Abstract:
Exemplary embodiments include a method for reducing access contention in a flash-based memory system, the method including selecting a chip stripe in a free state, from a memory device having a plurality of channels and a plurality of memory blocks, wherein the chip stripe includes a plurality of pages, setting the ship stripe to a write state, setting a write queue head in each of the plurality of channels, for each of the plurality of channels in the flash stripe, setting a write queue head to a first free page in a chip belonging to the channel from the chip stripe, allocating write requests according to a write allocation scheduler among the channels, generating a page write and in response to the page write, incrementing the write queue head, and setting the chip stripe into an on-line state when it is full.

Description:
BACKGROUND 
       [0001]    The present invention relates to flash-based memory systems, and more specifically, to methods and systems for reducing access contention in flash-based memory systems, or other memory systems exposing similar properties as flash-based memory systems. 
         [0002]    Flash memory is a non-volatile computer storage that can be electrically erased and reprogrammed. Flash-based storage devices such as solid-state drives (SSD) have hardware characteristics in which reads and writes are typically performed in page-sized chunks, typically 2 to 4 KB in size. Erases are typically performed in full blocks where a block typically includes of 64 to 128 pages. Flash memory includes both NOR types and NAND types. Generally, there exist two different types of NAND flash chips: the type based on single-level cells (SLC) store one bit and the type based on multi-level cells maintain multiple voltage levels in order to store more than one bit. A 4 KB page in an SLC-based flash chip has typical read and write times of 25 and 600 μs, respectively. Erasing a full block takes significant longer amount of time and can take 7 ms in enterprise-grade flash chips. These read/write/erase characteristics are valid irrespective of the workload. In contrast, in hard disk drive (HDD)-based storage systems, the seek time limits the random access performance. However, in flash-based storage devices cells must first be erased before they can be programmed (e.g., written). Therefore, the common technique used to hide the block erase latency is to always write data out-of-place and erasing of blocks is deferred until garbage collection is initiated. When an erase command is issued, the chip is busy until the operation completes and there is no way to read or write on this chip during this time, which is referred to as “blocking erase”. The out-of place write strategy requires a special layer called the Flash Translation Layer (FTL), which maintains the mapping between logical block addresses (LBA) and the actual physical page/block addresses (PBA) in the Flash memory. 
         [0003]    Access time to the flash-based storage device can still expose delay variations.  FIG. 1  illustrates a plot  100  of probability density function versus latency for a simulated prior art system.  FIG. 2  illustrates a plot  200  of cumulative distribution function versus latency for a simulated prior art system.  FIGS. 1 and 2  illustrate a flash simulator in a high load scenario. In the example, when a block is being erased, a subsequent read request on the same chip might have to wait up to 7 ms to be serviced. Similarly, a read request can be delayed up to 600 us by an ongoing write request. Such significant delays are in certain environments not acceptable. In addition, certain countries (e.g., Japan) even force maximum delay bounds (˜5-10 ms) for specific IT applications. Hence the potential total delays may exceed the required delay bounds. Moreover, traditional DRAM memory technologies don&#39;t have the same restrictions as flash and hence provide significant more homogeneous access delays. Flash cache solutions try to replace expensive battery-backed DRAM memory, called NVRAM, with Flash. Hence, the characteristics of such a flash cache are similar to the ones of those memory technologies. It is therefore beneficial but not trivial to provide such characteristics with Flash-based memory technologies. 
       SUMMARY 
       [0004]    Exemplary embodiments include a method for reducing access contention in a flash-based memory system or a memory systems exposing similar properties as flash-based memory systems, the method including selecting a chip stripe in a free state, from a memory device having a plurality of channels and a plurality of memory blocks, wherein the chip stripe includes a plurality of pages, setting the chip stripe to a write state, setting a write queue head in each of the plurality of channels, for each of the plurality of channels in the flash stripe, setting a write queue head to a first free page in a chip belonging to the channel from the chip stripe, allocating write requests according to a write allocation scheduler among the channels, generating a page write and in response to the page write, and incrementing the write queue head. 
