencing entries in a radix tree and/or other data structure that uses a read-copy-update and/or other lock-free mechanism to coordinate access. Speculatively-referenced entries may be individually locked and/or verified to exist when collecting the stored values for corresponding portion tracking data structures. In some examples, if a portion is already locked, invalid, and/or unable to be verified, the portion may be skipped. Skipping these portions may be advantageous and/or may improve efficiency of collecting values in example systems where locked and/or invalid portions are unlikely to be good candidates to reclaim. 
       FIG.  14 A  and  FIG.  14 B  illustrate example mappings and/or associations between address spaces, file data, local primary memory, and/or external primary memory for example systems in which some portions of an address space are mapped to local primary memory and/or some portions of the address space are associated with external primary memory. As illustrated in  FIG.  14 A , an address space  1402  may include and/or map to a mapped portion  1404 , such as with an anonymous memory mapping. In some examples, the address space  1402  may be a virtual address space. One or more data structures may be updated, modified, and/or replaced to indicate that the portion of the address space  1402  corresponding to the mapped portion  1404  includes and/or is mapped to anonymous memory. As portions  1406  of the address space  1402  for the mapped portion  1404  are accessed and/or reclaimed, the portions  1406  may become mapped to one or more local memory portions  1408  and/or associated with one or more portions  1410 ,  1412  of one or more regions  214   a ,  214   b  of external primary memory. In some examples, one or more portions  1408  of local primary memory may be associated with one or more portions of external primary memory  1414 , such as when writing to the portion of external primary memory  1414  (such as prior to reclaiming the portion of local primary memory  1408  with operations, such as described for  FIG.  13 B ) and/or when reading from the portion of external primary memory  1414  (such as in response to a page fault and/or other event, such as described for  FIG.  13 A ). 
     As portions  1406  of the mapped portion  1404 , other portions of the address space  1402 , and/or other portions of other address spaces are accessed, the mappings and/or associations illustrated for  FIG.  14 A  may change. For example, other mappings to other portions of local primary memory may be formed using operations similar to those described for  FIG.  13 A . Alternatively or in addition, mappings to portions of local primary memory may be replaced with associations with external primary memory using operations similar to those described for  FIG.  13 B . 
     As illustrated in  FIG.  14 B , an address space  1416  may include and/or map to a mapped portion  1418 , such as with a file-backed memory mapping. The corresponding file may be a pseudo file or a conventional file. The address space  1416  may be the same address space or a different address space as the address space  1402  illustrated in  FIG.  14 A . In some examples, the address space  1416  may be a virtual address space. The mapped portion  1418  may be the same as or different than the mapped portion  1404  illustrated in  FIG.  14 A . The mapped portion  1418  may map to a mapped file portion  1420  of a file and/or may be included in the file data  1422  for a file. In the example illustrated in  FIG.  14 B , the file data  1422  represents the entire contents of the file. 
     As portions of the address space  1416  for the mapped portion  1418  are accessed and/or reclaimed, some portions  1424  of the address space  1416  may become mapped to one or more local memory portions  1426  and/or some portions  1428 ,  1430  of the file data  1422  may become associated with one or more portions  1432 ,  1434  of one or more regions  214   a ,  214   b  and/or of one or more external memory allocations. For example, a portion  1424  of the mapped portion  1418  in the address space  1416  may map to the local memory portion(s)  1426 . A reference and/or address to the local memory portion  1426  may be stored in a page table entry corresponding to the portion  1424  of the mapped portion  1418  in the address space  1416 . Alternatively, or in addition, a portion  1436  of the mapped filed portion  1420  in the file data  1422  may map to the local memory portion  1426 . For example, a reference to the local memory portion  1426  may be stored in the file data  1422  and/or in a data structure related to the file and/or file data  1422 , such as the data structure associating portions of a corresponding file with portions of external primary memory and/or with some other backing store. 
     As portions of the mapped portion  1418 , other portions of the address space  1416 , and/or other portions of other address spaces are accessed, the mappings and/or associations illustrated for  FIG.  14 B  may change. For example, other mappings to other portions of local primary memory may be formed using operations similar to those described for  FIG.  13 A . Alternatively or in addition, mappings to portions of local primary memory may be replaced with associations with external primary memory using operations similar to those described for  FIG.  13 B . 
       FIG.  15    illustrates an example flow diagram for relocating one or more portions of external primary memory. Operations for relocating one or more portions of external primary memory may begin ( 1502 ) in response to a triggering event and/or condition. For example, an administrator may request that all data from a first region and/or external memory allocation be migrated to a second region and/or external memory allocation. This may be desirable in examples where a memory appliance  110  that includes the first region may need to be taken offline for maintenance, repairs, upgrades, replacement, and/or for any other reason. In other examples, an administrator may request that all data from a memory appliance  110  be migrated to other memory appliances and/or to other locations. 
     Upon starting the operations for relocating one or more portions of external primary memory, the client logic  312 , region access logic  212 , and/or another logic may select ( 1504 ) one or more portions to relocate. In some examples with a triggering event, the portion(s) may be specified by the triggering event. Alternatively or in addition, the portion(s) may be selected via some other strategy. For example, the client logic  312  and/or another logic may select all previously-allocated portions in one or more regions  214  and/or external memory allocations that may have been selected to be destroyed. In another example, the client logic  312  and/or another logic may select all previously-allocated portions in one or more memory appliances  110  that may need to be taken offline for maintenance, repairs, upgrades, replacement, and/or for any other reason. In another example, the client logic  312  and/or another logic may select all previously-allocated portions in one or more parts of one or more regions  214  and/or external memory allocations that are to be shrunk, truncated, hole-punched, resized, and/or marked not present. 
     Upon selecting the one or more portions to relocate, the client logic  312 , region access logic  212 , and/or another logic may select ( 1506 ) one or more destination portions. There may be the same number of destination portions, more destination portions, or fewer destination portions than portions of external primary memory to relocate. For example, there may be one destination portion of size equal to or larger than the sum of the sizes of all portions to relocate. In other examples, there may be multiple destination portions. In still other examples, for each portion to relocate, there may be a corresponding same-sized destination portion. In other examples, there may be only one portion to relocate and one destination portion. 
     In some examples, the destination portion(s) may be included in one or more regions  214  and/or external memory allocations. The destination portion(s) may be selected and/or the region(s)  214  and/or external memory allocation(s) in which the portion(s) are selected may be selected using any of the methods and/or logic described herein, such as for provisioning, allocation, selection, slab allocation, allocation strategies, policies, passed functions, steps, and/or rules, etc. Alternatively or in addition, the portion(s) may be selected and/or the region(s)  214  and/or external memory allocation(s) in which the portion(s) are selected may be selected using any selection strategy known now or later discovered. In examples where more than one portion of external primary memory is to be relocated, the same region  214  and/or external memory allocation may be selected for multiple portions to be relocated and/or one or more different regions and/or external memory allocations may be selected for one or more portions to be relocated. In other examples, the one or more of the destination portion(s) may be included in local primary memory, such as in the memory  310  of the client  130 . 
     Upon selecting the one or more destination portions, the client logic  312 , the region access logic  212 , and/or another logic may copy ( 1508 ) data from the portion(s) to relocate to the destination portion(s). In some examples, the data may be copied using client-side memory access, via one or more RDMA operations, and/or via any other one or more suitable data transfer operations. In these and/or other examples, the data may be optionally encrypted, decrypted, compressed, decompressed, and/or may undergo data translation before, during, and/or after being copied ( 1508 ). Other examples of data translation may be described elsewhere in this disclosure. In some examples, the encryption, decryption, compression, decompression, and/or data translation may be performed in whole or in part by one or more communication interfaces  230 ,  330 ,  430 , one or more processors  240 ,  340 ,  440 , other devices, and/or other logic. 
     Upon copying the data, the client logic  312 , region access logic  212 , and/or another logic may update, modify, and/or replace ( 1510 ) one or more portion associations. Updating, modifying, and/or replacing the association(s) may include disassociating one or more addresses with one or more of the portions to relocate. Alternatively, or in addition, updating, modifying, and/or replacing the associations may include removing one or more identifiers of the portions to relocate from one or more data structures, such as one or more page tables and/or the data structure associating portions of a corresponding file with portions of external primary memory and/or with some other backing store, as described elsewhere in this disclosure. Alternatively or in addition, updating, modifying, and/or replacing the associations may include associating one or more addresses with one or more of the destination portions and/or adding one or more identifiers of the destination portions to the one or more data structures, such as one or more page tables and/or the data structure associating portions of a corresponding file with portions of external primary memory and/or with some other backing store. Alternatively or in addition, one or more identifiers may not be updated, modified, and/or replaced, such as if the identifiers are used with one or more other data structures to identify the portions to relocate and/or the destination portions. For example, the one or more other data structures may be updated, modified, and/or replaced to identify the destination portions instead of the portions to be relocated. Alternatively or in addition, no updates/modifications/replacements may be needed, such as if the other data structure(s) include a hash table (or a data structure with similar utility), and/or if the portion(s) to relocate and the destination portion(s) are both identifiable from the same identifiers using the one or more data structures. Upon updating, modifying, and/or replacing ( 1510 ) the one or more portion associations, the operations for relocating one or more portions of external primary memory may be complete ( 1512 ). 
