# Patent

## Title
Emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array

## Abstract

Emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array, including: inserting, into a buffer utilized by a direct memory access (‘DMA’) engine of a first storage array controller, a data transfer descriptor describing data stored in memory of the first storage array controller and a location to write the data to memory of the second storage array controller; retrieving, in dependence upon the data transfer descriptor, the data stored in memory of the first storage array controller; and writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor.

## Background
BACKGROUND OF THE INVENTION
Field of the Invention
The field of the invention is data processing, or, more specifically, methods, apparatus, and products for emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array.
Description of Related Art
Modern storage systems can include many storage devices that are accessed via multiple controllers. Communication between the controllers may be facilitated through the use of special purpose adapters, cables, and other computer hardware. Such adapters, cables, and other computer hardware, however, are often expensive and consume valuable resources such as space, power, and the like. Furthermore, incorporating the controllers into a single form factor can make the inclusion of such adapters, cables, and other computer hardware impossible.
SUMMARY OF THE INVENTION
Methods, apparatuses, and products for emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array, including: inserting, into a buffer utilized by a direct memory access (‘DMA’) engine of a first storage array controller, a data transfer descriptor describing data stored in memory of the first storage array controller and a location to write the data to memory of the second storage array controller; retrieving, in dependence upon the data transfer descriptor, the data stored in memory of the first storage array controller; and writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of example embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of example embodiments of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 sets forth a block diagram of a system configured for emulating an RDMA link between storage array controllers in a storage array according to embodiments of the present invention.
FIG. 2 sets forth a block diagram of a storage array controller useful in emulating an RDMA link between controllers in a storage array according to embodiments of the present invention.
FIG. 3 sets forth a block diagram of a system for emulating an RDMA link between storage array controllers in a storage array according to embodiments of the present invention.
FIG. 4 sets forth a flow chart illustrating an example method for emulating an RDMA link between controllers in a storage array according to embodiments of the present invention.
FIG. 5 sets forth a flow chart illustrating an example method for emulating an RDMA link between controllers in a storage array according to embodiments of the present invention.



DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
Example methods, apparatuses, and products for emulating an RDMA link between controllers in a storage array in accordance with the present invention are described with reference to the accompanying drawings, beginning with FIG. 1. FIG. 1 sets forth a block diagram of a system configured for emulating an RDMA link between controllers in a storage array according to embodiments of the present invention. The system of FIG. 1 includes a number of computing devices (164, 166, 168, 170). Such computing devices may be implemented in a number of different ways. For example, a computing device may be a server in a data center, a workstation, a personal computer, a notebook, or the like.
The computing devices (164, 166, 168, 170) in the example of FIG. 1 are coupled for data communications to one or more storage arrays (102, 104) through a storage area network (‘SAN’) (158) as well as a local area network (‘LAN’) (160). The SAN (158) may be implemented with a variety of data communications fabrics, devices, and protocols. Example fabrics for such a SAN (158) may include Fibre Channel, Ethernet, InfiniBand, Serial Attached Small Computer System Interface (‘SAS’), and the like. Example data communications protocols for use in such a SAN (158) may include Advanced Technology Attachment (‘ATA’), Fibre Channel Protocol, SCSI, iSCSI, HyperSCSI, and others. Readers of skill in the art will recognize that a SAN is just one of many possible data communications couplings which may be implemented between a computing device (164, 166, 168, 170) and a storage array (102, 104), and readers will further appreciate that any other data communications coupling is well within the scope of embodiments of the present invention.
The local area network (160) of FIG. 1 may also be implemented with a variety of fabrics and protocols. Examples of such fabrics include Ethernet (802.3), wireless (802.11), and the like. Examples of such data communications protocols include Transmission Control Protocol (‘TCP’), User Datagram Protocol (‘UDP’), Internet Protocol (‘IP’), HyperText Transfer Protocol (‘HTTP’), Wireless Access Protocol (‘WAP’), Handheld Device Transport Protocol (‘HDTP’), Session Initiation Protocol (‘SIP’), Real Time Protocol (‘RTP’), and others as will occur to those of skill in the art.
The example storage arrays (102, 104) of FIG. 1 provide persistent data storage for the computing devices (164, 166, 168, 170). Each storage array (102, 104) depicted in FIG. 1 includes a plurality of storage array controllers (106, 112). Each storage array controller (106, 112) may be embodied as a module of automated computing machinery comprising computer hardware, computer software, or a combination of computer hardware and software. The storage array controllers (106, 112) may be configured to carry out various storage-related tasks. Such tasks may include writing data received from the one or more of the computing devices (164, 166, 168, 170) to storage, erasing data from storage, retrieving data from storage to provide the data to one or more of the computing devices (164, 166, 168, 170), monitoring and reporting of disk utilization and performance, performing Redundant Array of Independent Drives (‘RAID’) or RAID-like data redundancy operations, compressing data, encrypting data, and so on.
Each storage array controller (106, 112) may be implemented in a variety of ways, including as a Field Programmable Gate Array (‘FPGA’), a Programmable Logic Chip (‘PLC’), an Application Specific Integrated Circuit (‘ASIC’), or computing device that includes discrete components such as a central processing unit, computer memory, and various adapters. Each storage array controller (106, 112) may include, for example, a data communications adapter configured to support communications via the SAN (158) and the LAN (160). Although only one of the storage array controllers (112) in the example of FIG. 1 is depicted as being coupled to the LAN (160) for data communications, readers will appreciate that both storage array controllers (106, 112) may be independently coupled to the LAN (160). Each storage array controller (106, 112) may also include, for example, an I/O controller or the like that couples the storage array controller (106, 112) for data communications, through a midplane (114), to a number of storage devices (146, 150), and a number of non-volatile Random Access Memory (‘NVRAM’) devices (148, 152).
Each NVRAM device (148, 152) may be configured to receive, from the storage array controller (106, 112), data to be stored in the storage devices (146). Such data may originate from any one of the computing devices (164, 166, 168, 170). In the example of FIG. 1, writing data to the NVRAM device may be carried out more quickly than writing data to the storage device. The storage array controller (106, 112) may be configured to effectively utilize the NVRAM devices (148, 152) as a quickly accessible buffer for data destined to be written to the storage devices. In this way, the latency for write requests may be significantly improved relative to a system in which the storage array controller writes data directly to the storage devices (146, 150).
The NVRAM devices may be implemented with computer memory in the form of high bandwidth, low latency random access memory (‘RAM’). In such an embodiment, each NVRAM device is referred to as ‘non-volatile’ because each NVRAM device may receive or include a unique power source that maintains the state of the RAM after main power loss to the NVRAM device (148, 152). Such a power source may be a battery, one or more capacitors, or the like. During the power loss, the NVRAM device (148, 152) may be configured to write the contents of the RAM to a persistent storage, such as the storage devices (146, 150).
A ‘storage device’ as the term is used in this specification refers to any device configured to record data persistently. The term ‘persistently’ as used here refers to a device's ability to maintain recorded data after loss of a power source. Examples of storage devices may include mechanical, spinning hard disk drives, solid-state drives (“Flash drives”), and the like.
The storage array controllers (106, 112) of FIG. 1 may be configured for emulating an RDMA link between each other according to embodiments of the present invention. An RDMA link between the storage array controllers (106, 112) may be embodied, for example, as an InfiniBand link between the storage array controllers (106, 112). InfiniBand is a computer-networking communications standard used in high-performance computing. A physical InfiniBand link between two storage array controllers (106, 112) may be created, for example, through the use of cabling that runs between two storage array controllers (106, 112). Each of the storage array controllers (106, 112) may include a special purpose InfiniBand data communications adapter for receiving such cabling. Readers will appreciate that other RDMA links between storage array controllers (106, 112) may be emulated according to embodiments of the present invention. For example, the embodiments of the present invention may emulate an RDMA over Converged Ethernet (“RoCE”) link between the two storage array controllers (106, 112), an iWARP link between the storage array controllers (106, 112), and so on.