         [0005]    Additional exemplary embodiments include a computer program product for reducing access contention in a flash-based memory system, the computer program product including instructions for causing a computer to implement a method, the method including selecting a chip stripe in a free state, from a memory device having a plurality of channels and a plurality of memory blocks, wherein the chip stripe includes a plurality of blocks, and blocks include a plurality of pages, setting the ship stripe to a write state, setting a write queue head in each of the plurality of channels, for each of the plurality of channels in the flash stripe, setting a write queue head to a first free page in a chip belonging to the channel from the chip stripe, allocating write requests according to a write allocation scheduler among the channels, generating a page write and in response to the page write, and incrementing the write queue head. 
         [0006]    Further exemplary embodiments include a memory device, including a plurality of channels having a write allocation scheduler, at least one chip stripe communicatively coupled to each of the plurality of channels, the at least one chip stripe has a free state, a write state, an online state, and an erase state, wherein the at least one chip stripe is configured to be set from the free state to the write state to allocate write requests, and is further configured to be set from the write state to the online state to serve read requests. 
         [0007]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  illustrates a plot of probability density function versus latency for a simulated prior art system; 
           [0010]      FIG. 2  illustrates a plot cumulative distribution function versus latency for a simulated prior art system; 
           [0011]      FIG. 3  illustrates an exemplary flash architecture in which exemplary access contention reduction methods can be implemented; 
           [0012]      FIG. 4  illustrates an exemplary state diagram showing the exemplary four states and respective transitions as described herein; 
           [0013]      FIG. 5  illustrates a flow chart of a method for reducing access contention in flash-based memory systems in accordance with exemplary embodiments; 
           [0014]      FIG. 6  illustrates a flow chart for an exemplary garbage collection method; and 
           [0015]      FIG. 7  illustrates a flow chart for an exemplary garbage collection method illustrating further detail from  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In exemplary embodiments, the systems and methods described herein reduce/eliminate access contention caused by erase and write delays of flash-based memory systems. The exemplary embodiments described herein discuss flash-based memory systems as an illustrative example. It is understood that in other exemplary embodiments, the methods described herein can be applied to any memory system that exposes similar properties in terms of access times and blocking write or erase operations than flash-based memory systems. In exemplary embodiments, flash-based storage device architecture is capable of delivering stringent delay bounds for read and write requests. In addition, the architecture provides efficient garbage collection and wear-leveling, especially for flash cache solutions. In exemplary embodiments, the systems and methods described herein completely hide additional delays from preceding operations on the same chip by decoupling read, write, and erase operations. To protect read and writes on a chip from additional delays caused by a preceding erase operation on the same chip, read and write operations are only done on chips where no erases are taking place while still providing access to all information stored, (i.e., the full LBA space of the device). Similarly, methods described herein can be implemented to protect read operations from being penalized by erase and write delays of preceding requests on the same chip. In exemplary embodiments, read operations are protected from penalization implementing an architecture, where flash chips are attached to channel busses and grouped into chip stripes among channels. This architecture is combined with a write strategy based on a per channel free-block queue with a write page allocator and a garbage-collection algorithm. 
         [0017]    A flash-based storage device such as a Flash Cache or an SSD includes a number of channels to which Flash chips are attached. Each channel has a low-complexity channel controller that manages all requests to the chips. The channels are controlled by a dedicated channel controller that performs read, write, and erase operations on chips and potentially other operations such as adding or verifying error correction information. The channel controllers are controlled by the main controller that performs the main tasks such as LBA-to-PBA mapping, garbage collection, free and bad-block management.  FIG. 3  illustrates an exemplary flash architecture  300  in which exemplary access contention reduction methods can be implemented. As described herein, the architecture  300  includes a main controller  305  for multiple lower complexity channel controllers  310 ,  315 ,  320 ,  325 . In exemplary embodiments, the main controller  305  maintains one free-block queue and a write queue head for each channel. The free-block queue holds all blocks that can be used for writing. The write queue head points to the first free page in the block where data is currently being written. The free-block has been previously taken from the free-block queue of the channel, as further described herein. In exemplary embodiments, each channel controller  310 ,  315 ,  320 ,  325  includes a respective channel of chips. As illustrated, by way of example, controller  310  controls channel  1 , controller  315  controls channel  2 , controller  320  controls channel C- 1 , and controller  325  controls channel C. In addition, each channel controller  310 ,  315 ,  320 ,  325  is communicatively coupled to its respective channel by a channel bus  311 ,  316 ,  321 ,  326 . Each channel also includes chips, which are generally shown as “Chip”. In exemplary embodiments, the write mode strips (e.g., Chip Stripe n) can include write head chips. In addition, the GC stripes (e.g., Chip Stripe n+2) can include garbage collector head chips. 