     Alternatively or in addition to the operations described herein, other operations may be performed to relocate one or more portions of external primary memory. For example, the client logic  312 , the application logic  314 , the allocation logic  412 , and/or another logic may transmit the request to migrate a region and/or the request to restructure an external memory allocation, which are described elsewhere in this disclosure. The request to migrate a region and/or the request to restructure an external memory allocation may cause one or more region(s)  214  and/or one or more external memory allocations to be relocated from one memory appliance  110  to another. By causing the one or more region(s)  214  and/or one or more external memory allocations to be relocated, one or more portions included in the one or more region(s)  214  and/or one or more external memory allocations may also be relocated. Upon completing the request to migrate a region and/or the request to restructure an external memory allocation, the client logic  312 , region access logic  212 , and/or another logic may update, modify, and/or replace ( 1510 ) one or more portion associations, as described for  FIG.  15   . Alternatively or in addition, one or more portion associations may not be updated, modified, and/or replaced, such as if the portion associations are defined independently of which memory appliance(s)  110  include the one or more regions  214  and/or external memory allocations. 
     Methods and/or systems may be provided that provide fork-safe access to data on memory appliances or other devices accessed via memory-mapped I/O (Input/Output). Memory-mapped I/O is a mechanism in which communication with an I/O device is accomplished by accessing memory that is mapped to the I/O device. For example, memory and/or registers of the I/O device may be mapped to (or associated with) memory addresses accessible by a CPU (central processing unit) or other processor. The mapped memory addresses may be referred to as virtual addresses and/or virtual memory addresses. When any virtual memory address is accessed by the CPU, the virtual memory address may reference a portion of physical RAM in primary memory and/or memory of the I/O device. Thus, instructions executed by the CPU that access the virtual memory address may also access the I/O device. 
     The memory-mapped I/O may involve a memory-mapped file. The memory-mapped file may be a segment of virtual memory which has been assigned to a portion of a file or a pseudo file. Accordingly, the memory-mapped file may be accessed via memory-mapped I/O. A memory-mapped file comprises virtual memory backed by a file or pseudo file. The memory-mapped file may be created, in some examples, by invoking mmap( ) on a POSIX-compliant Unix operating system. 
     By way of an example, a system may provide one or more pseudo files in a filesystem; a user and/or a process may open the one or more of the pseudo files; and memory map (for example, mmap) operations may be performed on the one or more pseudo files. A pseudo file may be a logical entity accessible through a file interface, where the pseudo file may be accessed or otherwise used like a file through the file interface, but the pseudo file may not actually be a file stored in a traditional file system. For example, reads and writes to the pseudo file may be translated into reads and writes to one or more of the memory appliances instead of accessing data in the traditional file system. In addition to accessing the pseudo file directly through the file interface, the pseudo file may be accessed through a memory operation on a virtual memory address mapped to the pseudo file. Alternatively or in addition to the pseudo file, the memory map operations may be performed on any other type of file. 
     The file may be a regular file in a filesystem, a special file, a block device file, a character device file, a pseudo file, any other type of file, and/or any other interface that can be memory-mapped. The file may be backed by any medium capable of holding data, such as a solid state memory, a random access memory (RAM), a dynamic random access memory (DRAM), a static random access memory (SRAM), a flash memory, a read-only memory (ROM), an electrically erasable programmable read-only memory (EEPROM), a flash memory, V-NAND, Z-NAND, a phase change memory, 3D XPoint memory, a memristor memory, a solid state storage device, a magnetic disk, tape, and/or any other media capable of holding data known now or later discovered. In one example, the file may include an interface to enable memory mapping to a peripheral that enables access to solid state memory, such as a PCIe-attached flash-memory peripheral. In another example, the file may include an interface, such as an interface in a virtual filesystem that provides access to a corresponding area of the memory  210  in the memory appliance  110 , such as the region  214 . As such, reading or writing data to a specified offset within the file may cause the virtual filesystem to read or write data from the corresponding offset within the memory  210  of the memory appliance  110 . Similarly, when the file is memory-mapped, page faults in memory backed by the file may cause the virtual filesystem to read data from the corresponding offset within the memory  210  of the memory appliance  110 , and/or writeback to the file may cause the virtual filesystem to write data to the corresponding offset within the memory  210  of the memory appliance  110 . 
     The capability to memory map the file may be provided by an operating system. A memory allocation wrapper around this memory map capability may be provided through which a user and/or a process may allocate memory from the memory appliance to use as its application memory. The memory allocation wrapper or interface may be used without re-compiling the application, such as if the memory allocation wrapper and/or related logic is configured to override and/or take the place of interfaces and/or programmatic procedures that would otherwise be provided by another logic, such as the operating system or a system library. 
     In order to implement a complete memory allocation solution using memory-mapped files, the system may address inherent differences in how memory-mapped files are treated versus anonymous memory. Anonymous memory is memory usually obtained from the operating system by memory allocation interfaces, such as malloc( ), mmap( ), brk( ), and/or sbrk( ) in C programs. Specifically, when a process forks using anonymous memory, the child process effectively obtains a copy of the parent process&#39;s anonymous memory at the time of the fork (most modern operating systems achieve this via copy-on-write semantics). The copying of anonymous memory means that changes to the anonymous memory in either the child process or the parent process after the fork do not appear in the other respective process&#39;s view of the memory. A process may fork for different reasons. In one example, a process may fork in order to achieve greater parallelization in doing some computation. In another example, a process may fork in order to start processes that do different parts of a larger task. In another example, a process may fork in order to start other processes requested by a user (such as a login shell forking, and then executing a program typed by the user into the shell&#39;s command prompt). Without the ability to support proper forking semantics, a memory allocation solution using memory-mapped files may be severely limited. The system may provide fork-safe allocation from memory-mapped files as described in U.S. Non-Provisional patent application Ser. No. 15/076,561, entitled “FORK-SAFE MEMORY ALLOCATION FROM MEMORY-MAPPED FILES WITH ANONYMOUS MEMORY BEHAVIOR”, which published as U.S. Patent Application Publication 2016/0283127 A1, and which is hereby incorporated by reference. 
     While some capabilities of the memory allocation interface and/or any other data interface(s) may have been described separately in order to simplify the explanations, it should be understood that any one or more capabilities may be present in an example embodiment. For example an embodiment may include any one or more of: allocation at the client device  130 , coordinated allocation, fork-safe allocation, allocation of local-primary memory, and/or allocation of external primary memory in response to a memory allocation request. Alternatively and/or in addition, the example embodiment may include conversion of anonymous memory to file-backed memory, conversion of file-backed memory to anonymous memory, handling of page faults, reclaiming of local primary memory, relocating one or more portions of external primary memory, and/or any other capability described herein. 
     In a sixth example, the data interface may include a memory swapping interface. The memory swapping interface may include an API. The memory swapping interface may enable the application logic  314  to remove infrequently accessed data from primary memory. In one example implementation, the application logic  314  may be an operating system, or a portion of an operating system, such as a kernel and/or a memory management subsystem. For example, the memory swapping interface may be available to, or part of, the swap implementation. In another example, the memory swapping interface may be an interface through which the swap implementation is invoked. The memory swapping interface may include a block-level interface. The memory swapping interface may include a swap device. Alternatively, or in addition, the memory swapping interface may include a character-level interface. For example, the memory swapping interface may emulate a swap file using a character device interface and/or a block device interface. In another example, the memory swapping interface may emulate a swap file using a filesystem interface. The filesystem interface may include an interface which may allow the application logic and/or any other logic to perform read and/or write operations without storing the read/written data in a page cache. Other examples may include any other suitable interface for storing and/or retrieving portions of memory (such as the memory  310  of the client  130 ) to be swapped into/out-of memory. The memory swapping interface may be made available to all applications in the client  130 , such as the application logic  314 , or to a subset of applications. The memory swapping interface may include a transcendental memory interface. For example, the memory swapping interface may include a front-swap interface. The front-swap interface may include one or more interfaces to initialize the front-swap interface, to store one or more pages, to get one or more pages, to invalidate one or more pages, and/or to invalidate multiple pages. 
