# Patent

## Title
Advanced management of a non-volatile memory

## Abstract

A method of managing a non-volatile memory device, the method comprising: receiving data sectors; writing each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device; maintaining a management data structure that comprises location metadata about a location of each data sector in the buffer; and merging, if a criterion is fulfilled and before the buffer becomes full, data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer.

## Background
RELATED APPLICATIONS
This application is a continuation in part of U.S. patent application Ser. No. 13/434,083, filing date Mar. 29, 2012, titled “SYSTEM AND METHOD FOR FLASH MEMORY MANAGEMENT”, which claims priority from U.S. provisional application Ser. No. 61/485,397, filing date May 12, 2011, the disclosures of which are expressly incorporated by reference herein in their entireties.


BACKGROUND
Flash memory devices store information with high density on Flash cells with ever smaller dimensions. In addition, Multi-Level Cells (MLC) store several bits per cell by setting the amount of charge in a cell. Flash memory devices are organized into (physical) pages. Each page includes a section allocated for data (512 bytes-8 Kbytes and expected larger in the future) and a small amount of spare bytes (64-512 or more bytes for every page) for storing redundancy and metadata. The redundancy bytes are used to store error correcting information, for correcting errors which may have occurred during flash lifetime and the page read process. Each program operation is performed on an entire page. A number of pages are grouped together to form an Erase Block (erase block). A page cannot be erased unless the entire erase block which contains it is erased.
One common application of flash memory devices is Secure Digital (SD) cards and embedded Multi-Media Cards (eMMC). An SD/eMMC card may typically contain flash memory devices and a flash memory controller. The controller translates commands coming in through the SD/eMMC interface into actions (Read/Write/Erase) on the flash memory devices. The most common SD/eMMC commands may be Read and Write commands of one or more sectors, where a sector may be, but is not limited to, a sequence of 512 bytes. The Read or Write commands may be of a single sector or multiple sectors. These commands may refer to logical addresses. These addresses may then be redirected to new addresses on the flash memory which need not directly correspond to the logical addresses that might be referenced by the Read or Write commands. This is due to memory management that may be carried out by the flash memory controller in order to support several features such as wear-leveling, bad block management, firmware code and data, error-correction, and others. The erase function is performed on an entire erase block. Because of this functionality, before the data of a certain block may be replaced such as during a write function, the new data must be written in an alternative location before an erase can occur, to preserve the integrity of the stored data.
Due to the small dimensions of a typical SD/eMMC card and the price limitations, the controller may typically have only a small RAM available for storage. The small size of the RAM memory limits the type of memory management which may be carried out by the controller with regard to the data stored in the flash memory device and received from the interface.
The controller may typically manage the memory at the erase block level, because managing data of small particle sizes becomes difficult. That is, the logical memory space may be divided into units of memory contained within a single erase block or some constant multiple of erase blocks, such that all logical sector addresses within each said unit of memory may be mapped to the same erase block or some constant multiple thereof.
This type of management has the drawback that for writing random access data sectors to memory or other memory units smaller than an erase block, erase blocks must be frequently rewritten. Because of the characteristics of flash memory, each new piece of information is written into an empty page. In flash memory a page may not be rewritten before the entire erase block is erased first.
If a portion of the memory unit contained within an erase block may need to be rewritten, it is first written into a freshly allocated erased erase block. The remaining, unmodified, contents of the erase block may then be copied into the new erase block and the former erase-block may be declared as free and may be further erased. This operation may be referred to as “sealing” or “merging”. The operation involves collecting the most recent data of a logical block and then merging it with the rest of the block data in a single erase block. Thus, even if a single sector from an erase block is rewritten, a complete erase block would be rewritten.
This may result in causing a significant degradation in the average write speed. It may also impose a significant delay in the response time between random write sector operations. It also may cause excessive P/E (program/erase) cycling, which may be problematic in new generations of flash memory devices where the number of P/E cycles is limited to a few thousand or even a few hundreds.
