# Patent

## Title
System and method for cache data recovery

## Abstract

A system comprises a first host device, a second host device, and first and second cache controllers. A cache controller includes a cache memory interface, a first peripheral interface that communicates with the first host device, a second peripheral interface that communicates with the second host device, logic circuitry that loads a cache command from a cache command memory of the first host device, loads a cache command from a cache command memory of the second cache controller, and performs the cache commands, and error checking circuitry that detects an uncorrectable error in a first cache controller/memory pair and indicates the uncorrectable error condition to at least one of the first and second host devices. At least one of the first host device or the second host device writes contents of the cache memory of the second cache controller/memory pair to a main memory in response to the indication.

## Background
CLAIM OF PRIORITY
This application is a Continuation application of U.S. application Ser. No. 14/495,115, filed Sep. 24, 2014, the contents of which are hereby incorporated by reference in its entirety and the benefit of priority is claimed herein.


TECHNICAL FIELD
The present disclosure relates generally to networked storage systems, and more particularly, in some embodiments, to subsystems for facilitating data storage and access in flash based networked storage systems.
BACKGROUND
The large amounts of information generated daily challenge data handling facilities as never before. In the context of today's information generation, data is being generated at rates perhaps thousands or tens of thousands of times greater than was the data-generation rate in the 1990s. Historically, large volumes of data sparked explosive growth in data communications. Responses to growing amounts of data generation centered on improving the movement of data based in increased transmission data rates to enhance throughput in communication channels. For instance, transmission pipelines grew from a few tens of megabits-per-second (Mb/s) transmission rates to several tens of gigabits-per-second (Gb/s) rates during the 1990s.
In the same period, typical storage devices, such as hard disk drives (HDDs), when amassed in sufficient numbers, might accommodate large volumes of data, but the rates at which data may be stored and retrieved have not scaled at the same rate as the volume of data stored on the devices has increased. Data access rates for HDDs are at similar orders of magnitude today as they were in the 1990s.
Fundamental storage subsystems have not integrated technology to enable scaling of effective data storage at the same rate that data generation is growing. Hence the challenge to systems handling large volumes of data is not likely to be alleviated by the combination of contemporary HDD technology with high-speed data transmission channels. In order to handle and manage big data, information processing facilities will be pressured to utilize larger volumes of storage with higher performance rates for capturing and accessing data.



BRIEF DESCRIPTION OF THE DRAWINGS
Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which:
FIG. 1 is an illustrative schematic block diagram of a network storage system according to some embodiments;
FIG. 2 is a block diagram showing a plurality of Flash storage modules coupled to a Flash memory controller, which in turn, is coupled to the first and second packet routing networks according to some embodiments;
FIG. 3 is a block diagram showing a dual inline memory module containing DRAM and flash memory, as may be used in example embodiments;
FIG. 4 is a block diagram showing an example of interfaces to a cache controller according to some embodiments;
FIG. 5 shows a flow diagram of an example of a method of managing access to cache memory using two cache controllers according to some embodiments;
FIG. 6 is a block diagram of portions of an example of a cache controller according to some embodiments;
FIG. 7 shows a diagram of an example of a sequence of communications for loading cache commands in cache controllers according to some embodiments;
FIGS. 8 and 9 show diagrams of examples of a sequence of communications for cache read commands performed by a cache controller according to some embodiments;
FIGS. 10 and 11 show diagrams of examples of a sequence of communications for cache write commands performed by a cache controller according to some embodiments;
FIGS. 12A, 12B, and 12C illustrate example flow diagrams showing caching principles, policies, or rules implemented by the cache management of the system according to some embodiments.