     An interface to initialize the front-swap interface may initialize metadata. The metadata may include offsets within the external memory allocations and/or the regions for where to find the data from zero or more pages. The metadata may include one or more data structures to facilitate fast lookup of the offsets. For example, the metadata may include a hash table, a red-black tree, a radix tree, and/or any other data structure known now or later discovered. The one or more data structures may be indexed and/or may include an index based upon an identifier for the zero or more pages. The metadata may be included in the memory of the client. Alternatively, or in addition, the metadata may be included in the external memory allocations, regions referenced by the external memory allocations, in the region metadata  215 , and/or in the external allocation metadata  318 . 
     An interface to store one or more pages may perform client-side memory access to write data from the page(s) to corresponding portion(s) of one or more external memory allocations and/or one or more regions referenced by the one or more external memory allocations. Alternatively, or in addition, the interface to store one or more pages may update metadata indicating the presence and/or offset of the data from the page(s) in the external memory allocations and/or the regions. The interface to store one or more pages may perform client-side memory access to read and/or write the metadata from/to one or more locations within the external memory allocations and/or regions referenced by the external memory allocations. 
     A interface to get one or more pages may perform client-side memory access to read data into the page(s) from corresponding portion(s) of the external memory allocations and/or the regions referenced by the external memory allocations. The interface to get one or more pages may utilize the metadata and/or the one or more data structures to determine the offset for where to find the data from the page(s). The interface to get one or more pages may perform client-side memory access to read and/or write the metadata from/to one or more locations within the external memory allocations and/or regions referenced by the external memory allocations. 
     A interface to invalidate one or more pages may update metadata indicating the non-presence of the data from the page(s) in the external memory allocations and/or the regions. Updating the metadata may include updating the one or more data structures. The interface to invalidate one or more pages may perform client-side memory access to read and/or write the metadata from/to one or more locations within the external memory allocations and/or regions referenced by the external memory allocations. Alternatively, or in addition, the interface to invalidate one or more pages may perform client-side memory access to overwrite data from the page(s) to one or more corresponding portion(s) of the external memory allocations and/or the regions referenced by the external memory allocations. 
     An interface to invalidate multiple pages may update metadata indicating the non-presence of the data from the multiple pages in the external memory allocation and/or the regions. The multiple pages may be all pages associated with a specified swap area, swap device, swap partition, and/or swap file. Updating the metadata may include updating the one or more data structures. For example, updating the metadata may include emptying and/or removing one or more data structures. The interface to invalidate a page may perform client-side memory access to read and/or write the metadata from/to a location within the external memory allocations and/or regions referenced by the external memory allocations. Alternatively, or in addition, the interface to invalidate multiple pages may perform client-side memory access to overwrite data from the multiple pages to one or more corresponding portions of the external memory allocations and/or the regions referenced by the external memory allocations. 
     In a seventh example, the data interface may include a memory caching interface. The memory caching interface may include an API. The memory caching interface may enable the application logic  314  to store data from secondary memory in external memory. For example, the memory caching interface may store data from secondary memory in regions, in an external memory allocation, and/or in the regions referenced by the external memory allocation. In one example implementation, the application logic  314  may be an operating system, or a portion of an operating system, such as a kernel and/or a page cache subsystem. Data from secondary memory may include data from a block-level interface, from a block device interface, from a file system, and/or from any other form of secondary memory. In one example, data from secondary memory may include pages of data from a file system. The memory caching interface may be made available to all applications in the client  130 , such as the application logic  314 . The memory caching interface may include a page-caching interface. Alternatively, or in addition, the memory may include a transcendental memory interface. For example, the memory caching interface may include a clean-cache interface. The clean-cache interface may include one or more interfaces to initialize a file system cache, to put one or more pages, to get one or more pages, to invalidate one or more pages, and/or to invalidate multiple pages. 
     An interface to initialize a file system cache may initialize metadata. The metadata may include offsets within the external memory allocations and/or the regions for where to find the data from zero or more pages. The metadata may include one or more data structures to facilitate fast lookup of the offsets. For example, the metadata may include a hash table, a red-black tree, a radix tree, and/or any other data structure known now or later discovered. The one or more data structures may be indexed and/or may include an index based upon an identifier for the zero or more pages, an identifier for the file system, an identifier for a file system object, any other identifier relevant to the data being stored in external memory, and/or a combination of multiple identifiers, such as a concatenation and/or hash of identifiers. The file system object may be an inode, a file, a directory, and/or any other representation of data in a file system. The metadata may be included in the memory of the client  130 . Alternatively, or in addition, the metadata may be included in the external memory allocations, regions referenced by the external memory allocations, in the region metadata, and/or in the external allocation metadata. Alternatively, or in addition, the interface to initialize a file system cache may initialize a file system cache for a shared and/or clustered file system. Alternatively, or in addition, the memory caching interface may include a separate interface to initialize a file system cache for a shared and/or clustered file system. 
     An interface to put one or more pages may perform client-side memory access to write data from the page(s) to corresponding portion(s) of one or more external memory allocations and/or one or more regions referenced by the one or more external memory allocations. Alternatively, or in addition, the interface to put one or more pages may update metadata indicating the presence and/or offset of the data from the page(s) in the external memory allocations and/or the regions. The interface to put one or more pages may perform client-side memory access to read and/or write the metadata from/to one or more locations within the external memory allocations and/or regions referenced by the external memory allocations. 
     An interface to get one or more pages may perform client-side memory access to read data into the page(s) from corresponding portion(s) of the external memory allocations and/or the regions referenced by the external memory allocations. The interface to get one or more pages may utilize the metadata and/or the one or more data structures to determine the offset for where to find the data from the page(s). The interface to get one or more pages may perform client-side memory access to read and/or write the metadata from/to one or more locations within the external memory allocations and/or regions referenced by the external memory allocations. 
     An interface to invalidate one or more pages may update metadata indicating the non-presence of the data from the page(s) in the external memory allocations and/or the regions. Updating the metadata may include updating the one or more data structures. The interface to invalidate one or more pages may perform client-side memory access to read and/or write the metadata from/to one or more locations within the external memory allocations and/or regions referenced by the external memory allocations. Alternatively, or in addition, the interface to invalidate one or more pages may perform client-side memory access to overwrite data from the page(s) to corresponding portion(s) of the external memory allocations and/or the regions referenced by the external memory allocations. 
     An interface to invalidate multiple pages may update metadata indicating the non-presence of the data from the multiple pages in the external memory allocation and/or the regions. The multiple pages may be all pages associated with a specified block device interface, file system, and/or file system object. Updating the metadata may include updating the one or more data structures. For example, updating the metadata may include emptying and/or removing one or more data structures. The interface to invalidate multiple pages may perform client-side memory access to read and/or write the metadata from/to a location within the external memory allocations and/or regions referenced by the external memory allocations. Alternatively, or in addition, the interface to invalidate multiple pages may perform client-side memory access to overwrite data from the multiple pages to one or more corresponding portions of the external memory allocations and/or the regions referenced by the external memory allocations. 
     In an eighth example, the data interface may include a hardware-accessible interface. The hardware-accessible interface may be a physically-addressable interface. A physically-addressable interface may be an interface which provides access to the underlying data using physical addresses, such as the physical addresses used on an address bus, a CPU interconnect, a memory interconnect, and/or on a peripheral interconnect. Alternatively or in addition, the hardware-accessible interface may be a virtually-addressable interface. For example, the hardware-accessible interface may be addressed using IO-virtual addresses. IO-virtual addresses may be translated to physical addresses by one or more address translation logics. Examples of address translation logics include memory management units (MMUs), input-output memory management units (IO-MMUs), translation lookaside buffers, and/or any other logic capable of translating virtual addresses to physical addresses known now or later discovered. 
     The hardware-accessible interface may enable a hardware application component to access data of a region. Alternatively or in addition, the hardware-accessible interface may enable the hardware application component to access data of one or more of the regions referenced by an external memory allocation. Alternatively or in addition, the hardware-accessible interface may enable the hardware application component to access data of the external memory allocation. The hardware application component may be a processor, a GPU, a communication interface, a direct memory access controller, an FPGA, an ASIC, a chipset, a compute module, a hardware accelerator module, a hardware logic, and/or any other physical component that accesses memory. The hardware application component may be included in the application logic  314 . The hardware-accessible interface may include a hardware client component. A hardware client component may be and/or may include a processor, a GPU, an MMU, an IO-MMU, a communication interface, such as the one or more communication interfaces  330 , a direct memory access controller, an FPGA, an ASIC, a chipset, a compute module, a hardware accelerator module, a hardware logic, a memory access transaction translation logic, any other hardware component, and/or a combination of multiple hardware components. The hardware client component may be included in the client logic  312 . The hardware client component, the hardware application component, and/or the one or more communication interfaces may be embedded in one or more chipsets. The hardware client component may include a memory and/or cache. The memory and/or cache of the hardware client component may be used to hold portions of the data of external memory allocations and/or regions. Alternatively, or in addition, the hardware client component may utilize a portion of the memory of the client to hold portions of the data of external memory allocations and/or regions. 