The controller is used to manage the overhead described above, and must always keep track of the data associated with each logical address and the actual memory location. This is usually achieved by implementing a mapping method between the logical address space assigned to the data and the actual memory storage location of the data.
Several methods may be implemented to execute such a mapping. Two approaches implement mapping systems that rely on block mapping and page mapping, respectively. In an approach using block mapping, each physical block in the flash memory is mapped to a contiguous logical memory block of the same data size (LBA). In this approach when one page in some logical block is updated, the entire associated physical block must be copied to a fresh block, and the new data must be written in place of the obsolete copy. A merge may be an operation where the original content of a logical block is merged with the new data to form a new up to date copy of the block. This up to date copy is the data block that is associated with a logical data block assigned to the data contained within. In the second approach, each logical page of a logical block is mapped to an arbitrary physical page where two pages belonging to the same logical block can reside in different physical blocks of the flash memory. The second approach requires additional complexity in terms of the amount of management data and memory overhead required for the physical memory to logical address mapping tables. For memory applications where severe limitations exist on available control memory, this approach is less appropriate. Flash memories such as SD/eMMC have limited amount of memory overhead and the first mapping approach, or variants thereof are more practical.
SUMMARY
According to an embodiment of the invention various methods, non-transitory computer readable media and systems may be provided and are described in the specification.
A method of managing a non-volatile memory device, the method may include: receiving data sectors; wherein each data sector belongs to a memory space subset out of multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks; writing each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device; maintaining a management data structure that may include location metadata about a location of each data sector in the buffer; and merging, if a criterion is fulfilled and before the buffer becomes full, data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer.
The set of logical memory blocks may include a single logical memory block.
The method may include allocating data blocks to the multiple memory space subsets; wherein the allocating may include maintaining at least one non-full data block to each memory space subset.
The method may include partitioning a memory space to the multiple memory space subsets.
The method may include partitioning the memory space to uneven sized memory space subsets.
The method may include partitioning the memory space to evenly sized memory space subsets.
The method may include determining a number (n) of the multiple memory space subsets in response to at least one out of: (a) an aggregate amount of data sectors received between consecutive merging operations; (b) a time required to write to the sequential portion all data blocks stored in the buffer, and (c) an amount of gap required to prevent a loss of data sectors due to a temporary absence of data blocks ready to receive new data sectors.
The method may include: computing, for each suggested number of memory space subsets out of multiple suggested numbers, a quantity in response to: (a) a number of transaction timeout durations required to perform a merge of a logical memory block; (b) an instantaneous number of logical memory blocks which have at least one associated data sectors in the buffer; (c) a number of data sectors in a data block of the buffer; (d) the suggested number of memory space subsets; (e) a time duration for writing a data sector; (f) a number of data blocks in the buffer, and (g) a time duration for performing a merge of a logical memory block; and selecting, out of the multiple suggested numbers of memory space subsets, a number that yields a highest value quantity.
The method may include partitioning the memory space so that each memory space subset may include an integral number of logical memory blocks.
The criterion is fulfilled when a number of free data block of the buffer falls below a predetermined threshold.
The method may include determining the predetermined threshold in response to: (a) a number of transaction timeout durations required to perform a merge of data sectors that belong to a certain logical memory block; (b) an instantaneous number of logical memory blocks which have at least one associated data sector in the buffer; (c) a number of memory sectors in a data block of the buffer, and (d) a number of memory space subsets.
The method may include selecting at least one logical memory blocks that form the set of logical memory blocks that stores the data that is being merged
The method may include selecting the set so that the set may include at least one logical memory blocks that is associated with a maximum amount of data blocks.
The selecting of the set may include: determining a permutation transformation on a set of logical block indices; selecting a set of indices; and computing the logical block indices by employing the permutation on the set of indices.
The merging of the data sectors into the sequential portion may include: merging data sectors into a plurality of erase block sections of data; writing the plurality of erase block sections of data to a plurality of erase blocks of the sequential portion; and creating for each data sector a pointer in the data management structure, the pointer corresponds to an associated logical address and to a location of an erase block in which the data sector is stored.