         [0018]    Besides the channel structure, chips are virtually grouped into so called chip stripes, shown as Chip Stripe  1 , Chip Stripe  2 , Chip Stripe n, Chip Stripe n+1, Chip Stripe n+2, Chip Stripe N−1, Chip Stripe N. As such, the first chip in each channel belongs to the first chip stripe and the like. In exemplary embodiments, a chip stripe is in one of four states: online, garbage collection (GC), free, or write state.  FIG. 4  illustrates an exemplary state diagram  400  showing the exemplary four states and respective transitions as described herein. In the example shown in  FIG. 3 , Chip Stripe  1 , Chip Stripe  2 , Chip Stripe N−1 and Chip Stripe N are “Online”, implemented for read requests, Chip Stripe n is a write mode stripe, Chip Stripe n+1 is free, and Chip Stripe n+2 is in a GC state. These states are illustrative, and other states are contemplated in other exemplary embodiments. 
         [0019]    In exemplary embodiments, when a chip stripe is in online state, data can be read from all its chips. A chip stripe in free state holds chips with all blocks freshly erased and ready to be written. A chip stripe in write state consists of chips on which data is currently being written. In a preferred embodiment there is only one chip stripe in write state at any time. However, in certain circumstances it can make sense to have more than one chip stripe in write state as described further herein. 
         [0020]    In exemplary embodiments, chip stripe in garbage-collection state (GC state) includes chips currently being cleaned up in order to prepare them for writing. Preferably there is no more than one chip stripe in GC state. As such, GC can occur in a single chip in each channel at any time. In addition, there can be more than one chip stripe in GC state, which is desirable if GC is accelerated. The number of chip stripes in GC state can also be dynamic (i.e., adaptive to the workload). A significant amount of storage space is reserved in the GC state and hence can not be used to store actual data during GC. In exemplary embodiments, the GC state can have sub-states, such as a “cleaning” and an “erasing” state. The cleaning state denotes the state where yet valid pages are moved to new locations in the write stripe and the erasing state denotes the state where blocks are actually being erased after all valid pages have been moved to new locations. In the cleaning state, a user read request can still be served from the chip stripe. Once the chip stripe changed to the erasing state, all data on that chip stripe has been moved out and must be served from the new location. It is desirable to prevent serving user read requests from a stripe in erase state. As such, read request can be served from a new location. In exemplary embodiments, erasing can be interrupted on a single chip but continue on the entire stripe. 
         [0021]      FIG. 5  illustrates a flow chart of a method  500  for reducing access contention in flash-based memory systems in accordance with exemplary embodiments. In exemplary embodiments, when the method  500  commences all blocks are free and hence all chip stripes are in the free state at block  505 . At block  510 , before data can be written to the device, a free chip stripe is selected and set into write mode. For example, the first free chip stripe on the flash chip can be selected, but any other free chip stripe can be selected at block  510 . At block  515 , for each channel, a free block is chosen from the corresponding free block queue. In exemplary embodiments, the first free block in the given chip stripe of each channel is chosen but any other free block in the given chip stripe and channel could be chosen as well. At block  520 , for each channel, the write queue head is set to the first free page in the chip belonging to that channel from the chip stripe in write mode. At block  525 , the method  500  checks whether the device has been stopped. If so, the method  500  terminates. If the device is still running, at block  530 , a write request is then allocated according to the write allocation scheduler among the channels. In exemplary embodiments, a round-robin scheduler can be implemented, which distributes the write workload evenly among all available channels. Depending on the desired properties, other schedulers can be envisaged in other exemplary embodiments. At block  535 , once a page has been written, the write queue head is incremented in order to set it to the next free page in the block. At block  540 , the method  500  checks if a chip stripe is full or not. In case the chip stripe is full, block  545  determines if there are enough free stripes available according to a preconfigured value. If there are enough remaining chip stripes at block  545 , then the method continues as block  555 . If there are not enough remaining chip stripes at block  545 , then garbage collection method  600  (See  FIG. 6  herein) is triggered at block  550  in order to reclaim space from invalidated pages in blocks that have been written previously and method  500  continues at block  555 . Note that in a preferred embodiment, garbage collection method  600  runs independently and in parallel to method  500 . At block  555 , it is known that the current stripe is full and it is set into on-line state. Method  500  then selects the next higher chip stripe that is free in block  510 . In other exemplary embodiments, the selection of the next write chip stripe can be performed according to some policy. In case the chip stripe is not yet full, block  560  checks if the current block is full or not. In case the block is not yet full, the method  500  continues at block  525 . When a block is full, at block  565 , garbage collection is triggered in case there are not enough free blocks according to a predefined value in block  570  and the next free block is taken from the free block queue associated to this channel at block  575 . In exemplary embodiments, the free-block queue can be maintained by keeping the blocks in the free block queue in the same order as their address in the chip (while removing all bad blocks). 