DESCRIPTION OF EMBODIMENTS
Introduction
The following description is presented to enable any person skilled in the art to create and use a computer system that provides high speed access to data storage devices, particularly Flash storage devices. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, in the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention might be practiced without the use of these specific details. In other instances, well-known data structures and processes are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. Identical reference numerals may be used to represent different views of the same item in different drawings. Flow diagrams in drawings referenced below are used to represent processes. A computer system is configured to perform some of these processes. The flow diagrams that represent computer-implemented processes include modules that represent the configuration of a computer system according to computer program code to perform the acts described with reference to these modules. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
System Overview
FIG. 1 is an illustrative schematic block diagram of a network storage system 100 in accordance with some embodiments. The system includes redundant components that permit recovery from component failures, for example. More specifically, the system 100 includes redundant first and second packet routing networks, referred to as the “routing networks” or “networks,” 102-1, 102-2 that route packets between endpoints. First and second general purpose management processors 116-1, 116-2 are coupled as endpoints to each of the networks 102-1, 102-2. The first and second general purpose management processors 116-1, 116-2 may be implemented as one or more server boards and are associated with respective first and second non-transitory local memory devices (hereinafter “local memory devices” or “local memory”) 117-1, 117-2. A plurality of Flash solid state (storage) drive (SSD) circuits (hereinafter “Flash circuits” or “Flash”) 110-1 to 110-8 are coupled as endpoints to each of the routing networks 102-1, 102-2. Input/Output (I/O) circuits 112-1 to 112-4 are coupled as endpoints to each of the routing networks 102-1, 102-2 and act as communication interfaces between the routing networks 102-1, 102-2 and an external network 104. Storage access networks 108-1 through 108-4 provide access to the Flash circuits 110-1 through 110-8. Storage I/O interface circuits 114-1 to 114-4 are coupled as endpoints to each of the routing networks 102-1, 102-2 and act as communication interfaces between the routing networks 102-1, 102-2 and storage access networks 108-1 through 108-4. First and second packet processing circuits 118-1, 118-2 are coupled as endpoints to each of the network 102-1, 102-2. The first and second packet processing circuits are configured to impart one or more services to packets transported over the networks 102-1, 102-2. In some embodiments, the service imparted by the first and second packet processing circuits includes a cache service, and the first and second packet processing circuits 118-1, 118-2 are coupled to cache storage circuitry 106-1, 106-2, which in some embodiments includes Flash-backed DRAM circuits. In some embodiments, the service imparted by the first and second packet processing circuits includes one or more of encryption/decryption, duplication/de-duplication, compression/de-compression, replication, and snapshot, for example.
In accordance with some embodiments, the network “fabric” of the first and second packet routing networks 102-1, 102-2 is compliant with the PCI Express Base Specification (hereinafter “PCIe”) released by the PCISIG (PCI Special Interest Group). See, PCI Express Technology, Comprehensive Guide to Generations 1.x, 2.x and 3.0, by M. Jackson and R. Budruk, 2102, Mindshare, Inc. PCIe specifies point-to-point bidirectional serial communication paths between endpoints over switches and connection lines. Information is transmitted in packets between endpoints over the routing networks 102-1, 102-2. A PCIe network includes serial connection lines 103-1, 103-2 commonly referred to as “links” that are capable of sending and receiving information at the same time. More specifically, information transmitted through either one or the other of the routing networks 102-1, 102-2 is encapsulated in packets that include routing information that indicates a source endpoint and a destination endpoint. According to the PCIe specification and in accordance with some embodiments, a link can include one or more serial transmit and serial receive connection pairs. Each individual pair is referred to as a “lane.” A link can be made up of multiple lanes. Each lane uses differential signaling, sending both positive and negative versions of the same signal. Advantages of differential signaling include improved noise immunity and reduced signal voltage. Each endpoint device coupled to one or both of the routing networks 102-1, 102-2 includes “core” logic that implements one or more functions. A device that is a component of a typical PCIe compliant network can have multiple functions, up to eight in some embodiments, each implementing its own configuration space.
The first management processor 116-1 is used to configure the first packet routing network circuit 102-1 to provide point-to-point communication between components operably coupled to it. The second management processor 116-2 is used to configure the second packet routing network circuit 102-2 to provide point-to-point communication between components operably coupled to it. In some embodiments, the first and second management processors 116-1, 116-2 configure point-to-point routing within the first and second packet routing networks. In other words, for a given pair of resource circuits, a fixed route among switches in the internal network circuits 102-1 or 102-2 is configured to transmit packets between the pair.
The PCIe specification specifies use of a root complex to configure a PCIe compliant network. A root complex includes interface circuitry (e.g., processor interface, DRAM interface) that couples a management processor and the rest of a PCIe network. Management processor 116-1 includes first and second root complexes 122-1, 122-2 that act as interfaces between processor 116-1 and network circuits 102-1 and 102-2. Management processor 116-2 includes second and third root complexes 122-3, 122-4 that act as interfaces between processor 116-2 and network circuits 102-1 and 102-2. The term “root” is used to indicate that the root complex is disposed at a root of an inverted tree topology that is characteristic of a hierarchical PCIe compliant network.
FIG. 2 is an illustrative schematic diagram showing a plurality of Flash storage modules 113-1 to 113-N coupled to a Flash memory controller 180, which in turn, is coupled to the first and second packet networks 102-1, 102-2. It will be appreciated that for simplification of explanation, FIG. 1 shows the system 100 with only a few representative Flash circuits 110-1 to 110-8. However, in some embodiments, a system 100 can include a large number “M” of Flash endpoints, such as two-hundred Flash endpoints for example. Moreover, as shown in FIG. 2, in some embodiments each endpoints 110-1 to 110-N can include a double stacked Flash storage module that includes multiple individual Flash integrated circuits (ICs) 182 arranged on opposite surfaces of a printed circuit board (PCB) (not shown). In accordance with some embodiments, the Flash circuit modules 110-1 to 110-N share the same form factor, and in some embodiments, the Flash circuit modules' form factor is compliant with a dual inline memory module (DIMM) format. A memory controller 180 is coupled to the routing networks 102-1, 102-2 and manages the flow of data going to and from the Flash ICs on the Flash storage module of the endpoints.
Referring again to FIG. 1, the I/O interface circuits 112-1 to 112-4 provide high-speed connections between the external network 104 (e.g., InfiniBand, Fibre Channel, and/or Ethernet) and the first switch network circuitry 102-1, 102-2. The I/O circuitry provides protocol conversion, including packet format conversion, during high-speed data communication between the external network 104 and the first switch network circuitry 102-1, 102-2. In some embodiments, the external network I/O interface circuits 112-1 to 112-4 are implemented as network interface cards commonly referred to as NICs, which include circuits that are configured to transform packets to suitable formats as they pass between the external network 104 and the routing networks 102-1, 102-2.
The storage I/O interface circuits 114-1 to 114-4 manage the distribution of data across the Flash storage circuits 110-1 to 110-8. In some embodiments, the storage I/O interface circuits are configured to implement a file system used to control how data is stored in and retrieved from storage devices. In some embodiments, the storage I/O interface circuits 114-1 to 114-4 are implemented as RAID controllers configured to organize data across multiple storage devices, such as Flash storage devices 110-1 to 110-8. The term RAID refers to data storage schemes that combine multiple disk drive components into a logical unit for the purposes of data redundancy and performance improvement. Persons skilled in the art will appreciate that Flash storage, sometimes referred to as solid-state drive (SSD) is a data storage device using integrated circuit assemblies as memory to store data persistently. Each of the storage access switch networks 108-1 to 108-4 provides point-to-point connections 124-1 to 124-4, respectively, using a serial protocol that moves data to and from the Flash storage devices 110-1 to 110-8. In some embodiments, the storage access switch networks 108-1 to 108-4 use a protocol that includes the SAS (Serial Attached SCSI) protocol. In general, according to the SAS protocol, there are three types of SAS devices: initiators (e.g., RAID controllers), target storage devices (e.g., Flash circuits), and expanders. An initiator device attaches to one or more target storage devices, to create a SAS domain. In some embodiments, the storage I/O interface circuits, implemented as RAID controllers, act as SAS initiators. In accordance with some embodiments, the Flash storage circuits 110-1 to 110-8 act as SAS targets. Using expanders (e.g., low-cost, high-speed switches), the number of targets attached to an initiator can be increased to create a larger SAS domain.
Communication paths 126-5, 126-6 couple storage I/O interface circuit 114-1 to exchange data with storage access switch networks 108-1 and 108-3. Communication paths 126-1, 126-2 couple storage I/O interface circuit 114-2 to exchange data with storage access switch circuits 108-2 and 108-4. Communication paths 126-3, 126-4 couple storage I/O interface circuit 114-3 to exchange data with storage access network circuits 108-3 and 108-1. Communication paths 126-7, 126-8 couple storage I/O interface circuit 114-4 to exchange data with storage access switch networks 108-4 and 108-2. Thus, all Flash circuits 110-1 to 110-8 are accessible via the first internal network circuit 102-1 via the storage I/O interface circuits 114-1, 114-2 coupled to it, and all Flash circuits 110-1 to 110-8 are accessible via the second internal network circuit 102-2 via the storage I/O interface circuits 114-3, 114-4 coupled to it.
In some embodiments, the first and second packet processing circuits 118-1, 118-2 are implemented as field programmable gate array (FPGAs). FPGA circuitry often can impart services with less latency delay, and therefore, faster than a typical general purpose management processor, for example, since the programmable logic can be programmed in advance to dedicate specific hardware circuitry to provide the services. Programmable hardware logic such as FPGA circuitry often can perform operations faster than, for example, a general purpose processor which often uses software interrupts to transition between different operations. Alternatively, in accordance with some embodiments, one or more of the packet processing circuits can include a special purpose processor, an application specific integrated circuit (ASIC), or an array of processors configured to run software to perform a given service.
The first and second packet processing circuits 118-1, 118-2 of FIG. 1 also are directly coupled to each other so that the same data can be cached at both. In some embodiments, a communication path 120 coupling the first and second programmable logic circuits includes a circuit connection compliant with a high speed network communication protocol. In some embodiments, the communication path 120 complies with the Ethernet protocol.
The first programmable logic circuit 118-1 is operably coupled to first cache storage circuitry 106-1. The second programmable logic circuit 118-2 is operably coupled to second cache storage circuitry 106-2. In some embodiments, the first and second cache circuits include DRAM circuits. More particularly, in some embodiments the first and second cache circuits include Flash-backed DRAM circuits in which Flash circuits are coupled to stored data persistently in the event of failure of a corresponding DRAM circuit.
Cache Memory
Cached data and cache metadata can be stored in Flash backed-up DRAM included in cache storage circuitry 106-1 and 106-2. FIG. 3 is a block diagram illustrating a dual inline memory module (DIMM) containing DRAM and flash memory, which in accordance with some embodiments, can be plugged into a standard DDR3 DIMM socket. In some embodiments, during operation the module behaves similar to a standard DDR3 DRAM DIMM, however, upon the occurrence of the certain events, data may be copied between the DRAM and flash memory of the cache storage circuitry 106-1 and 106-2. The entire contents of on-board DRAM may be written to the flash back-up memory upon the occurrence of any of the following:

    
    
        A power supply fault is detected.
        A SAVE command is received via an inter-integrated circuit (I2C) interface on the DIMM socket.
        An interrupt is received via a pin on the DIMM socket.
    
    


The contents of flash back-up memory are written to DRAM upon occurrence of any of the following events:

    
    
        If configured to do so: After power up if a valid saved image is detected in flash.
        A RESTORE command is received via the I2C interface on the DIMM socket.
    
    


Reconfigurable Circuitry
The packet processing circuits 118-1 and 118-2 of FIG. 1 can be configured to include a cache controller to provide cache management services. The circuitry of a cache controller may include a state machine to perform the functions described. This offloads cache control functions to hardware from software to increase cache speed. A cache controller may also be capable of accommodating other hardware accelerators for off-loading CPU software functions to hardware. Some examples of these functions include encryption/decryption, duplication/de-duplication, compression/de-compression, processing, replication, and snapshot.
The software driver (e.g., of a management processor) and cache controller allow for 1:1 redundancy of data storage between cache memory and system flash memory. Data redundancy may also be applied to the cached data by having two cache controllers operate together to mirror the cache data. If one cache controller fails the other cache controller can operate independently, and cached data is preserved without any data loss.
FIG. 4 shows an example of interfaces to a cache controller. A cache controller can include two PCIe interfaces, PCIe CHO and PCIe CH1. A cache controller can appear as a PCIe end-point on two RCs (one end-point per RC). One cache controller and one PCIe switch may reside on one switch card or switch board of a switch fabric. Hosts on the server boards can communicate with the cache controllers via the PCIe switches. A PCIe interface may provide a raw bandwidth of 64 gigabits/sec (64 Gbps). Channel 0 (PCIe CH0) can be electrically coupled to a PCIe switch local to the switch board of the cache controller and Channel 1 (PCIe CH1) can be electrically coupled to a PCIe switch remote from the switch board of the cache controller. A cache controller can include a serial link for inter-cache controller communication between switch boards. In an example, a serial link can include four lanes and have aggregate bandwidth of 10 Gbps.
Each cache controller may include an interface to DRAM that includes the storage space of the cache modules. In the example shown in FIG. 4, the cache controller includes two interfaces labeled DDR3 CH0 and DDR3 CH 1. Two dual in-line memory modules (RDIMMs) can be connected to the cache controller for data and cache table storage. In certain variations, one RDIMM can store 8 gigabytes (8 GB). Other sizes are possible such as 16 GB or 32 GB for example.
FIG. 5 shows a flow diagram of an example of a method 500 of managing access to cache memory using two (e.g., redundant) cache controllers. Each cache controller performs cache management tasks for a cache memory. The cache memory may include one or more cache modules 106-1 and 106-2 shown in FIG. 1. At block 505 of FIG. 5, a first cache command is posted in a first command buffer by a first host device and a second cache command is posted in a second command buffer by a second host device. A host device may be a management processor 116-1 and 116-2 in FIG. 1 and a cache command may be posted by a host executing at the management processor. The command buffers may reside in the host devices and can be separate from the cache controller.
Referring again to FIG. 5 at block 510, the first cache command and the second cache command are loaded into a buffer memory of the first cache controller, and the first cache command and the second cache command are loaded into a buffer memory of the second cache controller. In some examples, the writing of the command buffers initiates the performance or action of a cache controller. At block 515, the order of execution of the first and second cache commands is synchronized in the first and second cache controllers. The synchronizing of the order of execution of the first and second cache commands can include communicating an indication between the first and second cache controllers that the loading of the first and second cache commands is completed. The synchronizing can include communicating the size of the loaded cache commands between the first and second cache controllers. This information may be used by the two cache controllers to coordinate the setting of a pointer for the command buffer memories.
At block 520, both the first and second cache commands are performed using both the first cache controller and the second cache controller. The contents of the data storage of the first cache memory and the second cache memory are substantially identical after the first and second cache commands are performed by both of the first and second cache controllers. This results in redundancy of the cache memory contents. If one cache memory fails or cache data is otherwise corrupted, a duplicate exists. The non-failing cache can operate independently and cached data is preserved without loss of data.
FIG. 6 shows a block diagram of portions of an example of a cache controller 650. Also shown are representations of a left host device 665a and a right host device 665b. An example of a host device is a management processor 116 in FIG. 1 that may be implemented as a server board. The designation left and right are arbitrary and the designation could also be, among other things, a number such as host device 0 and host device 1. The host devices also show representations of a cache command memory 667a,b or command circular ring buffer, read data buffers 668a,b and write data buffers 669a,b. 
In the example shown in FIG. 6, each cache controller 650 includes a cache memory interface 655 (labelled DDR3 I/F for “DDR3 DRAM DIMM Interface”). As shown in the example of FIG. 4, a cache controller 650 may have multiple interfaces to cache memories or cache modules. The cache controller 650 also includes an inter-cache controller communication link 683. The inter-cache controller communication link 683 may be a serial link that provides bidirectional communication with the other cache controller. The cache controller 650 includes two peripheral interfaces. A first peripheral interface 640a is used to communicate with the first host device 665a and a second peripheral interface 640b is used to communicate with the second host device 665b. In some variations, the peripheral interfaces 640a,b are PCIe interfaces. A cache controller 650 and a routing network 102 of FIG. 1 can reside on a switch card, and hosts running on a management processor 116 communicate with the cache controller via the routing network. The first peripheral interface (PCIe-0) can be connected to a first host device via a PCIe interface local to the cache controller 650 and the second peripheral device (PCIe-1) can be connected to the second host device via a PCIe interface remote from the cache controller 650.
The cache controller 650 can include logic circuitry 605. In some variations, the logic circuitry includes hardware circuits that implement a state machine. The logic circuitry may be reconfigurable or non-reconfigurable. The logic circuitry controls operation of the cache controller 650. For instance, the logic circuitry may load a cache command from the cache command memory of the first host device and load a cache command from a cache command memory of the second cache controller. The logic circuitry may trigger the pulling and execution of a cache command when an indication is received that a cache command is loaded into at least one of the cache command memories 667a,b. 
Each host (e.g., left (L) host and right (R) host) posts cache commands to its own command ring buffer within memory of the host device. As a host posts cache commands to the command ring buffer, it updates a tail pointer of the ring to a register within the cache controller memory space. Each cache controller can include a first-in-first-out memory buffer 610 (FIFO) to store cache commands loaded or pulled from the first and second host devices. The cache controller 650 pulls cache commands from the command ring buffers into its on-board FIFOs. This can be done as memory-reads (MEMRD) to the host memory, with the length of the MEMRD command set to the PCIe maximum transmission unit (MTU) or the number of commands on the ring, preferably whichever is less. After each “command pull” the cache controller 650 waits for the other cache controller to perform the same command pull. The logic circuitry 605 can be configured to reorder cache commands received from the hosts and stored in the FIFO of the cache controller to match an execution order of commands of the FIFO of the other cache controller. This ordering ensures that the same sequence of cache commands is performed by both of the cache controllers for commands from both of the left and right hosts.