The storage array controllers (106, 112) of FIG. 1 may be configured for emulating an RDMA link between each other according to embodiments of the present invention by inserting, into a buffer utilized by a DMA engine in a first storage array controller (106), a data transfer descriptor. The DMA engine may be embodied, for example, as an embedded DMA controller that resides within a computer processor in the storage array controller (106). The DMA engine may offload the responsibility of performing memory copies from the computer processor by performing direct memory accesses of main memory independently of any processor. Although the DMA engine may be embedded within a processor, the DMA engine may also be embodied as a special purpose controller that is mounted on the same motherboard as the computer processor, but physically separate from the computer processor itself.
The buffer utilized by each DMA engine may be embodied as a data structure used to store descriptors that generally describe data transfer operations to be performed by the DMA engine. For example, the buffer in the first storage array controller (106) may be used to store a data transfer descriptor that describes data stored in memory of the first storage array controller (106) and a location to write the data to memory of the second storage array controller (112). In such an example, the DMA engine may operate by retrieving descriptors from the buffer and performing the data transfer operations described by the descriptors.
The storage array controllers (106, 112) of FIG. 1 may be further configured for emulating an RDMA link between each other according to embodiments of the present invention by the first storage array controller (106) retrieving the data stored in memory of the first storage array controller (106). The memory of the first storage array controller (106) may be embodied, for example, as main memory in the form of RAM. A DMA engine of the first storage array controller (106) may retrieve the data stored in memory of the first storage array controller by performing a direct memory access where the DMA engine accesses the memory independently of any processor within the first storage array controller (106).
The DMA engine within the first storage array controller (106) may retrieve the data stored in memory of the first storage array controller (106) in dependence upon the data transfer descriptor. As described above, the data transfer descriptor may include information that describes data stored in memory of the first storage array controller (106). Such information that describes data stored in memory of the first storage array controller (106) can include, for example, the address of the data stored in memory of the first storage array controller (106), the size of the data stored in memory of the first storage array controller (106), and so on. The DMA engine within the first storage array controller (106) may be configured to utilize such information to retrieve the data stored in memory of the first storage array controller (106), for example, by reading an amount of data that is equal to the size of the data as described by the data transfer descriptor from the address of the data as described by the data transfer descriptor.
The storage array controllers (106, 112) of FIG. 1 may be further configured for emulating an RDMA link between each other according to embodiments of the present invention by writing the data into the memory of the second storage array controller (112). The data stored in memory of the first storage array controller (106) may be written into the memory of the second storage array controller (112) via a serial computer expansion bus that communicatively couples a processor in the first storage array controller (106) and a processor in the second storage array controller (112). Such a serial computer expansion bus may be embodied, for example, as a Peripheral Component Interconnect Express (‘PCIe’) Non-Transparent Bridge (‘NTB’) that communicatively couples the processor in the first storage array controller (106) and the processor in the second storage array controller (112). In such an example, the processor in the first storage array controller (106) and the processor in the second storage array controller (112) may each include a port that is connected to the PCIe NTB, such that bidirectional communication may occur directly between the processors. In such an example, a passive interconnect such as a passive midplane may physical reside between the processors, but no active components are required to route communications between the processors. Readers will appreciate that other physical interconnects between the storage array controllers (106, 112) may be utilized in accordance with embodiments of the present invention.
The DMA engine within the first storage array controller (106) may write the data into the memory of the second storage array controller (112) by performing a direct memory access of the memory in the second storage array controller (112). In such an example, the data may be written into the memory of the second storage array controller (112) in dependence upon the data transfer descriptor. As described above, the data transfer descriptor can include information describing a location to write the data to memory of the second storage array controller (112). The DMA engine of the first storage array controller (106) may utilize such information by writing the data into the memory location in the second storage array controller (112) that is described in the data transfer descriptor.
Although the example illustrated in FIG. 1 depicts an embodiment where a first storage array controller (106) is connected to a first set of storage devices (146) and NVRAM devices (148) by a first midplane (114) and a second storage array controller (112) is connected to a second set of storage devices (150) and NVRAM devices (152) by a second midplane (116), other arrangements are contemplated. For example, each of the storage array controllers (106, 112) may be connected to each of the storage devices (146, 150) and NVRAM devices (148, 152) by a single midplane. Readers will appreciate that the arrangement of computing devices, storage arrays, networks, and other devices making up the example system illustrated in FIG. 1 are for explanation, not for limitation. Systems useful according to various embodiments of the present invention may include different configurations of servers, routers, switches, computing devices, and network architectures, not shown in FIG. 1, as will occur to those of skill in the art.