     The hardware client component may respond to and/or translate attempts to access virtual addresses, physical addresses, logical addresses, IO addresses, and/or any other address used to identify the location of data. Alternatively, or in addition, the hardware client component may participate in a cache coherency protocol with the hardware application component. In a first example, the hardware client component may respond to attempts of the hardware application component to access physical addresses by accessing data included in the memory and/or cache of the hardware client component. In a second example, the hardware component may interface with a CPU interconnect and handle cache fill requests by reading data from the memory and/or cache included in the hardware client component. In a third example, the hardware client component may redirect and/or forward attempts of the hardware application component to access physical addresses to alternate physical addresses, such as the physical addresses of the portion of the memory  310  of the client  130  utilized by the hardware component. In a fourth example, the hardware client component may translate attempts of the hardware application component to access physical addresses into client-side memory access. For example, the hardware client component may interface with the CPU interconnect and handle cache fill requests by performing client-side memory access to read the requested data from the external memory allocation. Alternatively, or in addition, the hardware client component may handle cache flush requests by performing client-side memory access to write the requested data to the external memory allocation. Alternatively, or in addition, the hardware client component may handle cache invalidate requests by updating the memory and/or cache of the hardware client component to indicate the non-presence of the data indicated by the cache invalidate requests. In a fifth example, the hardware client component may translate attempts of the hardware application component to access IO addresses into client-side memory access. For example, the hardware client component may interface with a peripheral interconnect, such as PCI Express, and may respond to requests to read a portion of the IO address space by reading data from the memory included in the hardware client component, by reading the portion of the memory and/or cache of the client utilized by the hardware component, and/or by performing client-side memory access to read the requested data from the external memory allocation. In another example, the hardware client component may interface with a memory-fabric interconnect, such as Gen-Z, and may respond to memory read operations by reading data from the memory included in the hardware client component, by reading the portion of the memory  310  and/or cache of the client  130  utilized by the hardware component, and/or by performing client-side memory access to read the requested data from the external memory allocation. 
     In a ninth example, the data interface may include an interface to enable peripheral devices of the client  130  to access the external memory allocations. For example, the data interface may include a Graphics Processing Unit (GPU) accessible interface. The GPU accessible interface may enable a GPU application to access data of a region. Alternatively or in addition, the GPU accessible interface may enable the GPU application to access data of one or more of the regions referenced by an external memory allocation. Alternatively or in addition, the GPU accessible interface may enable the GPU application to access data of the external memory allocation. The GPU application may be an application logic, such as the application logic, executable on one or more processors of a GPU. The GPU may be included in the client. The GPU may include a client-side memory access interface that may enable the GPU application and/or the GPU accessible interface to perform client-side memory access using the one or more communication interfaces included in the client. The client-side memory access interface may be a GPUDirect, which is a trademark of NVIDIA Corporation, RDMA interface. The GPU accessible interface may include any one or more data interfaces. The GPU accessible interface may provide the one or more data interfaces to the GPU application. Examples of data interfaces included in the GPU-accessible interface and/or provided to the GPU application may be: an API, a block-level interface, a character-level interface, a memory-mapped interface, a memory allocation interface, a memory swapping interface, a memory caching interface, a hardware-accessible interface, any other interface used to access the data of the external memory allocations and/or of the regions, and/or a combination of data interfaces. 
     The client logic  312 , the application logic  314 , and/or another logic may be notified prior to, during, and/or after the size of the external memory allocation and/or the region  214  is changed (such as by the request to resize the region  214  and/or the request to resize an external memory allocation). Alternatively or in addition, one or more data interfaces may be updated. In one example, the size of a block device exposed by the block-level interface may be reduced before reducing the size of a corresponding external memory allocation and/or region  214 . In another example, the size of a file exposed by the character-level interface may be increased after increasing the size of the corresponding external memory allocation and/or region  214 . In another example, the capacity of a memory-allocation interface may be grown after increasing the size of the corresponding external memory allocation and/or region  214 . The memory-allocation data structures and/or other data structures may be updated to correspond to the new capacity. 
     In examples where reducing the size of the corresponding external memory allocation and/or region  214  would cause in-use portions of the external memory allocation and/or region  214  to be discarded, the client logic  312 , the application logic  314 , and/or another logic may relocate the data of the in-use portions upon being notified. For example, with a memory-allocation interface, the client logic  312  may allocate one or more different portions of the external memory allocation and/or region  214 , may copy the data of the in-use portions to the different portions, and/or may map the different portions to the virtual address space in place of the in-use portions. The client logic  312  may prevent access to in-use portions while copying and/or mapping to preserve data integrity. For example, the client logic  312  may mark the corresponding page table entries as read-only prior to copying and/or mapping. Alternatively or in addition, the client logic  312  may hold one or more lock primitives while allocating, copying, and/or mapping, such as lock primitives protecting changes to page table entries and/or mapping data structures of an operating system. Alternatively or in addition, the client logic  312  may not prevent access to the in-use portions and/or may iteratively copy the in-use portions until all portions have been copied without being modified. 
     Alternatively or in addition, the client logic  312 , the application logic  314 , and/or another logic may be notified prior to, during, and/or after discarding data at a location other than the end of the region  214 , such as when configuring the communication interface  230  to treat the discarded data as not-present. Alternatively or in addition, one or more data interfaces may be updated. In one example, corresponding portions of a cache (such as a page cache) may be discarded. In another example, where discarding the data would cause in-use portions of the external memory allocation and/or region  214  to be discarded, the client logic  312 , the application logic  314 , and/or another logic may relocate the data of the in-use portions upon being notified, such as described in this disclosure. Alternatively or in addition, the client logic  312  may cause the discarded portions to become not allocable with the memory-allocation interface. For example, the client logic  312  may allocate the portions being discarded, such as with a balloon allocator, prior to the data being discarded. Alternatively or in addition, the client logic  312  may update the memory-allocation data structures and/or other data structures to indicate that the portions are not present and/or not allocable. 
     Ownership of, management of, administration of, control of, and/or access to all or part of the external memory allocation and/or the region may be transferred from one client to another. Ownership, management, administration, control, and/or access may be represented by an association in the metadata of the external memory allocation and/or the region with the client, an account on the client, and/or a virtual machine on the client. Ownership, management, administration control, and/or access may be the ability of one or more entities (such as one or more clients  130  and/or logics) to read from, write to, and/or manipulate the external memory allocation and/or the region. A capability to transfer ownership, management, administration, control, and/or access from one client to another may facilitate moving the application logic from one client to another client more efficiently and/or more rapidly. For example, the application logic may include a virtual machine that is to be migrated from a hypervisor running on a first client to a hypervisor running on a second client. When migrating data of the virtual machine, the hypervisors or other component may elect not to migrate data that is stored in the external memory allocation and/or the region. The hypervisors or other component may instead transfer ownership of, management of, administration of, control of, and/or access to all or part of the external memory allocation and/or the region from the first client to the second client. For example, the allocation logic  412  may update the metadata to transfer the ownership, management, administration, control, and/or access. By updating the metadata to transfer ownership, management, administration, control, and/or access, the corresponding data stored in the external memory allocation and/or the region may be effectively migrated from the hypervisor of the first machine to the hypervisor of the second machine without copying the data. In examples where access to the data of the external memory allocation and/or the region  214  is controlled by an encryption key and/or another identifier, the encryption key and/or other identifier may be provided to the second machine. Alternatively, or in addition, if data translation (such as encryption and/or decryption) is being performed for the external memory allocation and/or for the region  214 , the encryption key(s), data translation method, and/or other parameters for data translation may be provided to the second machine. Alternatively, or in addition, ownership of, management of, administration of, control of, and/or access to all or part of the external memory allocation and/or the region may be associated with the virtual machine that is being migrated, and the ownership, management, administration, control, and/or access may be transferred implicitly as the virtual machine is migrated. Alternatively, or in addition, prior to migrating the virtual machine, the hypervisor and/or the virtual machine may elect to discard cached copies of data that are otherwise stored in the external memory allocation and/or the region, which may reduce the total amount of data to be migrated with the virtual machine. Ownership of, management of, administration of, control of, and/or access to all or part of the external memory allocation and/or the region may be transferred from one client to another by sending, for example, a request to modify settings for the region to the region access logic of each memory appliance which includes the regions for which ownership, management, administration, control, and/or access is being transferred. 