The cache controller may include a first content addressable memory (CAM) 615 internal to the cache controller 650. The cache controller may include a second CAM 620 including associative memory configured to describe the contents of the cache memory. The first CAM is used to store one or more queues containing cache commands. These queues can be viewed as Virtual Queues (VQs) that include a sequence of cache commands for a specified cache entry index for the cache memory. After re-ordering in a FIFO, cache commands are placed within the VQs based on the command cache key. A cache key is used to locate an entry in the cache and is included in field of a command word to indicate the cache key used in the cache operation. If the command is a Cache Query command, the Cache Key field includes the number of entries returned by the Cache Query command and an index of the last returned valid cache entry. A cache key may be assigned its own queue, and the queues may be serviced in a round-robin fashion. After the cache keys are assigned to queues, the cache command keys of the received commands are matched to the keys assigned to the queues. If a match is found for the key of the received command, the key is placed in the matching queue. If no match is found for a key of a received command, a new queue is created if space allows. When all commands of a queue are completed, the queue is deleted. The first CAM 615 can be relatively small (e.g., large enough to hold 256 command entries or 64 queues). The second CAM 620 can be relatively large (e.g., 4 million entries) and can be used to store metadata for the cache memory contents.
After a cache command operation is fully completed and data is written to either DRAM or host memory, the cache controller may write a status word to the host memory indicating the status of the completed command (an error status can be reported at this stage to indicate a failure during command execution). Once each host receives the status word from both cache controllers, it can free the data buffer memory within its memory space. Although only one cache controller returns cache read data to the host for cache read command, both cache controllers should return the same completion status indication to the host. It is an error condition when only one cache controller returns completion status or if the returned completion statuses are not the same.
FIG. 7 shows a diagram of an example of a sequence of communications for loading cache commands in cache controllers. The Left Host (L-Host) posts cache commands to its command ring buffer and the Right Host (R-Host) posts commands to its command ring buffer. At 705, the Left Host updates a tail pointer (TL_PR update) of the command ring buffer to a register within the left cache controller memory space (L-FPGA-RO-FIFO) and Right Host updates a tail pointer of the command ring buffer to a register within the right cache controller memory space (R-FPGA-RO-FIFO). The tail pointers may be updated using a peripheral interface between the hosts and the cache controllers. The sequence shows the Left Host sending a tail pointer first. This may happen when the Left Host is given priority over the Right Host. At 710, the size of the loaded cache commands is communicated between the left and right cache controllers. This communication may be implemented using a serial link between cache controllers.
Each host posts cache commands to the companion cache controller. At 715, the Left Host updates a tail pointer (TL_PR update) of the command ring buffer to a register within the right cache controller memory space (R-FPGA-RO-FIFO) and Right Host updates a tail pointer of the command ring buffer to a register within the left cache controller memory space (L-FPGA-RO-FIFO). At 720, the size of the cache commands is communicated between the left and right cache controllers and matched by the cache controllers.
At 725, each cache controller sends a command pull request to each host and cache commands are pulled from each host device buffer memory by each cache controller. At 730, an indication the completion of loading of the first and second cache commands is communicated between the left and right cache controllers. At 735, the cache controller transfers the cache commands into virtual queues. The cache commands are shown queued by the left cache controller first and right cache controller second.
The cache commands include cache read (CR) and cache write (CW) commands. FIG. 6 shows read data buffers 668a,b for the host devices 665a,b respectively. For each cache read operation, 4 kilobytes (4 KB) of data is read from the DRAM and returned to the host memory. The block 4 KB is only one option for the block read size and other block sizes are possible. In some examples, the cache controller 650 includes direct memory access engine 625 (DMA). The block of read data is returned to the host memory from the DRAM by the DMA engine. A CR command includes one or more bits to indicate which cache controller will return the read data. If there are two cache controllers designated as left and right, this may include one L/R bit to designate left or right. This prevents both cache controllers from writing the same data to the same read buffer in host memory. Preventing the redundant write preserves the bandwidth at the PCIe and DRAM interfaces.
FIG. 8 shows a diagram of an example of a sequence of communications for cache read commands performed by the left cache controller. The example shown involves a cache hit for a CR command from the left host. At 805, the CR command is executed out of the virtual queue (e.g., in the first CAM 615 of FIG. 6) and CAM is searched (e.g., the second CAM 620) to determine the status of the data in the cache memory. At 810, a block read of cache memory is performed by the left cache controller. The block read can be performed using the DDR3 I/F to DRAM. A block of data (e.g., a 4 KB block) is transferred to buffer memory (e.g., read buffer memory 668a in FIG. 6) and a status word is written to memory of the Left Host. At 815, a second block read is performed for the Left Host. The example in FIG. 8 also includes a CR command for the Right Host, and shows two block reads 820 performed by the left cache controller and transferred to memory of the Right Host together with status words for the read operations.
FIG. 9 shows a diagram of an example of a sequence of communications for cache read commands performed by the right cache controller. As explained above, both cache controllers load the cache commands from each of the Left and Right hosts, and both cache controllers perform the commands. Only one cache controller may return cache read data to a host for a cache read command, but both cache controllers return a completion status word.
Returning to FIG. 6, write data buffers 669a,b are shown for host devices 665a,b respectively. Cache write data can be pre-fetched from the write data buffer 669a,b. In certain variations, the pre-fetching is performed by the DMA 625. Pre-fetching ensures no waiting at the DRAM interface for a PCIe MEMRD operation. The write data is written to the DRAM once the DRAM address is obtained from second CAM 620.
FIG. 10 shows a diagram of an example of a sequence of communications for cache write (CW) commands performed by the left cache controller. After cache commands are pulled from the hosts and transferred from the FIFOs to the virtual queues, a data pull request originates from a virtual queue. At 1005, two data pull requests are sent to the Left Host and CAM is updated accordingly. Each request is for a block of data (e.g., a request for 4 KB of data). At 1010, data pull requests are sent to the Right Host. At 1015, a block of data is pulled by the left cache controller from memory of the Left Host. The data is written to DRAM of cache memory and a completion status word is returned to the Left Host. At 1020, a second block of data is pulled by the left cache controller from memory of the Left Host and a second completion status word is returned to the Left Host when the data is written to cache memory. At 1025, two blocks of data are pulled by the left cache controller from the Right Host. The data is written to cache by the left cache controller and completion status is returned to the Right Host.