Emulating an RDMA link between controllers in a storage array in accordance with embodiments of the present invention is generally implemented with computers. In the system of FIG. 1, for example, all the computing devices (164, 166, 168, 170) and storage controllers (106, 112) may be implemented to some extent at least as computers. For further explanation, therefore, FIG. 2 sets forth a block diagram of a storage array controller (202) useful in emulating an RDMA link between controllers in a storage array according to embodiments of the present invention.
The storage array controller (202) of FIG. 2 is similar to the storage array controllers depicted in FIG. 1, as the storage array controller (202) of FIG. 2 is communicatively coupled, via a midplane (206), to one or more storage devices (212) and to one or more NVRAM devices (214) that are included as part of a storage array (216). The storage array controller (202) may be coupled to the midplane (206) via one or more data communications links (204) and the midplane (206) may be coupled to the storage devices (212) and the NVRAM devices (214) via one or more data communications links (208, 210). The data communications links (204, 208, 210) of FIG. 2 may be embodied, for example, as a PCIe bus.
The storage array controller (202) of FIG. 2 includes at least one computer processor (232) or ‘CPU’ as well as RAM (236). The computer processor (232) may be connected to the RAM (236) via a data communications link (230), which may be embodied, for example, as a high speed memory bus such as a Double-Data Rate 4 (‘DDR4’) bus.
Stored in RAM (214) is an operating system (246). Examples of operating systems useful in storage array controllers (202) configured for emulating an RDMA link between controllers in a storage array according to embodiments of the present invention include UNIXTM, LinuxTM, Microsoft WindowsTM, and others as will occur to those of skill in the art. Also stored in RAM (236) is an array management module (248), a module of computer program instructions useful in emulating an RDMA link between controllers in a storage array according to embodiments of the present invention. The array management module (248) may be configured to perform steps such as: inserting, into a buffer utilized by a direct memory access (‘DMA’) engine of the storage array controller (202), a data transfer descriptor describing data stored in memory of the storage array controller (202) and a location to write the data to memory of a second storage array controller; retrieving, in dependence upon the data transfer descriptor, the data stored in memory of the storage array controller (202); writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor; and other steps that will be described in greater detail below as being performed by the storage array controller generally. Readers will appreciate that while the array management module (248) and the operating system (246) in the example of FIG. 2 are shown in RAM (168), many components of such software may also be stored in non-volatile memory such as, for example, on a disk drive, on a solid-state drive, and so on.
The storage array controller (202) of FIG. 2 also includes a plurality of host bus adapters (218, 220, 222) that are coupled to the processor (232) via a data communications link (224, 226, 228). Each host bus adapter (218, 220, 222) may be embodied as a module of computer hardware that connects the host system (i.e., the storage array controller) to other network and storage devices. Each of the host bus adapters (218, 220, 222) of FIG. 2 may be embodied, for example, as a Fibre Channel adapter that enables the storage array controller (202) to connect to a SAN, as an Ethernet adapter that enables the storage array controller (202) to connect to a LAN, and so on. Each of the host bus adapters (218, 220, 222) may be coupled to the computer processor (232) via a data communications link (224, 226, 228) such as, for example, a PCIe bus.
The storage array controller (202) of FIG. 2 also includes a host bus adapter (240) that is coupled to an expander (242). The expander (242) depicted in FIG. 2 may be embodied as a module of computer hardware utilized to attach a host system to a larger number of storage devices than would be possible without the expander (242). The expander (242) depicted in FIG. 2 may be embodied, for example, as a SAS expander utilized to enable the host bus adapter (240) to attach to storage devices in an embodiment where the host bus adapter (240) is embodied as a SAS controller.
The storage array controller (202) of FIG. 2 also includes a switch (244) that is coupled to the computer processor (232) via a data communications link (238). The switch (244) of FIG. 2 may be embodied as a computer hardware device that can create multiple endpoints out of a single endpoint, thereby enabling multiple devices to share what was initially a single endpoint. The switch (244) of FIG. 2 may be embodied, for example, as a PCIe switch that is coupled to a PCIe bus (238) and presents multiple PCIe connection points to the midplane (206).
The storage array controller (202) of FIG. 2 also includes a data communications link (234) for coupling the storage array controller (202) to other storage array controllers. Such a data communications link (234) may be embodied, for example, as a QuickPath Interconnect (‘QPI’) interconnect, a PCIe NTB interconnect, and so on.
Readers will recognize that these components, protocols, adapters, and architectures are for illustration only, not limitation. Such a storage array controller may be implemented in a variety of different ways, each of which is well within the scope of the present invention.
For further explanation, FIG. 3 sets forth a block diagram of a system for emulating an RDMA link between storage array controllers (302, 304) in a storage array according to embodiments of the present invention. Each of the storage array controllers (302, 304) depicted in FIG. 3 includes a processor (306, 308) and memory (318, 320), which may be embodied as RAM or other form of computer memory. In the example depicted in FIG. 3, the processors (306, 308) and the memory (318, 320) may be communicatively coupled via a memory bus, and other components not depicted here (e.g., a memory controller) may be included in the system to facilitate access of the memory (318, 320) by the processors (306, 308). Readers will appreciate that although each storage array controller (302, 304) depicted in FIG. 3 includes only a single processor (306, 308), each of the storage array controllers (302, 304) may include additional processors that can access the memory (318, 320).