     One or more of the hypervisor, the virtual machine, and/or any other component (such as a container, a jail, a zone, the client logic, the application logic, the allocation logic, the region access logic, and/or any other logic) may elect to allocate additional external memory for use by the virtual machine, container, jail, and/or zone using the methods described in this document. For example, the hypervisor or another component may allocate an additional external memory allocation and/or an additional region and assign the additional external memory to the virtual machine, container, jail, and/or zone. Alternatively, or in addition, the virtual machine, container, jail, zone, and/or other component may allocate an additional external memory allocation and/or an additional region for use by the virtual machine, container, jail, and/or zone. Alternatively or in addition, one or more of the hypervisor, the virtual machine, and/or another component may resize an existing external memory allocation and/or region. Allocating additional external memory for use by the virtual machine, container, jail, and/or zone may be done in place of or in addition to allocating additional local memory for use by the virtual machine, container, jail, and/or zone. For example, if not enough local memory is available to satisfy the demand of an application running within the virtual machine, container, jail, and/or zone, additional external memory may be allocated for use by the virtual machine, container, jail, and/or zone in order to satisfy all or part of the demand. Using external memory may avoid a need to otherwise migrate the virtual machine, container, jail, and/or zone to a different client to satisfy the virtual machine&#39;s, container&#39;s, jail&#39;s, and/or zone&#39;s demand for memory in cases where not enough local memory is available to allocate for use by the virtual machine, container, jail, and/or zone. 
       FIG.  16    illustrates an example flowchart for migrating an application logic from a first client to a second client. One or more application logics  314  may allocate external memory, as described herein, while operating on a first client and subsequently be migrated to a second client. 
     In one example, the one or more application logics  314  being migrated may include a container, a jail, and/or a zone and/or may be included in a container, a jail, and/or a zone. In another example, the one or more application logics  314  may include a virtual machine and/or may be included in a virtual machine. In another example, the one or more application logics  314  may include applications executing with an operating system. 
     The client logic  312  of the first client  130  (a first client logic  312   a ) may receive a request to migrate one or more application logics  314  ( 1602 ). In response to the request, the first client logic  312   a  may flush cached data ( 1604 ) for portions allocated by the application logic(s)  314  to the region  214  and/or to the external memory allocation X 1 , X 2 , X 3  ( 1604 ). Flushing cached data ( 1604 ) may include flushing dirty data from the memory  310  of the client  130 . Alternatively or in addition, flushing cached data ( 1604 ) may include discarding cached copies of data that are otherwise stored in the external memory allocation X 1 , X 2 , X 3  and/or the region  214 . Alternatively or in addition, discarding cached copies of data may be performed later, such as when migrating the application logic(s) ( 1612 ). Alternatively or in addition, if the memory of the application logic(s)  314  is backed by a file and/or secondary storage, the first client logic  312   a  may flush the cached data to the file and/or secondary storage ( 1604 ). The first client logic  312   a  may track which portions of the memory have been written after flushing ( 1606 ). 
     Flushing the cached data ( 1604 ) may be performed one or more times. For example, the first client logic  312   a  may iteratively flush the cached data ( 1604 ) and/or may track ( 1606 ) which portions have been re-dirtied while flushing. The first client logic  312   a  may iterate until some end point is reached ( 1608 ), such as: if none of the data is dirty after an iteration, if more data is dirty after performing an iteration than before starting the iteration, if a maximum number of iterations has been performed, etc. After the end point is reached, the application logic(s)  314  may be paused ( 1610 ) and/or migrated ( 1612 ) to the second client. 
     The application logic(s)  314  may be migrated ( 1612 ) using currently known or later discovered techniques for migrating application logic(s)  314  between physical machines. For example, the application logic(s)  314  may be migrated by shutting down and restarting at the second client, by using Checkpoint-Restore in Userspace (CRIU), by using LXD Live Migration, etc. In examples where the memory-allocation data structures include a reference to the first client and/or to a process identifier within the first client, migrating the application logic(s)  314  ( 1612 ) may include updating the memory-allocation data structures and/or other data structures to reference the second client and/or a process identifier within the second client. Alternatively, the memory-allocation data structures and/or other data structures may not be updated, such as if the process identifier will be the same after migrating. 
     Migrating the application logic(s)  314  ( 1612 ) may include copying the memory of the application logic(s)  314  from the first client to the second client. Copying the memory may be performed between when the application logic(s)  314  are paused on the first client ( 1610 ) and when resumed on the second client ( 1616 ), such as when migrating the application logic(s) ( 1612 ). Alternatively or in addition, copying the memory may be performed prior to pausing the application logic(s)  314  ( 1610 ), such as with a pre-copy. Copying the memory prior to pausing the application logic(s)  314  ( 1610 ) may be performed one or more times. For example, the first client logic  312   a  may iteratively copy the memory and/or may track which portions have been written to while copying. The pre-copy may be repeated until some end point is reached, such as: if none of the data has been written during a pre-copy iteration, if more data remains to be copied after performing a pre-copy iteration than before starting the iteration, if a maximum number of iterations has been performed, etc. Alternatively or in addition, copying the memory may be performed after the application logic(s)  314  are resumed on the second client ( 1616 ), such as with a post-copy. With a post-copy, portions of the memory may be copied on-demand, such as during a page fault. Alternatively or in addition, ownership, management, administration, control, and/or access to the memory of the application logic  314  may be transferred from the first client to the second client, such as described elsewhere in this disclosure. Some or all of the memory of the application logic(s)  314  may not be copied, such as if the memory of the application logic(s)  314  is stored in external memory and/or is accessible from the second client. For example, the client logic  312  may discard cached copies of data that are otherwise stored in the external memory allocation and/or the region. Discarding cached copies of data may reduce the total amount of data to be migrated with the application logic(s)  314 . 
     Information about external memory allocated by the application logic(s)  314  may be accessed by the client logic  312  of the second client (a second client logic  312   b ) from the memory-allocation data structures and/or may be used to map the corresponding portions of the region  214  and/or of the external memory allocation X 1 , X 2 ,X 3  to the virtual address space of the application logic(s)  314  at the second client ( 1614 ). Upon mapping the portions to the application logic(s)  314  at the second client, the application logic(s)  314  may be resumed ( 1616 ). 
       FIG.  17    illustrates a memory architecture diagram of an example system providing multiple tiers of memory. The system may provide one or more tiers of memory. A tier may be a collection of memory with a common set of attributes, such as price, capacity, latency, bandwidth, operations per second, physical locality, network locality, logical locality, and/or any other attributes of the memory and/or of the device containing the memory. The attributes of a tier involving memory of a memory appliance may include any of the characteristics and/or configurations of the memory appliance  110 . 
     The attributes of one tier may differ from those of another tier. In one example, price and performance may decrease for lower tiers while capacity increases. This may enable the system to naturally demote data from higher levels to lower levels as other data proves to be used more often and/or more recently. 
     In at least one example, the highest-level tiers may be provided by the hardware of the client  130 . For example, level  1  may be provided by the L 1  cache of the processor of the client  130 , level  2  may be provided by the L 2  cache of the processor of the client  130 , level  3  may be provided by the L 3  cache of the processor of the client  130 , level  4  may be provided by the memory  310  of the client  130 , and/or another level may be provided by the backing store  360  of the client  130 . 
     In at least one example, one or more tiers may be provided by one or more memory appliances. For example: level  5  may be provided by one or more memory appliances with very low latency and/or high bandwidth; level  6  may be provided by one or more memory appliances with higher latency, lower bandwidth, and/or higher capacity; level  7  may be provided by the backing store of one or more memory appliances and/or of the client. 
     A logic, such as the client logic  312  and/or the region access logic  212  may cause data for one or more portions of the region  214  to be migrated to lower-numbered tiers by causing the data of the portions to be faulted-in at the desired level. In one example, the client logic  312  may attempt to read the data, causing the data to be loaded into the memory of the client  312  and/or into one or more levels of processor cache of the client. Alternatively, or in addition, the client logic  312  may pre-fetch the data, such as by issuing a pre-fetch request with an operating system of the client. The pre-fetch request may be a memory advisory request, indicating that the client logic will need the data. In another example, the client logic  312  may send a pre-fetch request to the region access logic  212 . Such as described in  FIG.  10 A  of “PAGING OF EXTERNAL MEMORY”, which published as US Patent App Publication 2016/0077761, and which is hereby incorporated by reference, the pre-fetch request may cause the data to be loaded into the memory  210  of the memory appliance  110 . In another example, the client logic  312  may send a pin request to the region access logic  212 . Such as described for  FIG.  10 B  of “PAGING OF EXTERNAL MEMORY”, the pin request may cause the data to be loaded into the memory  210  of the memory appliance  110 . 