FIG. 11 shows a diagram of an example of a sequence of communications for CW commands performed by the right cache controller. Cache write commands from the Left Host and the Right Host are performed by both the left and write cache controllers. This results in duplicate cache data in the cache memories.
Overview of Cache Management Service
FIGS. 12A-12C are illustrative drawings showing example flow diagrams representing caching principles, policies, or rules implemented by the software driver (e.g., of a host on a server board) and cache controller according to some embodiments. The software driver performs cache capacity management operations to ensure that the cache modules always have sufficient capacity to handle I/O requests. If cache capacity is not adequately monitored and optimized, for example, there may not be enough space to fully handle a new write request. Cache management for cache modules are performed independently of each other.
Referring to FIG. 12A, it is determined whether it is time to perform cache capacity maintenance at block 802. Maintenance may be a continuous background operation, a periodic background operation, or on a need basis type of operation. Maintenance frequency can be a system setting, user setting, or dynamic setting based on current operating conditions of the system 100 of FIG. 1. If maintenance is initiated (‘yes’ branch of block 802), then at block 804 the cache controller determines whether the current data storage capacity of the cache modules 106-1 or 106-2 (depending on which set of cache modules is associated with the given cache controller of the packet processing circuit 118-1 and 118-2) is at or above a pre-set maximum capacity level. The pre-set maximum capacity level is a certain value that is pre-set by the system 100 or user, and represents the portion of the total data storage capacity of the cache modules that can be occupied while having a “safe” amount of available space in case, for example, an unexpectedly large write request is received. Examples of pre-set maximum capacity level include, but are not limited to, 70%, 80%, or some other value. In some embodiments, the pre-set maximum capacity level may be adjusted over time as more system usage information becomes available. Instead of expressing the pre-set maximum capacity level as a percentage of the total data storage capacity, for example, it is understood that it can be expressed as a minimum available or reserved free space.
If the current cache capacity is below the pre-set maximum capacity level (‘no’ branch of block 804), then the flow diagram returns to block 802. Otherwise the current cache capacity is too close to the pre-set maximum capacity level, and some of the stored data needs to be moved to the flash modules 110 and evicted or erased from the cache modules 106-1 or 106-2 (depending on which set of cache modules is associated with the given cache controller) (‘yes’ branch of block 804).
Next at block 806, the cache controller determines what data to displace from the associated cache modules according to a set of cache eviction rules, such as according to a Least Recently Used algorithm for example that evicts data that is the least used. Data that is “pinned” stays within the cache module based on a user specified directive.
Once the cache line(s) to empty are identified, at block 808 the cache controller sends data stored in those cache line(s) to the flash modules 110-1 to 110-8 for storage. Such data is erased, emptied, or evicted from those particular cache line(s) at block 810. The flow diagram then returns to block 802. Thus, the cache capacity of cache modules 106-1 or 106-2 (depending on which set of cache modules is associated with the given cache controller) is maintained at or below the pre-set maximum capacity level. It is understood that blocks 808 and 810 may occur simultaneously of each other.
FIG. 12B illustrates operations performed by the software driver and cache controller of the cache management subsystem in connection with a data read request from any of the I/O circuits 112-1 to 112-4. At block 820, a software driver of a host (e.g., a general purpose management processor 116) receives a read request originating from one of the I/O circuits 112-1 to 112-4. Assuming normal operation, the software driver handles read requests originating from I/O circuit 112-1 to 112-4 using the cache controllers included in the packet processing circuits 118-1, 118-2. The data request includes a particular memory address location of the flash modules 110-1 to 110-8 at which the requested data is stored. A command for the data request may be posted in a command buffer of a host device for loading by the cache controllers.
Next at block 822, a look-up of the key in a cache table is performed to determine whether the requested data exists in the cache modules 106-1, 106-2. If a matching cache key is found (a cache hit, or the ‘yes’ branch of block 822), the cache controllers access the data corresponding to the matching cache tag from the cache module 106-1, 106-2 and sends the retrieved data to the software driver and the data is sent to the originating I/O circuit at block 824. The retrieved data is the requested data in the read request. The tracking metrics (e.g., according to a least recently used algorithm) is updated at block 826. If the retrieved data was previously written to the cache module 106-1, 106-2 (in a previous write request) and such data was not evicted from the cache module due to cache management operations (see FIG. 12A), then such data is present in the cache module for later access such as the present read request. When data is present in the cache module there is no need to retrieve the data from the flash modules 110-1 to 110-8. Data retrieval from a DRAM cache is significantly faster than from flash-based memory, upwards of a thousand times faster using cache than flash.
If no matching cache key is found (a cache miss, or the ‘no’ branch of block 822), the requested data is not present in the cache modules 106-1, 106-2 and is retrieved from the flash modules. At block 828, the cache controllers initiate retrieval of the requested data from the appropriate flash modules. Both cache controllers load the cache read command posted by the software driver of host. Both cache controllers perform the command, but only one cache controller may return the cache read data to the host.
Next at block 830, a system setting (or user specified setting) is checked to see whether the requested data retrieved from one or more of the flash modules 110-1 to 110-8, should be copied to the cache modules. If the system is set to not copy the data to cache modules (‘no’ branch of block 830) then the flow diagram proceeds to block 834. Otherwise the retrieved data is copied to the cache modules (′yes' branch of block 830 and block 832). The retrieved data is also sent to the I/O circuit, one of 112-1 to 112-4 that made the read request at block 834. The associated tracking metrics (e.g., for a least recently used algorithm) are also updated at block 836.
FIG. 12C is an illustrative flow diagram representing operations performed by the software driver and cache controller of the cache management subsystem in connection with a data write request from any of the I/O circuits 112-1 to 112-4 in accordance with some embodiments.
At block 840, the software driver of a host receives a write request originating from one of the I/O circuits 112-1 to 112-4, and the request is then passed onto the cache controllers included in the packet processing circuits 118-1, 118-2 (assuming normal operational state of the system 100). The data request includes the data to be written as well as a particular memory address location of the flash modules at which the data is to be written.
At block 842, the software driver determines whether the data associated with the write request is exceptional. While the default rule is to store all data associated with write requests to the cache modules packet processing circuits 118-1, 118-2 and then at some later point in time copy data from the cache modules to the flash modules 110-1 to 110-8, one or more exceptions to the default rule may be implemented. One or more exception criteria may be a system setting or user specified setting. For example, the exception may comprise there being no exception to the default rule. As another example, data exceeding a certain size (e.g., data that if written to the cache modules may exceed the cache capacity or likely to exceed the pre-set maximum capacity level) may warrant storing directly in the flash modules without first storing in the cache modules. As still another example, the write request or the data associated with the write request itself may specify that the data will be rarely accessed (e.g., is archival data) or has a certain characteristic that warrants being stored directly in the flash modules 110-1 to 110-8 without first being stored in the cache modules 106-1, 106-2.