The method further may include performing a cleaning process that may include writing data sectors that are associated with a group of one or more logical memory blocks and are stored at one or more data blocks to one or more other data blocks.
The method may include updating pointers associated with the data sectors to reflect the cleaning process.
The method may include determining the one or more other data blocks.
The method may include determining the one or more other data blocks by: computing a quantity in response to (a) an instantaneous number of logical memory blocks which have at least one associated data sector in the buffer; (b) a number of data blocks in the buffer, and (c) a predefined constant; computing, based on said quantity and on at least one previously computed quantities, an integer number of logical memory blocks to be cleaned.
The method may also include selecting (based on said number of logical blocks to be cleaned or regardless of the number of logical blocks to be cleaned), the one or more other data blocks.
The selecting of the logical memory blocks may include: determining a permutation transformation on a set of logical memory block indices; selecting a set of indices; and computing logical memory block indices by employing the permutation on the set of indices.
The selecting of a set of indices may include the selecting of a contiguous set of integers starting from a next index after an index which was last used for either merging or cleaning.
The selecting of a set of indices may include the selecting of a contiguous set of integers starting from the next index after the index which was last used for either merging or cleaning, wherein indices which correspond to logical block indices which were never merged since they were last cleaned are omitted.
The method may include copying content of a first data block to a second data block.
The copying may be executed in response to a fullness level of the first data block.
A method of managing a non-volatile memory device, the method may include: receiving data sectors; wherein each data sector belongs to a memory space subset out of multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of memory space fragments; wherein each memory space fragment may include logical memory blocks; writing each data sector into a data block that is allocated to a memory space fragment that is associated with the data sector; wherein the data block belongs to a sub-buffer; wherein multiple sub-buffers form a buffer of the non-volatile memory device; maintaining a management data structure that may include location metadata about a location of each data sector in the buffer; and merging, if a criterion is fulfilled and before a certain sub-buffer becomes full, data sectors stored at different data blocks of the certain sub-buffer, the data sectors belong to a same set of logical memory blocks, wherein the merging is done into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer.
According to an embodiment of the invention there may be provided a method of managing a non-volatile memory device, the method may include determining an integer number n; partitioning the user space into n user space subsets; opening n data blocks of a buffer of the non-volatile memory device for writing and associating each data block with a user space subset; receiving data sectors; writing each data sector into one of the said n data blocks according to the logical address of the said data sector; creating pointers in a data management structure for each data sector, corresponding to an associated logical address and a storage location of the data sector in the buffer; detecting that a first predefined criterion is fulfilled; wherein the first predefined criterion is fulfilled before the buffer becomes full; opening, in response to the detecting, a new data block and associating it with one of the user space subsets; detecting that a second predefined criterion is fulfilled; wherein the second predefined criterion is fulfilled before the buffer becomes full; determining, in response to the detecting that the second predefined criterion is fulfilled, a set of logical blocks to be merged; writing, in response to the detecting and to the determining, data sectors corresponding to the determined logical blocks, from the buffer to a primary non-volatile data storage memory of a non-volatile memory device; wherein the non-volatile memory device mandates erasing erase blocks of data.
The method wherein partitioning of the user space into n user space subsets is done by dividing all logical blocks to n subsets, such that each subset includes an integral number of logical blocks.
The first predefined criterion may be fulfilled when a data block out of multiple data blocks of the buffer becomes full.
The second predefined criterion may be fulfilled when the number of free data block out of multiple data blocks of the buffer falls below some threshold.
The method may include determining of said threshold in response to (a) a number of transaction timeout durations required to perform a merge of a logical block; (b) an instantaneous number of logical blocks which have at least one associated data page in the buffer; (c) a number of memory pages in a data block of the buffer, and (d) a number of user space subsets.
The buffer of the non-volatile memory device may be an extended random sector buffer.