     Alternatively, or in addition, a logic, such as the client logic  312  and/or the region access logic  212  may cause the data for one or more portions of the region  214  to be migrated away from lower-numbered tiers by unpinning the corresponding portions of memory and/or by causing the portions to be invalidated and/or reclaimed at the desired level. Causing the portions to be invalidated and/or reclaimed at the desired level may be as described elsewhere in this document. Alternatively, or in addition, the client logic  312  may send an unpin request to the region access logic  212 . Such as described for  FIG.  10 C  of “PAGING OF EXTERNAL MEMORY”, the unpin request may cause the data to be unpinned from the memory of the memory appliance  110 . Alternatively, or in addition, the client logic  312  may send a reclaim request to the region access logic. Such as described for  FIG.  10 D  of “PAGING OF EXTERNAL MEMORY”, the reclaim request may cause the portions to be invalidated and/or reclaimed from the memory of the memory appliance  110 . 
     Alternatively, or in addition, the operating system of the client and/or of the memory appliance  110  may cause the data for one or more portions of the region  214  to be migrated away from lower-numbered tiers by causing the portions to be invalidated and/or reclaimed. 
     The client  130 , the memory appliance  110 , and the management server  120  may be configured in any number of ways. In one example, the memory appliance  110  may be included in a computer. For example, the processor may be the CPU of the computer, the memory may be the memory of the computer, and the computer may include the communication interface  330 . Alternatively or in addition, the memory appliance  110  may be a peripheral of a computer, including but not limited to a PCI device, a PCI-X device, a PCIe device, an HTX (HyperTransport eXpansion) device, or any other type of peripheral, internally or externally connected to a computer. 
     In a second example, the memory appliance  110  may be added to a computer or another type of computing device that accesses data in the memory appliance  110 . For example, the memory appliance  110  may be a device installed in a computer, where the client  130  is a process executed by a CPU of the computer. The memory in the memory appliance  110  may be different than the memory accessed by the CPU of the computer. The processor in the memory appliance  110  may be different than the CPU of the computer. 
     In a third example, the memory appliance  110 , the client  130 , and/or the management server  120 , may be implemented using a Non-Uniform Memory Architecture (NUMA). In NUMA, the processor may comprise multiple processor cores connected together via a switched fabric of point-to-point links. The memory controller may include multiple memory controllers. Each one of the memory controllers may be electrically coupled to a corresponding one or more of the processor cores. Alternatively, multiple memory controllers may be electrically coupled to each of the processor cores. Each one of the multiple memory controllers may service a different portion of the memory than the other memory controllers. 
     In a fourth example, the processor of the memory appliance  110 , the client  130 , and/or the management server  120  may include multiple processors that are electrically coupled to the interconnect, such as with a bus. Other components of the memory appliance  110 , the client  130 , and/or the management server  1202 , such as multiple memories included in the memory, the communication interface, the memory controller, and the storage controller may also be electrically coupled to the interconnect. 
     In a fifth example, the external memory system may include multiple memory appliances, multiple regions, multiple region metadatas, multiple management servers, multiple external memory allocation metadatas, multiple allocation logics, multiple client logics, and/or multiple application logics. 
     In a sixth example, the client  130  may provide additional services to other systems and/or devices. For example, the client  130  may include a Network Attached Storage (NAS) appliance. Alternatively or in addition, the client  130  may include a Redundant Array of Independent Disks (RAID) head. Alternatively or in addition, the client  130  may provide file-level access to data stored in the memory appliance  110 . Alternatively, or in addition, the client  130  may include a database, such as an in-memory database. 
     In a seventh example, multiple clients may utilize one or more memory appliances as shared memory. For example, the clients may include or interoperate with an application logic that relies on massive parallelization and/or sharing of large data sets. Examples of application logic that may use massive parallelization include logic that performs protein folding, genetic algorithms, seismic analysis, or any other computationally intensive algorithm and/or iterative calculations where each result is based on a prior result. The application logic  314  may store application data, application state, and/or checkpoint data in the regions of the one or more memory appliances and/or in an external memory allocation. The additional capabilities of the one or more memory appliances, such as low latency access and persistence to the backing store, may be exploited by the clients in order to protect against application crashes, a loss of power to the clients, or any other erroneous or unexpected event on any of clients. The clients may access the one or more memory appliances in a way that provides for atomic access. For example, the client-side memory access operations requested by the clients may include atomic operations, including but not limited to a fetch and add operation, a compare and swap operation, or any other atomic operation now known or later discovered. An atomic operation may be a combination of operations that execute as a group or that do not execute at all. The result of performing the combination of operations may be as if no operations other than the combination of operations executed between the first and last operations of the combination of operations. Thus, the clients may safely access the one or more memory appliances without causing data corruption. 
     The application logic  314 , the client logic  312 , the allocation logic  412 , the observer logic  218 , and/or the region access logic  212  may be co-located, separated, or combined. The actions performed by combined logic may perform the same or similar feature as the aggregate of the features performed by the logics that are combined. In a first example, all five logics may be co-located in a single device. In a second example, the region access logic  212  and the observer logic  218  may be combined into a single logic. In a third example, the client logic  312  and the observer logic  218  may be combined into a single logic. In a fourth example, the client logic  312  and the region access logic  212  may be combined. In a fifth example, the observer logic  218  may be in a device different from the memory appliance  110 , such as the management server  120  and/or a metadata server. A metadata server may be one or more hardware and/or software entities that may participate in the processing of operations, but may not directly handle the data stored in the memory appliance  110 . The metadata server may track statistics, coordinate persistence, coordinate data duplication, and/or perform any other activity related to the memory access operations. In a sixth example, the region access logic  212  and the allocation logic  412  may be combined into a single logic. In a seventh example, the client logic  312  and the allocation logic  412  may be combined into a single logic. In an eight example, the client logic  312  and the application logic  314  may be combined into a single logic. Other combinations of the various components are possible, just a few of which are described here. 
     The application logic  314 , the client logic  312 , the allocation logic  412 , the observer logic  218 , and/or the region access logic  212  may include computer code. The computer code may include instructions executable with the processor. The computer code may be written in any computer language now known or later discovered, such as C, C++, C#, Java, or any combination thereof. In one example, the computer code may be firmware. Alternatively or in addition, all or a portion of the application logic  314 , the client logic  312 , the allocation logic  412 , the observer logic  218 , the region access logic  212  and/or the processor may be implemented as a circuit. For example, the circuit may include an FPGA (Field Programmable Gate Array) configured to perform the features of the application logic  314 , the client logic  312 , the allocation logic  412 , the observer logic  218 , and/or the region access logic  212 . Alternatively, or in addition, the circuit may include an ASIC (Application Specific Integrated Circuit) configured to perform the features of the application logic  314 , the client logic  312 , the allocation logic  412 , the observer logic  218 , and/or the region access logic  212 . The circuit may be embedded in a chipset, a processor, and/or any other hardware device. 
     Alternatively, or in addition, a portion of the application logic  312 , the client logic  312 , the allocation logic  412 , the observer logic  218 , and/or the region access logic  212  and the processor may be implemented as part of the one or more communication interfaces or other hardware component. For example, the one or more communication interfaces or other hardware component may modify a portion of the memory when a write operation is performed. The observer logic  218  may periodically check the portion of memory and may take further action based on the contents of the portion and the region associated with the portion. The further action may include determining statistics related to the operations that are being and/or were performed, identifying portions that are being and/or have been written to and/or read from, persisting the contents of the portions to the backing store  260 , duplicating the contents of the portions to a different region, a different memory appliance, an external server, and/or a backup device, and/or taking any other action related to the operations. 
     A method may be provided for allocating memory in response to a memory allocation request, the method comprising: selecting whether to allocate local primary memory or external primary memory based on a determination of available memory, wherein a portion of local primary memory is used to hold a cached copy of data in the external primary memory. 
     Alternatively or in addition, method may be provided for allocating memory in response to a memory allocation request, the method comprising: selecting whether to allocate anonymous memory or file-backed memory, based on a determination of available local primary memory or available file space, wherein a portion of local primary memory is used to hold a cached copy of data of the file. 
     In some examples, an allocated portion is converted from anonymous memory to file-backed memory by copying the data to a new portion of local primary memory associated with the file. Alternatively or in addition, an allocated portion is converted from anonymous memory to file-backed memory without copying data of the allocated portion by associating the allocated portion with the file. 