In the example depicted in FIG. 3, each processor (306, 308) includes a DMA engine (310, 316). Each of the DMA engines (310, 316) may be embodied, for example, as special purpose computer hardware that enables the storage array controller (302, 304) to access main memory (318, 320) independently of the processor (306, 308) such that main memory (318, 320) can be accessed without the processor (306, 308) using programmed I/O. In the example depicted in FIG. 3, the DMA engines (310, 316) are depicted as being incorporated into the processors (306, 308), although in other embodiments the DMA engines (310, 316) may be embodied as a standalone DMA controllers. Readers will appreciate that although each of the processors (306, 308) depicted in FIG. 3 includes only a single DMA engine (310, 316), each of the processors (306, 308) may include additional DMA engines.
In the example depicted in FIG. 3, each processor (306, 308) includes a PCIe port (312, 314). Each of the PCIe ports (312, 314) may be embodied, for example, as physical connectors that enable the processor (306, 308) to couple to a PCIe bus. Readers will appreciate that although each of the processors (306, 308) depicted in FIG. 3 includes only a single PCIe port (312, 314), each of the processors (306, 308) may include additional PCIe ports. Readers will further appreciate that the processors (306, 308) may include additional physical connectors such that each of the processors (306, 308) may be directly coupled to each other via other data communications links.
For further explanation, FIG. 4 sets forth a flow chart illustrating an example method for emulating an RDMA link between controllers (402, 424) in a storage array according to embodiments of the present invention. An RDMA link between controllers (402, 424) may be embodied, for example, as an InfiniBand link between the two controllers (402, 424). InfiniBand is a computer-networking communications standard used in high-performance computing. A physical InfiniBand link between two storage array controllers (402, 424) may be created, for example, through the use of cabling that runs between two storage array controllers (402, 424) and is coupled to a special purpose InfiniBand data communications adapter. Readers will appreciate that other RDMA links between controllers (402, 424) may be emulated according to embodiments of the present invention. For example, the embodiments of the present invention may emulate an RoCE link between the two controllers (402, 424), an iWARP link between the two controllers (402, 424), and so on.
The example method depicted in FIG. 4 includes inserting (404), into a buffer (408) utilized by a DMA engine (418) in a first storage array controller (402), a data transfer descriptor (406). The DMA engines (418, 432) of FIG. 4 may be embodied, for example, as embedded DMA controllers that offload the responsibility for performing memory copies from the processors (416, 430) by performing direct memory accesses of main memory (414, 428) independently of any processor (416, 430). Although the DMA engines (418, 432) of FIG. 4 are depicted as being embedded within the processors (416, 430) themselves, the DMA engines (418, 432) may also be embodied as controllers that are mounted on the same motherboard as the processors (416, 430) but physically separate from the processors (416, 430) themselves.
The buffers (408, 426) utilized by the DMA engines (418, 432) may be embodied as data structures used to store descriptors that generally describe data transfer operations to be performed by the DMA engines (418, 432). For example, the buffer (408) in the first storage array controller (402) may be used to store a data transfer descriptor (406) that describes data (412) stored in memory (414) of the first storage array controller (402) and a location to write the data (412) to memory (428) of the second storage array controller (424). In such an example, the DMA engines (418, 432) may operate by retrieving descriptors from the buffers (408, 426) and performing the data transfer operations described by the descriptors.
The example method depicted in FIG. 4 also includes retrieving (410) the data (412) stored in memory (414) of the first storage array controller (402). The memory (414) of the first storage array controller (402) may be embodied, for example, as main memory in the form of RAM. In the example method depicted in FIG. 4, the DMA engine (418) may retrieve (410) the data (412) stored in memory (414) of the first storage array controller (402) by performing a direct memory access where the DMA engine (418) accesses the memory (414) independently of the processor (416).
In the example method depicted in FIG. 4, the DMA engine (418) may retrieve (410) the data (412) stored in memory (414) of the first storage array controller (402) in dependence upon the data transfer descriptor (406). As described above, the data transfer descriptor (406) may include information that describes data (412) stored in memory (414) of the first storage array controller (402). Such information that describes data (412) stored in memory (414) of the first storage array controller (402) can include, for example, the address of the data (412) stored in memory (414) of the first storage array controller (402), the size of the data (412) stored in memory (414) of the first storage array controller (402), and so on. In the example method depicted in FIG. 4, the DMA engine (418) may be configured to utilize such information to retrieve (410) the data (412) stored in memory (414) of the first storage array controller (402), for example, by reading an amount of data that is equal to the size of the data (412) as described by the data transfer descriptor (406) from the address of the data (412) as described by the data transfer descriptor (406).
The example method depicted in FIG. 4 also includes writing (420) the data (412) into the memory (428) of the second storage array controller (424). In the example method depicted in FIG. 4, the data (412) stored in memory (414) of the first storage array controller (402) may be written (420) into the memory (428) of the second storage array controller (424) via a serial computer expansion bus (422) that communicatively couples the processor (416) in the first storage array controller (402) and a processor (430) in the second storage array controller (424). Such a serial computer expansion bus (422) may be embodied, for example, as a PCIe NTB that communicatively couples the processor (416) in the first storage array controller (402) and the processor (430) in the second storage array controller (424). In such an example, the processor (416) in the first storage array controller (402) and the processor (430) in the second storage array controller (424) may each include a port that is connected to the PCIe NTB, such that bidirectional communication may occur directly between the processors (416, 430). A passive interconnect such as a passive midplane may physically reside between the processors (416, 430), but no active components are required to route communications between the processors (416, 430). Readers will appreciate that other physical interconnects between the storage array controllers (402, 424) may be utilized in accordance with embodiments of the present invention.