         [0022]    In exemplary embodiments, GC can be triggered when the last free chip stripe is changed to write state or earlier based on a configurable threshold (i.e., based on the total number of free blocks), the number of free chip stripes that should be available in the system. In exemplary embodiments, the larger the threshold, the higher is the reserved memory space that is put aside and hence decreases the overall capacity available for reading and writing. The threshold can be set larger than zero to better accommodate write bursts. Similarly GC can be stopped when enough free pages are available. In exemplary embodiments, the method  500  can be re-initiated as needed. 
         [0023]      FIG. 6  illustrates a flow chart for an exemplary garbage collection method  600 . Method  600  is triggered by method  500  in blocks  550  and  570  (see  FIG. 5  above). It first checks, whether GC is already running or not. In case GC is already running, the already running GC method  600  frees enough space and there is no need to start another instance of the method  600 . In case the GC is not running, garbage collection is commenced at block  610  and ceased at block  615 . The detailed GC method is described in method  700  (see  FIG. 7  herein). At block  620 , after the termination of method  700 , method  600  checks if enough free blocks and chip stripes are available or not. The method  600  triggers another GC round if this is not the case, method  600  ceases. 
         [0024]      FIG. 7  illustrates a flow chart for an exemplary garbage collection method  700  in detail. In exemplary embodiments, the GC method  700  chooses the oldest chip stripe when it initiates. At block  705 , the method  700  sets the selected chip stripe to the cleaning state. The method  700  then performs a series of blocks for all the pages in the selected chip stripe. At block  710 , the method  700  reads the page meta-data and LBA-to-PBA mapping information. At block  715 , the method  700  determines if the page is still valid. If the page meta-data and the LBA-to-PBA mapping information indicate that the page is still valid, then the page is written to a new place. As such, if the page is still valid at block  715 , then at block  720 , the method  700  reads the full page data. At block  725 , the method  700  obtains a target location for the page from the write allocation scheduler. In exemplary embodiments, the write allocation scheduler can allocate the new location in the same channel or in another one. Furthermore, if the channel controller is capable of moving data directly in a channel, then the channel controller is preferred to move the data as it doesn&#39;t consume resources from other busses. At block  730 , the method  700  writes the page to the new location. If at block  715 , the page is no longer valid, no action is taken and the space that the invalid page occupies is automatically reclaimed later in the method  700  because the invalid page is not moved and thus is later erased. At block  735 , the method determines if there are remaining pages. If at block  735 , there are remaining pages, then the method  700  continues at block  710 . If there are no remaining pages at block  735 , then at block  740 , the method  700  sets the chip stripe to the erase state. At block  745 , the method  700  erases all blocks in the chip stripe. In exemplary embodiments, the channel controllers can perform erase operation of the blocks in parallel. In addition, each channel controller can perform block erases on a chip in the background, such that the respective channel is not blocked while blocks are being erased. At block  750 , the method  700  changes the state of the chip stripe to the free state. At block  755 , the method  700  determines on which chip stripe GC will run next time. 