If the data associated with the write request is determined to be exceptional (‘yes’ branch of block 842), then such data is sent to the flash modules for writing 110-1 to 110-8 at block 844. Otherwise the data associated with the write request is not exceptional (‘no’ branch of block 842) and operations are performed to write to the cache modules 106-1, 106-2. At block 846, the cache table is checked for a key of the cache data containing the same flash memory address location as provided in the write request.
If a matching cache key is found (‘yes’ branch of block 846), this means that an older version of the data associated with the write request (or some data in general) is currently stored in the cache line(s) now intended for the data associated with the write request. The cache controllers facilitate overwriting the existing data at these cache line(s) with the data associated with the write request at block 848. Then the flow diagram proceeds to block 852. If no matching cache key is found (‘no’ branch of block 846), then the cache controllers facilitate writing the data associated with the write request to empty/available cache line(s) in the cache modules 106-1, 106-2 at block 850.
Next at block 852, the data associated with the write request is additionally copied to empty/available cache line(s) in the cache modules 106 associated with the other FPGA packet processing circuit. A cache command associated with the write request is posted by the software driver of the host and loaded by both cache controllers. In this way, the write request from the I/O circuit is formed into two identical requests, one going to the packet processing circuit 118-1 and the other to the packet processing circuit 118-2. Then the cache controller in each of the packet processing circuits can store the data associated with the write request (also referred to as write data) in its respective cache modules. At block 854, the associated tracking metrics are updated to reflect the addition of the data associated with the write request into certain cache line(s) of the cache modules.
Because flash modules 110-1 to 110-8 comprise the primary or permanent data storage medium for the storage system 100, the data associated with the write request, although already written to the cache modules 106-1, 106-2 (see blocks 848 and 850), is eventually written to the flash modules 110-1 to 110-8. Nevertheless, cache management of the system 100 is configured to intelligently perform data writes to the flash modules taking into account the characteristics of the flash modules. In order to prolong the usability of flash modules, which are limited to a certain number of writes before degrading, the cache management accumulates certain type of data corresponding to a plurality of write requests and then performs a single write of the accumulated data to flash modules rather than performing a write to flash modules for each write request. This means that if, for example, there are 25 write requests, instead of writing to flash modules 25 times or once for each of the 25 write requests, the data corresponding to these 25 write requests may be written at the same time and once (e.g., a single write operation) to the flash modules.
Accordingly, the cache management acts as a middleman between the I/O circuits 112-1 to 112-4 and flash modules 110-1 to 110-8 for every read and write requests from the I/O circuits. For all read and write requests, the presence of data associated with the read or write request in the cache modules 106-1, 106-2 is checked before the flash modules are involved. Based on the presence or absence of such data in the cache modules, the cache management performs optimization operations to complete the data requests significantly faster than is possible with flash modules alone. The cache management also prolongs the useful lifespan of flash modules by minimizing the number of writes to flash modules without sacrificing completeness of data being stored in the flash modules. Data associated with write requests are written to cache memory (prior to be written to flash modules) unless the data fits an exception. Data associated with read requests that are retrieved from the flash modules may or may not be written to cache modules corresponding to both packet processing circuits (depending upon a system or user setting). The cache management actively maintains the used storage capacity level of the cache modules at or below a pre-set capacity level (e.g., 70%, 80%, etc.) by evicting data stored in the cache modules that fit one or more eviction rules, as needed. An example of an eviction rule comprises evicting data that has the least amount of access (read or write) and moving it to the flash modules.
Recovery from Fault
The system provides for recovery in the event of a power failure in all or a portion of the cache management subsystem. An early warning of the power failure can be generated to initiate preparation for the loss of power. For instance an onboard circuit may generate an indication of a drop in voltage. As explained previously and shown in FIG. 3, the cache storage circuitry can include DRAM and back-up flash. The entire contents of on-board DRAM of the cache storage circuitry may be written to the on-board flash back-up memory of the cache storage circuitry upon the occurrence of the early warning of power failure. The system may include one or more super-capacitors to supply power to the cache module to complete the transfer operation. (A super-capacitor, sometimes called an ultra-capacitor, refers to a capacitor having a different dielectric material than a conventional capacitor (e.g., a non-solid dielectric material) and has an energy density much greater than the energy density of electrolytic capacitors (e.g., 10,000 times)). Any block reads or block writes to the DRAM may be halted by its corresponding cache controller, even if the cache controller is in the middle of performing the block read or block write.
As explained previously regarding the example of FIG. 6, each of the cache controllers of the system 100 in FIG. 1 can include a CAM 615 and a queue implemented in the CAM. The logic circuitry 605 may transfer, to the queue, cache commands posted by the first and second host devices 665a,b such that a sequence of commands in the queue of a first cache controller matches the sequence of commands in the queue of a second cache controller. The sequence of cache commands corresponds to a specified cache entry index for the cache memories.
According to some embodiments, each cache controller may include a port 685 to receive an indication of power failure. The port may be electrically connected to on-board power failure detection circuitry, or the port may be a communication port that provides a connection to a monitoring subsystem interface such as an I2C interface. In certain embodiments, the port may include a SAVE pin and the process of saving the contents of DRAM to the backup flash memory may begin in response to a signal received at the SAVE pin. The logic circuitry 605 of each cache controller may halt the transfer of posted cache commands to its queue in response to the indication of power failure and complete any cache commands in the queue.
Once power is restored after a power interruption, the host devices 665a,b (e.g., servers) recover from the loss of power. According to some embodiments, the host devices each include a port 687a,b to receive an indication of recovery from the power interruption or power failure. For instance, the host's port may be electrically connected to on-board power failure detection circuitry that provides the indication of recovery or the port may be electrically connected to a monitoring subsystem interface that provides the indication of recovery (e.g., via an I2C bus).
In response to receiving the indication, at least one of the host devices 665a,b enters a recovery mode and reads cache metadata of one or both of the cache controllers. The metadata can be included in CAM 620. In some embodiments, reading cache metadata can include posting a cache query (CQ) command. The CQ command may specify reading the next N (N being appositive integer) starting from M (where M is a CAM address). Using the metadata, the host device determines whether a cache entry of cache memory is modified from the corresponding copy of the cache entry in main memory (e.g., flash memory 110 in FIG. 1). This is often referred to as the cache entry being dirty. If the cache data is modified, the host device writes the cache entry to the main memory.