In the example method depicted in FIG. 4, the DMA engine (418) may write (420) the data (412) into the memory (428) of the second storage array controller (424) by performing a direct memory access of the memory (428) in the second storage array controller (424). Because the first storage array controller (402) and the second storage array controller (424) are coupled for communications, for example, via a PCIe NTB, a DMA engine (418) within the first storage array controller (402) may be capable of writing data directly to memory (428) in the second storage array controller (424). In such an example, the data (412) may be written (420) into the memory (428) of the second storage array controller (424) in dependence upon the data transfer descriptor (406). As described above, the data transfer descriptor (406) can include information describing a location to write the data (412) to memory (428) of the second storage array controller (424). The DMA engine (418) may utilize such information by writing (420) the data (412) into the memory location in the second storage array controller (424) that is described in the data transfer descriptor (406).
In the example method depicted in FIG. 4, inserting (404) the data transfer descriptor (406) into the buffer (408) utilized by the DMA engine (418) in the first storage array controller (402) may be carried out, for example, by management software executing on the storage array controller (402) forming the data transfer descriptor (406) and inserting (404) the data transfer descriptor (406) into the buffer (408) according to a predetermined insertion policy. For example, if the buffer (408) utilized by the DMA engine (418) is a FIFO buffer, the data transfer descriptor (406) may be inserted (404) into the end of the buffer (408). Alternatively, if the buffer (408) utilized by the DMA engine (418) is a LIFO buffer, the data transfer descriptor (406) may be inserted (404) into the front of the buffer (408). In such an example, the management software executing on the storage array controller (402) may determine the appropriate location within the buffer (408) to insert (404) the data transfer descriptor (406) for subsequent processing by the DMA engine (418).
In the example method depicted in FIG. 4, inserting (404) the data transfer descriptor (406) into the buffer (408) utilized by the DMA engine (418) in the first storage array controller (402) may include inserting (434) the data transfer descriptor (406) into the buffer (408) utilized by the DMA engine (418) of the first storage array controller (402) via virtualization hardware. The virtualization hardware may be embodied, for example, as computing components that support I/O virtualization. Such computing components that support I/O virtualization can include an Input/Output Memory Management Unit (‘IOMMU’), chipsets and BIOS that support virtualization technology, and so on.
In the example method depicted in FIG. 4, inserting (404) the data transfer descriptor (406) into the buffer (408) utilized by the DMA engine (418) in the first storage array controller (402) may alternatively include inserting (436) the data transfer descriptor (406) into a location within the buffer (408) where a completed data transfer descriptor resides. In the example method depicted in FIG. 4, the buffer (408) may be embodied as a circular buffer with a limited number of entries. As such, new data transfer descriptors may be inserted into the buffer (408) by overwriting a previous entry in the buffer (408).
In the example method depicted in FIG. 4, inserting (404) the data transfer descriptor (406) into the buffer (408) utilized by the DMA engine (418) in the first storage array controller (402) may also include associating (438) a static validity value with the data transfer descriptor (406). The static validity value may be embodied, for example, as a single bit value used to determine whether a particular data transfer descriptor in the buffer (408) has already been processed. For example, a value of ‘0’ may indicate that the associated data transfer descriptor has not been processed while a value of ‘1’ may indicate that the associated data transfer descriptor has been processed. Readers will appreciate that because the static validity value is ‘static’, the static validity value does not change. For example, when a data transfer descriptor is processed, the static validity value is not changed from ‘0’ to ‘1’. Once the static validity value has been set, the static validity value that is associated with a particular data transfer descriptor does not change.
In the example method depicted in FIG. 4, the static validity value that is associated with the data transfer descriptor (406) that is inserted (404) into the buffer (408) does not match a static validity value for the completed data transfer descriptor. Consider the example described above in which the static validity value is embodied as a single bit value. Assume that in such an example, the buffer (408) is a circular buffer that can store 1024 data transfer descriptors. In such an example, the first 1024 data transfer descriptors may be written sequentially into the buffer (408), and each data transfer descriptor may be associated with a static validity value of ‘0’, indicating that the associated data transfer descriptor has not been processed. As data transfer descriptors are processed, however, the static validity value of each data transfer descriptor is not changed from a ‘0’ to ‘1’, as such changes would consume processing cycles from the processor (416). Instead of changing each static validity value, when the buffer (408) is full and new data transfer descriptors are inserted (404) by overwriting already-processed data transfer descriptors, the meaning of the static validity values may be flipped. That is, when the buffer (408) is full and new data transfer descriptors are inserted (404) by overwriting already-processed data transfer descriptors, the meaning of the static validity values may be flipped such that a value of ‘0’ may indicate that the associated data transfer descriptor has been processed while a value of ‘1’ may indicate that the associated data transfer descriptor has not been processed. As such, each time a new data transfer descriptor is overwritten, the new data transfer descriptor will be associated with a static validity value that is different than the static validity value of the overwritten data transfer descriptor.