         [0025]    In exemplary embodiments, chip stripes can be split into sub-strips depending on the number of chips on a given channel. In addition, the stripe size (i.e., the number of chips in the same channel that belong to the same chip stripe) is selected to effect channel bandwidth. Bandwidth management is particularly desirable when the channel controller supports pipelining of read and write operations on multiple chips. Hence more than one chip on the same channel might be associated to the same write stripe. Moreover, different stripe sizes can be supported for each chip stripe state. 
         [0026]    In exemplary embodiments, user read requests can be protected from delays caused by ongoing erase and write operations on the same chip. User data being written in the write chip stripe is kept in the main controller&#39;s write cache until the full chip stripe has been written. User read requests for this freshly written data are then served from the write cache and hence potential delays due to preceding writes are eliminated. However, the main controller can require a huge write cache that is capable of holding a full chip stripe in order to perform these operations. In exemplary embodiments, in order to circumvent a huge write cache, two alternating write stripes can be implemented. When the write cache is de-staged to flash, the data is first written to the first write stripe, and then this write stripe is temporarily switched to the on-line state from where it can be accessed. The updates in the LBA-to-PBA map are maintained in a dedicated mapping block. The same data is then written to the second write stripe and a second LBA-to-PBA mapping block containing the changes is created. After completion of the write, the second write stripe is switched to the on-line state (the corresponding LBA-to-PBA mapping block is set active). The first chip stripe is switched back to write state and data can be de-staged from the write cache and so on. Once the write chip stripes are full, one of them is kept in on-line state, as well as the corresponding LBA-to-PBA mapping table. The other chip stripe can be erased and set to the free state or used for redundancy or parallel reads (same arguments as RAID 1). This type of operation doubles the total number of flash writes. 
         [0027]    In some configurations there might be only a few chips on a single channel. Maintaining a full chip stripe for GC then becomes expensive in terms of reserved storage capacity that can not be used to hold user data. In such a configuration it is beneficial to group multiple channels into channel groups and splitting the chip stripes into sub-stripes. Hence one sub-stripe only includes chips from a single channel group. Then the same algorithm described herein can be applied on a sub-stripe instead of a full chip stripe. This approach reduces the erase speed as fewer chips can be erased in parallel. However, if the write workload is not high this approach can be implemented. As such, the sub stripe sizes can be dynamically adjusted depending on the observed write workload. 
         [0028]    In exemplary embodiments, a channel as described herein can be either a physical or virtual channel. Several physical channels can be grouped into one virtual channel. Hence the systems and methods described herein apply to both, physical and virtual channels. 
         [0029]    In exemplary embodiments, the GC, free, on-line, and write states move sequentially through all chip stripes, which can be referred to as a circular buffer. A circular buffer architecture has the advantage to equally distribute the wear of blocks on chip granularity, hence reducing the complexity of the scheme by not requiring additional wear-leveling methods. In the examples described herein, a round-robin strategy for allocating pages to be written is a solution that can be implemented in a circular buffer. Nevertheless, due to bad blocks the write queue head may become out of sync between the different channels. In such a case, extra rounds can be inserted in the round-robin scheduler where only pages are allocated in queues that are behind the head of the foremost write queue head. 
         [0030]    In exemplary embodiments, in case workloads that can be separated into static and dynamic data, a circular buffer architecture may cause data to be unnecessarily moved around during GC. As such, another example of a policy that can be implemented is a window-based greedy reclaiming policy. The methods described herein can be adapted to fit into such a window-based greedy reclaiming policy by dynamically grouping chips into chip stripes as well as dynamically adapting the chip stripe size according to the information maintained by GC. 
         [0031]    Technical effects include completely hiding additional delays from preceding operations on the same chip by decoupling read, write, and erase operations. Furthermore, read and write operations are protected on a chip from additional delays caused by a preceding erase operation on the same chip. Technical effects further include ensuring that read and write operations are only done on chips where no erases are taking place while still providing access to all information stored. In addition, read operations are protected from being penalized by erase and write delays of preceding requests on the same chip. 
         [0032]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
         [0033]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
         [0034]    The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
         [0035]    While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.