A cache controller 650 can include error checking circuitry 689 such as error correcting code (ECC) or circuitry cyclic redundancy code (CRC) circuitry. In some examples, the error checking circuitry 689 calculates CRC bits for a block of cache data (e.g., a block of 4096 bytes of cache data) written to DRAM of the cache memory, and stores the CRC bits as cache metadata in the Cache DRAM in a section of DRAM that stores meta data corresponding to the cache entry. The loss of power may have caused incorrect data to be in the cache memory, either directly due to the loss of power at the DRAM or because a block write to DRAM of the cache storage circuitry may have been interrupted by the power failure.
When a block of cache data is read from the DRAM, CRC bits for the block of cache data are recalculated. The cache controller compares the recalculated CRC bits of the block of cache data to the CRC bits stored in metadata for the block of cache data. If the CRC bits do not match, the cache controller sends an indication of a CRC error to one or both of the host devices (e.g., using a status word). If a host device receives a CRC error from one cache controller as a result of a block read, the host device may attempt to read the same cache entry from the other cache memory using the other cache controller. If this other cache controller returns the cache entry without a CRC error it can be assumed that the read data can be written to the main memory (e.g., flash memory 110 in FIG. 1).
If both of the host devices write the contents of the cache memory of the second cache controller/memory pair to main memory in the recovery mode, both of the first and second host devices post cache commands to the second cache controller to access the data of the second (the surviving or error-free) cache memory. If both host devices are writing the contents of memory, ownership of the cache entries may be taken into account. According to some embodiments, for every write access to the main memory, the cache memory is checked (e.g., by a cache read command) to determine if the entry is in the cache and whether the cache entry is modified from contents of main memory or dirty. A dirty cache entry is flushed to the main memory if the cache entry is owned by the host device. If the cache entry is tagged for eviction from the cache memory and a host device determines that it is owned by the other host device, the host device may delay a write of a cache entry to the main memory. This gives the host device trying to evict the cache entry the chance to complete its eviction without interruption.
According to some embodiments, only one host device writes the contents of the cache memory of the second cache controller/memory pair to main memory in the recovery mode by posting cache commands to the second cache controller in response to the uncorrectable error condition. The host device may disable the checking of cache entry ownership before eviction of a cache entry. The host device determines whether a cache entry is modified from the corresponding copy of the cache entry in the main memory (a dirty entry), such as by posting or issuing a cache query command for example. If a cache entry is modified, the host device writes the cache entry to the main memory when the cache entry indicates ownership by the same or a different host device.
According to some embodiments, only a portion of the system may experience power failure. If power is lost to only one cache controller (e.g., the first cache controller), one or both of the host devices 665a,b flush valid entries from the other cache memory using the other cache controller (e.g., the second cache memory and the second cache controller). Valid entries in the operational cache memory are written to the main memory 110, and the entries are deleted in the cache memory. When the failing cache controller returns to operation, a host device may use one or more CQ commands to determine valid entries and to determine the ownership of the valid entries and flush valid entries from the returning cache controller/memory pair.
If power is lost to one server and the other server is still operational, the system 100 may treat the situation as though one cache controller has lost power as well. The operational or active server performs the functions of the recovery mode using the other cache controller. In some embodiments, the system may operate normally using the surviving host device and wait for the failing host device to come back online. When the failing host device becomes operable, it may flush those valid cache entries for which it is the owner.
The cache subsystem redundancy allows for recovery from an uncorrectable error condition detected at times other than a power failure. For instance, the error checking circuitry may detect an uncorrectable error in a first cache controller/memory pair (e.g., processing circuit 118-1 and cache storage circuitry 106-1 in FIG. 1) and generate an indication of the uncorrectable error to at least one of the first and second host devices 665a, 665b. In response to the indication, one or both of the host devices enters a recovery mode and write the contents of the cache memory of the second cache controller/memory pair (e.g., processing circuit 118-2 and cache storage circuitry 106-2 in FIG. 1) to main memory. In certain examples, uncorrectable errors are detectable at the peripheral interface 640a,b to a cache controller. If the peripheral interface is a PCIe interface, the CRC may be generated and checked according to PCIe standards. These types of CRC errors may indicate a failure in hardware circuitry of a cache controller and may cause the cache controller to be inoperable.
A device other than a host device or cache controller may detect the uncorrectable condition. In some embodiments, the system includes a system monitoring device or monitoring subsystem. A cache controller includes a communication interface or communication port 685 to communicate contents of one or more operating registers of the cache controller to the system monitoring device. In certain embodiments the communication port 685 is connected to an I2C interface that communicates with a system behaviour monitoring card (BMC). The system monitoring device detects an uncorrectable error condition at a cache controller using the contents of the one or more operating registers of the cache controller. The system monitoring device may communicate that an uncorrectable condition occurred to the host devices. One or both of the host devices writes the contents of the cache memory associated with the other cache controller in response to the communication.
There are other types of uncorrectable errors in addition to those described previously. In some embodiments, a host device detects a timeout of an expected response of a peripheral interface to a cache controller (e.g., PCIe interface 640a) as an uncorrectable error condition. In response to the timeout, one or both host devices may write contents of the cache memory of the other cache controller/memory pair (e.g., processing circuit 118-1 and cache storage circuitry 106-1) to the main memory.
As explained previously, a cache controller may include an inter-cache controller communication link 683 for bidirectional communication with the other cache controller. This communication can be used to synchronize execution of cache commands in a same order in each cache controller. Another type of uncorrectable error is when one cache controller fails to respond to an inter-cache communication with the other cache controller within a specified time duration. The logic circuitry 605 of the first cache controller may halt execution of cache commands when the other cache controller fails to respond. In certain embodiments, the first cache controller may halt pulling cache commands from the command buffers 667a,b of the host devices when the other cache controller fails to respond to a communication. The system monitoring device or a host device may determine whether the inter-cache controller communication link is not operating or whether the companion cache controller is not operating and generate a corresponding indication identifying the problem. If the cache controller is not operating, the card or board that contains the cache controller may have to be replaced. If the inter-cache controller communication link is not operating one or both of the cards or board containing the cache controllers may have to be replaced.
Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. One skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. Moreover, it will be appreciated that various modifications and alterations may be made by those skilled in the art without departing from the scope of the invention.
The Abstract is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.