     Alternatively or in addition, the data is read from swap backing store before associating the allocated portion with the file. Alternatively or in addition, an allocated portion is converted from file-backed memory to anonymous memory by copying the data to a second portion of local primary memory not associated with the file. Alternatively or in addition, an allocated portion is converted from file-backed memory to anonymous memory without copying data of the allocated portion by dis-associating the allocated portion from the file and updating one or more page table entries to reference the allocated portion. Alternatively or in addition, the determination is controllable by setting a configurable parameter indicating how much local primary memory or external primary memory to use. Alternatively or in addition, the determination is controllable by a usage indicator, which may be set as a configurable parameter indicating how memory will be used. (For example, class of usage such as: application, streaming, random access, etc.) 
     A method may be provided comprising: allocating a region of external primary memory in a memory appliance; allocating a portion of local primary memory; mapping an address within an address space to the portion of local primary memory, allocating, at a client device, a portion of external primary memory from the region of external primary memory; and reclaiming the portion of local primary memory by copying data from the portion of local primary memory to the portion of external primary memory and associating the address within the address space with the portion of external primary memory, wherein the data in the portion of the external primary memory is accessible via client-side memory access in which a communication interface of the memory appliance is configured to access the portion of external primary memory on the memory appliance. In some examples, the portion of external primary memory is not in an address space addressable by a processor of the client device. 
     In some examples, allocating the portion of external primary memory is included in reclaiming the portion of local primary memory. Alternatively or in addition, the region of external primary memory or the memory appliance is associated with the address space or with an application logic associated with the address space. Alternatively or in addition, the application logic is a virtualization instance. Alternatively or in addition, the application logic is a virtual machine, a container, a jail, or a zone. Alternatively or in addition, the region of external primary memory is provisioned to an entity associated with the address space. Alternatively or in addition, the portion of local primary memory is a portion of anonymous memory. Alternatively or in addition, the portion of local primary memory is a portion of a memory-mapped file. Alternatively or in addition, the region of external primary memory or the memory appliance is associated with the memory-mapped file. Alternatively or in addition, associating the address within the address space with the portion of external primary memory includes updating a page table data structure or a data structure that tracks one or more portions of memory that correspond to one or more portions of a file. 
     Alternatively or in addition, associating the address within the address space with the portion of external primary memory includes storing an identifier for the portion of external primary memory in a data structure. In some examples, the identifier indicates one or more regions of external primary memory or one or more memory appliances that contain the portion of external primary memory. Alternatively or in addition, the data structure is a first data structure, and wherein the identifier is usable with a second data structure to identify one or more regions of external primary memory or one or more memory appliances that contain the portion of external primary memory. Alternatively or in addition, the second data structure is a distributed hash table. Alternatively or in addition, the data is encrypted prior to or while writing to the portion of external primary memory. 
     In some examples, the portion of external primary memory is a first portion of external primary memory, the method further comprising: selecting a second portion of external primary memory; and copying data from the first portion of external primary memory or from the portion of local primary memory to the second portion of external primary memory. In some examples, the first portion of external primary memory and the second portion of external primary memory are included in different memory appliances. 
     In some examples, the method further comprises associating the address within the address space with the second portion of external primary memory. Alternatively or in addition, the client device is a first client device, wherein the portion of local primary memory is a first portion of local primary memory for the first client device, and further comprising: migrating an application logic from the first client device to a second client device; copying the data from the portion of external primary memory to a second portion of local primary memory for the second client device; an accessing the second portion of local primary memory by the application logic while operating with the second client device. 
     In some examples, the application logic is a virtualization instance. Alternatively or in addition, the application logic is a virtual machine, a container, a jail, or a zone. Alternatively or in addition, an offset of the portion of external primary memory and the address within the address space are not in a linear relationship relative to other portions of external primary memory and other addresses within the address space. 
     Alternatively or in addition, the portion of external primary memory is not in an address space addressable by a processor of the client device. Alternatively or in addition, the portion of external primary memory is not addressable by a processor of the client device. Alternatively or in addition, the portion of external primary memory is not addressable by a processor of the client device via a load instruction or a store instruction of the processor&#39;s instruction set. 
     In some examples, the portion of local primary memory is a first portion of local primary memory, and the method further comprises: allocating, in response to a memory access operation, a second portion of local primary memory; copying data from the portion of external primary memory to the second portion of local primary memory; and mapping the address within the address space to the second portion of local primary memory. 
       FIG.  18    illustrates a schematic diagram of an example of the system  100  in which a type of memory to allocate is selected from a group of memory types ( 310 ,  214 ) based on an available memory determination  1802 . In the illustrated example, a planned division of the local primary memory  310  is to provide 3% of local primary memory to an operating system, 90% of the local primary memory to applications (designated “main” in  FIG.  18   ), and 7% to a cache for the region  214  of external primary memory. Of course, the particular divisions of the local primary memory  310  and the corresponding percentages planned may be different in other examples. 
     The ratio of the size of the cache to the size of the region  214  of external primary memory in the illustrated example is 1:2. The ratio may be different in other examples. This means that the size of the region  214  is twice the size of the cache. For example, the client device  130  may include 128 gigabytes of local primary memory  310 . In such an example, 7% of the 128 gigabytes is to be reserved for the cache, so the size of the region  214  of external primary memory would be 18 gigabytes (7%×128 gigabytes×2). 
     With 7% of the 128 gigabytes reserved for the cache, that leaves 93% of the 128 gigabytes to the operating system and applications, which is 119 gigabytes. In this example, the size of the address space in the client  130  is (1) the amount of the local primary memory  310  reserved for the operating system and application, plus (2) the size of the region  214  of external memory (which is 119 gigabytes plus 18 gigabytes, or a total of 137 gigabytes). 
     In response to a memory allocation request, such as an invocation of malloc( ), the system  100  selects a type of memory to allocate from a group of memory types based on an available memory determination  1802 . In this example, the available memory determination  1802  is the amount of unallocated local primary memory  310 . If the available memory  1802  is greater than a margin  1804 , then the selected type of memory is local primary memory. Alternatively, if the available memory  1802  is less than or equal to the margin  1804 , then the selected type of memory is external primary memory. 
     An advantage of the system  100  shown in  FIG.  18    may be that local primary memory will be allocated before external primary memory, resulting in potentially a better performing system. Another advantage may be that even if the client device  130  allocates more memory than is available in local primary memory, swapping may be avoided so long as the total amount of allocated memory remains within (1) the amount of the local primary memory  310  reserved for the operating system and application, plus (2) the size of the region  214  of external memory (which is 119 gigabytes plus 18 gigabytes, or a total of 137 gigabytes). Another potential advantage of the system  100  may be that only the 18 gigabytes of external memory are needed to make 137 gigabytes available at the client device  130 . 
     In some examples, a method is provided comprising allocating a portion of memory by: selecting a type of memory to allocate in a client device from a group of memory types in response to a memory allocation request and/or in response to a request to access a portion of an address space addressable by a processor of the client device, wherein the selection of the type of memory to allocate is based on an available memory determination, wherein the group of memory types includes a local primary memory, wherein the group of memory types further includes an external primary memory and/or a memory-mapped file, wherein the external primary memory is memory that is located in a memory appliance external to the client device but is treated as primary memory at the client device; selecting a portion of the local primary memory, a portion of the external primary memory, and/or a portion of the memory-mapped file for the portion of memory to allocate at the client device depending on the selected type of memory; and mapping at least the selected portion to the address space. 
     The type of memory to allocate may be selected from the group of memory types including the local primary memory and the external primary memory, and wherein data in the external primary memory is accessible via client-side memory access in which a communication interface of the memory appliance is configured to access the data in memory of the memory appliance. 
     Alternatively or in addition, the type of memory to allocate may be selected from the group of memory types including the local primary memory and the memory-mapped file. 
     Alternatively or in addition, the type of memory to allocate may be selected in response to a memory allocation request, which includes an invocation of an operating system programmatic procedure configured to allocate memory for a caller of the operating system programmatic procedure. 
     Alternatively or in addition, the available memory determination may include a determination that an amount of allocated local primary memory is less than a limit and, therefore, the local primary memory is selected as the type of memory to allocate. 
     Alternatively or in addition, the available memory determination may include a determination that an amount of local primary memory used to cache the external primary memory and/or to cache the memory-mapped file is within a limit and, wherein the external primary memory and/or memory-mapped file is selected as the type of memory to allocate. 
     Alternatively or in addition, the available memory determination may include a determination that a ratio of an amount of allocated local primary memory to an amount of local primary memory used to cache the external primary memory and/or to cache the memory-mapped file is within a limit and, therefore, the local primary memory is selected as the type of memory to allocate. 