For further explanation, FIG. 5 sets forth a flow chart illustrating an additional example method for emulating an RDMA link between controllers (402, 424) in a storage array according to embodiments of the present invention. The example method depicted in FIG. 5 is similar to the example method depicted in FIG. 4, as the example method depicted in FIG. 5 also includes inserting (404) a data transfer descriptor (406) into a buffer (408) utilized by a DMA engine (418) in a first storage array controller (402), retrieving (410) the data (412) stored in memory (414) of the first storage array controller (402), and writing (420) the data (412) into the memory (428) of the second storage array controller (424).
The example method depicted in FIG. 5 also includes creating (506) a plurality of logical communications channels between the first storage array controller (402) and the second storage array controller (424). In the example method depicted in FIG. 5, each of the logical communications channels between the first storage array controller (402) and the second storage array controller (424) may be reserved for sending specific data and messages between the first storage array controller (402) and the second storage array controller (424). For example, a first data communications channel may be utilized to forward storage requests received by the first storage array controller (402) to the second storage array controller (424) when the second storage array controller (424) is serving as the primary controller, a second data communications channel may be utilized to forward storage requests received by the second storage array controller (424) to the first storage array controller (402) when the first storage array controller (402) is serving as the primary controller, a third data communications channel may be utilized to transmit data between the storage array controllers (402, 424) that can be used when failing over from a primary controller to a secondary controller, and so on.
In the example method depicted in FIG. 5, creating (506) a plurality of logical communications channels between the first storage array controller (402) and the second storage array controller (424) can include segmenting (508) a shared memory space into a plurality of memory segments, where each memory segment is associated with a distinct logical communications channel. As described above, the first storage array controller (402) and the second storage array controller (424) may be coupled by a PCIe NTB. In such an example, some dedicated memory may be utilized to support communications over the PCIe NTB. Such dedicated memory that is used to support communications over the PCIe NTB may serve as shared memory that is segmented (508) into a plurality of memory segments, wherein each memory segment is associated with a distinct logical communications channel. In such a way, sending data over the first logical communications channel may be carried out by writing the data to the memory segment associated with associated with the first logical communications channel, sending data over the second logical communications channel may be carried out by writing the data to the memory segment associated with associated with the second logical communications channel, and so on.
The example method depicted in FIG. 5 also includes receiving (504) a storage request (502) directed to the storage array. The storage request (502) may be embodied, for example, as a request to read data from some memory location within the storage array or a request to write data to some memory location within the storage array. The storage request (502) may also be embodied, for example, as an ‘extended copy request’ where an initiator requests that storage array moves data internally (thereby replacing a sequence of read and write requests), as a ‘reservation request’ where an initiator reserves a volume so that if another initiator accesses the volume an error is returned, as a ‘get LBA status request’ that allows an initiator to query thin provisioning status, and so on.
In the example method depicted in FIG. 5, writing (420) the data (412) into the memory (428) of the second storage array controller (424) can include forwarding (510), from the first storage array controller (402) to the second storage array controller (424) via a predetermined logical communications channel, the storage request (502). Forwarding (510) the storage request (502) from the first storage array controller (402) to the second storage array controller (424) via a predetermined logical communications channel may be carried out, for example, by writing the storage request (502) into the memory segment associated with the predetermined logical communications channel.
The example method depicted in FIG. 5 also includes signaling (512) that the data transfer descriptor (406) has been processed. Signaling (512) that the data transfer descriptor (406) has been processed may be carried out, for example, by the DMA engine (418) writing the data transfer descriptor (406) to a designated memory location utilized to store the most recently completed data descriptor. In such an example, the designated memory location utilized to store the most recently completed data descriptor may be periodically polled to detect the presence of a new descriptor. In alternative embodiments, signaling (512) that the data transfer descriptor (406) has been processed may be carried out through the use of active messaging, by triggering an event, and so on.
Example embodiments of the present invention are described largely in the context of a fully functional computer system for emulating an RDMA link between controllers in a storage array according to embodiments of the present invention. Readers of skill in the art will recognize, however, that the present invention also may be embodied in a computer program product disposed upon computer readable storage media for use with any suitable data processing system. Such computer readable storage media may be any storage medium for machine-readable information, including magnetic media, optical media, or other suitable media. Examples of such media include magnetic disks in hard drives or diskettes, compact disks for optical drives, magnetic tape, and others as will occur to those of skill in the art. Persons skilled in the art will immediately recognize that any computer system having suitable programming means will be capable of executing the steps of the method of the invention as embodied in a computer program product. Persons skilled in the art will recognize also that, although some of the example embodiments described in this specification are oriented to software installed and executing on computer hardware, nevertheless, alternative embodiments implemented as firmware or as hardware are well within the scope of the present invention.
It will be understood from the foregoing description that modifications and changes may be made in various embodiments of the present invention without departing from its true spirit. The descriptions in this specification are for purposes of illustration only and are not to be construed in a limiting sense. The scope of the present invention is limited only by the language of the following claims.