## Claims

1. A system comprising:
a first host device that includes a first host data buffer and a second host device that includes a second host data buffer;
first and second cache controllers, wherein each of the first cache controller and the second cache controller include:
a cache memory interface;
a first peripheral interface configured for communication with the first host device and a second peripheral interface configured for communication with the second host device;
a first-in-first-out memory buffer (FIFO) configured to store cache commands loaded from at least one of the first and second host devices; and
logic circuitry configured to load one or more cache commands over at least one of the first peripheral interface from a cache command memory of the first host device to the FIFO and the second peripheral interface from a cache command memory of the second host device to the FIFO, and to perform the cache commands, and to provide to a completion status signal to both the first and second host devices in response to at least one of the cache commands;

wherein the first host device is configured to free data in the first host data buffer in response to receiving matching completion status signals from both the first and second cache controllers in response to the at least one of the cache commands;
wherein the second host device is configured to free data in the second host data buffer in response to receiving matching completion status signals from both the first and second cache controllers in response to the at least one of the cache commands; and
wherein at least one of the first host device and the second host device is configured to write contents of at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to a main memory in response to at least one of receiving nonmatching completion status signals from the first and second cache controllers in response to the at least one of the cache commands and receiving no completion signal from at least one of the first and second cache controllers.

2. The system of claim 1, wherein a host device is configured to:
determine whether a cache entry is modified from the corresponding copy of the cache entry in the main memory; and
to write the cache entry to the main memory when the cache entry is modified and the cache entry indicates ownership of the cache entry by the host device.

3. The system of claim 1, wherein a host device is configured to delay a write of a cache entry to the main memory when the cache entry indicates ownership by a different host device and indicates that the entry is tagged for eviction from the second cache memory.

4. The system of claim 1, wherein the at least one of the first host device and the second host device is configured to write contents of at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to main memory is further configured to determine whether a cache entry is modified from the corresponding copy of the cache entry in the main memory and write the cache entry to the main memory when the cache entry indicates ownership by the same or a different host device.

5. The system of claim 1, further including error checking circuitry of a cache controller is configured to detect at least one of an uncorrectable memory error of a cache memory associated with the cache controller, or a cyclic redundancy code (CRC) error detected during a cache block read.

6. The system of claim 1, wherein a host device is configured to detect a timeout of an expected response of a peripheral interface to the first cache controller as an uncorrectable error condition and write contents of the cache memory of the second cache controller/memory pair to the main memory in response to the uncorrectable error condition.

7. The system of claim 1, wherein a cache controller includes an inter-cache controller communication link configured for bidirectional communication with the other cache controller to synchronize execution of cache commands in a same order as the other cache controller, and wherein the logic circuitry of the cache controller is configured to halt execution of cache commands when the other cache controller fails to respond to an inter-cache communication within a specified time duration.

8. The system of claim 1, including a system monitoring device, wherein a cache controller includes a communication interface configured to communicate contents of one or more operating registers of the cache controller to the system monitoring device and the system monitoring device is configured to detect an uncorrectable error condition at the cache controller using the contents of the one or more operating registers, and at least one of the first host device or the second host device is configured to write contents of the cache memory associated with the other cache controller in response to the uncorrectable error condition.

9. A method comprising:
posting a first cache command in a first command buffer by a first host device and posting a second cache command in a second command buffer by a second host device;
performing the first and second cache commands using a first cache controller of a first cache memory and performing the first and second cache commands using the second cache controller of a second cache memory, wherein contents of data storage of the first cache memory and the second cache memory are substantially identical after the performing of the first and second cache commands;
receiving at least one of nonmatching completion status signals from the first and second cache controllers at least one of the first and second host devices in response to the at least one of the cache commands and no completion signal from at least one of the first and second cache controllers;
detecting an uncorrectable error at the first cache controller; and
writing contents of at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to main memory in response to the step of receiving.

10. The method of claim 9, wherein writing contents of at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to main memory includes writing the contents of the at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to main memory by both of the first host device and the second host device.

11. The method of claim 10, including determining, using a host device, whether a cache entry is modified from the corresponding copy of the cache entry in the main memory, and writing the cache entry to the main memory using the host device when the cache entry is determined to be modified and the cache entry indicates ownership of the cache entry by the host device.

12. The method of claim 10, including delaying writing of a cache entry to the main memory when the cache entry indicates ownership by a different host device and indicates that the cache entry is tagged for eviction from the second cache memory.

13. The method of claim 9, wherein writing contents of at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to main memory includes writing the contents of the at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair to main memory by only one of the first host device or the second host device.

14. The method of claim 13, including, determining by the host device writing the contents of the at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair, whether a cache entry is modified from the corresponding copy of the cache entry in the main memory, and writing the cache entry to the main memory using the at least one of the cache memory of the second cache controller/memory pair and the cache memory of the first cache controller/memory pair when the cache entry is determined to be modified and the cache entry indicates ownership by the same or a different host device.

15. The method of claim 9, wherein detecting an uncorrectable error includes detecting at least one of:
an uncorrectable memory error of the first cache memory;
a cyclic redundancy code (CRC) error detected during a cache read command;
a timeout of an expected response of a first peripheral interface of the first cache controller configured for communication with the first host device or a second peripheral interface of the first cache controller configured for communication with the second host device; or
a CRC error detected at one or both of the first peripheral interface and the second peripheral interface.

16. The method of claim 9, including:
loading the first cache command and the second cache command into a buffer memory of a first cache controller of a first cache memory and loading the first cache command and the second cache command into a buffer memory of a second cache controller of a second cache memory;
synchronizing an order of execution of the first and second cache commands in the first and second cache controllers using an inter-cache communication link; and
halting execution of cache commands by the second cache controller when the first cache controller fails to respond to an inter-cache communication within a specified time duration.

17. The method of claim 9, including communicating, by the first cache controller and the second cache controller, contents of one or more operating registers to a system monitoring device, wherein detecting an uncorrectable error at the first cache controller includes detecting the uncorrectable error using the system monitoring device.