     Alternatively or in addition, the type of memory to allocate may be selected from the group of memory types including the local primary memory and the memory-mapped file, wherein the available memory determination includes a determination of available local primary memory and/or available file space in the memory-mapped file. 
     Alternatively or in addition, the allocated portion of memory may be converted from the local primary memory to the memory-mapped file by copying data in the selected portion of the local primary memory to a second portion of local primary memory that is associated with the memory-mapped file. 
     Alternatively or in addition, the allocated portion of memory may be converted from the local primary memory to the memory-mapped file without copying data of the selected portion of the local primary memory by associating the selected portion of the local primary memory with the memory-mapped file. In some examples, data may be read into the selected portion of the local primary memory from a swap backing store before associating the selected portion of the local primary memory with the memory-mapped file. 
     Alternatively or in addition, the allocated portion of memory may be converted from the memory-mapped file to the local primary memory by copying data from the portion of the memory-mapped file to a second portion of the local primary memory not associated with the memory-mapped file. 
     Alternatively or in addition, the allocated portion of memory may be converted from the memory-mapped file to the local primary memory without copying data of the allocated portion of memory by dis-associating the allocated portion of memory from the memory-mapped file and updating one or more page table entries to reference the allocated portion of memory. 
     Alternatively or in addition, the available memory determination is controlled by setting a configurable parameter indicating a limit on a usable amount of the local primary memory and/or the external primary memory. 
     In some examples, a client device may be provided comprising: a processor; and a local primary memory in communication with the processor, the local primary memory comprising: an application logic unit; and a client logic unit configured to: receive a memory allocation request from the application logic unit to allocate a portion of memory and/or receive a request to access a portion of an address space addressable by the processor; select a type of memory to allocate at the client device from a group of memory types in response to the memory allocation request and/or in response to the request to access the portion of the address space, wherein the selection of the type of memory to allocate is based on an available memory determination, wherein the group of memory types includes the local primary memory, wherein the group of memory types further includes an external primary memory and/or a memory-mapped file, wherein the external primary memory is memory that is located in a memory appliance external to the client device but is treated as primary memory at the client device; select a portion of the local primary memory, a portion of the external primary memory, and/or a portion of the memory-mapped file for the portion of memory to allocate at the client device depending on the selected type of memory; and map at least the selected portion to the address space. 
     The selection of the type of memory to allocate may be based on the available memory determination and on a configurable parameter indicating a manner in which the allocated portion of memory is to be used. 
     Alternatively or in addition, the client logic unit may be configured to map the at least the selected portion to the address space before the memory allocation request and/or the request to access the portion of the address space is received. 
     Alternatively or in addition, the client logic unit may include a swap implementation of an operating system. 
     Alternatively or in addition, the application logic unit may include a virtual machine, a container, a jail, or a zone. 
     In still other examples, a computer readable storage medium may be provided comprising computer executable instructions executable by a processor of a client device, the computer executable instructions comprising instructions executable to allocate a portion of memory by: selecting a type of memory to allocate in the client device from a group of memory types in response to a memory allocation request, wherein the selection of the type of memory to allocate is based on an available memory determination, wherein the group of memory types includes a local primary memory and an external primary memory, wherein the external primary memory is memory that is located in a memory appliance external to the client device but is treated as primary memory at the client device, wherein data in the external primary memory is accessible via client-side memory access in which a communication interface of the memory appliance is configured to access the data in memory of the memory appliance; selecting a portion of the local primary memory and/or a portion of the external primary memory for the portion of memory to allocate at the client device depending on the selected type of memory; and mapping at least the selected portion to an address space addressable by the processor. 
     The system may be implemented in many different ways. Each module or unit, such as the client logic unit, the region access unit, the allocation logic unit, the configuration unit, and/or any other logic described herein may be hardware or a combination of hardware and software. For example, each module may include an application specific integrated circuit (ASIC), a Field Programmable Gate Array (FPGA), a circuit, a digital logic circuit, an analog circuit, a combination of discrete circuits, gates, or any other type of hardware or combination thereof. Alternatively or in addition, each module may include memory hardware, such as a portion of the memory  210 , for example, that comprises instructions executable with the processor  240  or other processor to implement one or more of the features of the module. When any one of the module includes the portion of the memory that comprises instructions executable with the processor, the module may or may not include the processor. In some examples, each module may just be the portion of the memory  210  or other physical memory that comprises instructions executable with the processor  240  or other processor to implement the features of the corresponding module without the module including any other hardware. Because each module includes at least some hardware even when the included hardware comprises software, each module may be interchangeably referred to as a hardware module. 
     All of the discussion, regardless of the particular implementation described, is exemplary in nature, rather than limiting. For example, although selected aspects, features, or components of the implementations are depicted as being stored in memories, all or part of systems and methods consistent with the innovations may be stored on, distributed across, or read from other computer-readable storage media, for example, secondary storage devices such as hard disks, floppy disks, and CD-ROMs; or other forms of ROM or RAM either currently known or later developed. The computer-readable storage media may be non-transitory computer-readable media, which includes CD-ROMs, volatile or non-volatile memory such as ROM and RAM, or any other suitable storage device. 
     Furthermore, although specific components of innovations were described, methods, systems, and articles of manufacture consistent with the innovation may include additional or different components. For example, a processor may be implemented as a microprocessor, microcontroller, application specific integrated circuit (ASIC), discrete logic, or a combination of other type of circuits or logic. Similarly, memories may be DRAM, SRAM, Flash or any other type of memory. Flags, data, databases, tables, entities, and other data structures may be separately stored and managed, may be incorporated into a single memory or database, may be distributed, or may be logically and physically organized in many different ways. The components may operate independently or be part of a same program. The components may be resident on separate hardware, such as separate removable circuit boards, or share common hardware, such as a same memory and processor for implementing instructions from the memory. Programs may be parts of a single program, separate programs, or distributed across several memories and processors. 
     In some examples, when it is said that an interface is performing some action, the action may be performed by the interface directly, and/or the action may be performed by a logic which provides and/or implements the interface, such as by an interface-implementing logic. The interface-implementing logic may be any logic which provides the interface and/or implements the logic that performs actions described as being performed by the interface. The interface-implementing logic may be and/or may include any of the logics described herein. For example, if the actions described as being performed by one or more data interfaces are performed by the client logic  312 , then the interface-implementing logic for the corresponding data interface(s) may be and/or may include the client logic. In examples where the action is performed by the interface, the actions performed may be performed in response to an invocation of the interface. In examples where the action is performed by the interface-implementing logic, the actions performed may be performed in response to an invocation of the corresponding interface. 
     The respective logic, software or instructions for implementing the processes, methods and/or techniques discussed throughout this disclosure may be provided on computer-readable media or memories or other tangible media, such as a cache, buffer, RAM, removable media, hard drive, other computer readable storage media, or any other tangible media or any combination thereof. The tangible media include various types of volatile and nonvolatile storage media. The functions, acts or tasks illustrated in the figures or described herein may be executed in response to one or more sets of logic or instructions stored in or on computer readable media. The functions, acts or tasks are independent of the particular type of instructions set, storage media, processor or processing strategy and may be performed by software, hardware, integrated circuits, firmware, micro code, or any type of other processor, operating alone or in combination. Likewise, processing strategies may include multiprocessing, multitasking, parallel processing and/or any other processing strategy known now or later discovered. In one embodiment, the instructions are stored on a removable media device for reading by local or remote systems. In other embodiments, the logic or instructions are stored in a remote location for transfer through a computer network or over telephone lines. In yet other embodiments, the logic or instructions are stored within a given computer, CPU, GPU, or system. 
     The logic illustrated in the flow diagrams may include additional, different, or fewer operations than illustrated. The operations illustrated may be performed in an order different than illustrated. 
     A second action may be said to be “in response to” a first action independent of whether the second action results directly or indirectly from the first action. The second action may occur at a substantially later time than the first action and still be in response to the first action. Similarly, the second action may be said to be in response to the first action even if intervening actions take place between the first action and the second action, and even if one or more of the intervening actions directly cause the second action to be performed. For example, a second action may be in response to a first action if the first action sets a flag and a third action later initiates the second action whenever the flag is set. 
     To clarify the use of and to hereby provide notice to the public, the phrases “at least one of &lt;A&gt;, &lt;B&gt;, . . . and &lt;N&gt;” or “at least one of &lt;A&gt;, &lt;B&gt;, . . . &lt;N&gt;, or combinations thereof” or “&lt;A&gt;, &lt;B&gt;, . . . and/or &lt;N&gt;” are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, . . . and N. In other words, the phrases mean any combination of one or more of the elements A, B, . . . or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, “a” or “an” means “at least one” or “one or more.” 
     While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.