## Claims

1. A method of emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array, the method comprising:
inserting, into a buffer utilized by a direct memory access (‘DMA’) engine of a first storage array controller, a data transfer descriptor describing data stored in memory of the first storage array controller and a location to write the data to memory of a second storage array controller;
retrieving, in dependence upon the data transfer descriptor, the data stored in memory of the first storage array controller;
writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor; and
signaling that the data transfer descriptor has been processed.

2. The method of claim 1 further comprising creating a plurality of logical communications channels between the first storage array controller and the second storage array controller.

3. The method of claim 2 wherein creating the plurality of logical communications channels between the first storage array controller and the second storage array controller further comprises segmenting a shared memory space into a plurality of memory segments, each memory segment associated with a distinct logical communications channel.

4. The method of claim 2 further comprising:
receiving, by the first storage array controller, a storage request directed to the storage array; and
wherein writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor further comprises forwarding, from the first storage array controller to the second storage array controller via a predetermined logical communications channel, the storage request.

5. The method of claim 1 wherein inserting the data transfer descriptor into the buffer further comprises inserting the data transfer descriptor into the buffer utilized by the DMA engine of the first storage array controller via virtualization hardware.

6. The method of claim 1 wherein inserting the data transfer descriptor into the buffer further comprises:
inserting the data transfer descriptor into a location within the buffer where a completed data transfer descriptor resides; and
associating a static validity value with the data transfer descriptor, wherein the static validity value does not match a static validity value for the completed data transfer descriptor.

7. A storage array controller for emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array, the storage array controller comprising a computer processor, a computer memory operatively coupled to the computer processor, the computer memory having disposed within it computer program instructions that, when executed by the computer processor, cause the storage array controller to carry out the steps of:
inserting, into a buffer utilized by a direct memory access (‘DMA’) engine of the storage array controller, a data transfer descriptor describing data stored in memory of the storage array controller and a location to write the data to memory of a second storage array controller;
retrieving, in dependence upon the data transfer descriptor, the data;
writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor; and
signaling that the data transfer descriptor has been processed.

8. The storage array controller of claim 7 further comprising computer program instructions that, when executed by the computer processor, cause the storage array controller to carry out the step of creating a plurality of logical communications channels between the storage array controller and the second storage array controller.

9. The storage array controller of claim 8 wherein creating the plurality of logical communications channels between the first storage array controller and the second storage array controller further comprises segmenting a shared memory space into a plurality of memory segments, each memory segment associated with a distinct logical communications channel.

10. The storage array controller of claim 8 further comprising computer program instructions that, when executed by the computer processor, cause the storage array controller to carry out the step of receiving a storage request directed to the storage array; and
wherein writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor further comprises forwarding, to the second storage array controller via a predetermined logical communications channel, the storage request.

11. The storage array controller of claim 7 wherein inserting the data transfer descriptor into the buffer further comprises inserting the data transfer descriptor into the buffer utilized by the DMA engine of the storage array controller via virtualization hardware.

12. The storage array controller of claim 7 wherein inserting the data transfer descriptor into the buffer further comprises:
inserting the data transfer descriptor into a location within the buffer where a completed data transfer descriptor resides; and
associating a static validity value with the data transfer descriptor, wherein the static validity value does not match a static validity value for the completed data transfer descriptor.

13. A computer program product for emulating a remote direct memory access (‘RDMA’) link between controllers in a storage array, the computer program product disposed upon a non-transitory computer readable medium, the computer program product comprising computer program instructions that, when executed, cause a storage array controller to carry out the steps of:
inserting, into a buffer utilized by a direct memory access (‘DMA’) engine of the storage array controller, a data transfer descriptor describing data stored in memory of the storage array controller and a location to write the data to memory of a second storage array controller;
retrieving, in dependence upon the data transfer descriptor, the data stored in memory of the storage array controller;
writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor; and
signaling that the data transfer descriptor has been processed.

14. The computer program product of claim 13 further comprising computer program instructions that, when executed, cause the storage array controller to carry out the step of creating a plurality of logical communications channels between the storage array controller and the second storage array controller.

15. The computer program product of claim 14 wherein creating the plurality of logical communications channels between the storage array controller and the second storage array controller further comprises segmenting a shared memory space into a plurality of memory segments, each memory segment associated with a distinct logical communications channel.

16. The computer program product of claim 14 further comprising computer program instructions that, when executed, cause the storage array controller to carry out the step of receiving a storage request directed to the storage array; and
wherein writing the data into the memory of the second storage array controller in dependence upon the data transfer descriptor further comprises forwarding, to the second storage array controller via a predetermined logical communications channel, the storage request.

17. The computer program product of claim 13 wherein inserting the data transfer descriptor into the buffer further comprises inserting the data transfer descriptor into the buffer utilized by the DMA engine of the storage array controller via virtualization hardware.