The determining of an integer n may include: computing a quantity in response to (a) a number of transaction timeout durations required to perform a merge of a logical block; (b) an instantaneous number of logical blocks which have at least one associated data page in the buffer; (c) a number of memory pages in a data block of the buffer; (d) a number of user space subsets; (e) a time duration for writing a unit of data; (f) a number of data blocks in the buffer, and (g) a time duration for performing a merge of a logical block; repeating the computing for a set of numbers of user space subsets; and determining the value of n as the number of user space subsets for which the said computed quantity is highest.
The determining of the a set of logical blocks to be merged may include selecting some or all of the logical blocks belonging to the user space subset to which a maximum amount of data blocks are associated.
The writing of the data sectors from the buffer to the primary flash data storage memory may include: merging the data sectors into a plurality of erase block sections of data; writing the plurality of erase block sections of data to a plurality of erase blocks in the primary flash data storage memory; and creating for each data sector a pointer in the data management structure, corresponding to an associated logical address and a storage location of the erase blocks in the primary flash data storage memory.
A method of managing a non-volatile memory device, the method may include: receiving data sectors; writing each data sector into a data block of a buffer of the non-volatile memory device; creating pointers in a data management structure, for each data sector corresponding to an associated logical address and a storage location of the data sector in the buffer; detecting that a predefined criterion may be fulfilled; wherein the predefined criterion may be fulfilled before the buffer becomes full; determining a first set of logical blocks to be merged; writing, in response to the detecting and to the determining of a first set of logical blocks, data sectors corresponding to the set of logical blocks to be merged, from the buffer to a primary non-volatile data storage memory of the non-volatile memory device; wherein the non-volatile memory device mandates erasing erase blocks of data; determining a second set of logical blocks to be cleaned; and writing, in response to the detecting and to the determining of a second set of logical blocks data sectors corresponding to the set of logical blocks to be cleaned, from one storage location in the buffer to another storage location in the buffer.
The predefined criterion may be fulfilled when the number of free data block out of multiple data blocks of the buffer falls below some threshold.
The method may include determining of said threshold in response to (a) a number of transaction timeout durations required to perform a merge of a logical block; (b) an instantaneous number of logical blocks which have at least one associated data page in the buffer, and (c) a number of memory pages in a data block of the buffer.
The writing of the data sectors from the buffer to the primary flash data storage memory may include merging the data sectors into a plurality of erase block sections of data; writing the plurality of erase block sections of data to a plurality of erase blocks in the primary flash data storage memory; and creating for each data sector a pointer in the data management structure, corresponding to an associated logical address and a storage location of the erase blocks in the primary flash data storage memory.
The method wherein the writing of the data sectors from one storage location in the buffer to another storage location in the buffer may include: writing the data sectors into the next available storage location in the buffer; and creating pointers in a data management structure, for each data sector corresponding to an associated logical address and a storage location of the data sector in the buffer.
The method wherein determining a first set of logical blocks to be merged may include: computing a quantity in response to (a) an instantaneous number of logical blocks which have at least one associated data page in the buffer; (b) a number of data blocks in the buffer, and (c) a predefined constant fraction; computing, based on said quantity and on all previously computed such quantities, an integer number of logical blocks to be merged; and selecting, based on said number of logical blocks to be merged, a set of logical blocks to be merged.
The determining of a second set of logical blocks to be cleaned may include: computing a quantity in response to (a) an instantaneous number of logical blocks which have at least one associated data page in the buffer; (b) a number of data blocks in the buffer, and (c) a predefined constant faction; and computing, based on said quantity and on all previously computed such quantities, an integer number of logical blocks to be cleaned; and selecting, based on said number of logical blocks to be cleaned, a set of logical blocks to be cleaned.
The selecting of a set of logical blocks to be merged may include: determining a permutation transformation on the set of logical block indices; selecting a set of indices; and computing the logical block indices by employing the permutation on the set of indices.
The selecting of a set of logical blocks to be cleaned may include: determining a permutation transformation on the set of logical block indices; selecting a set of indices; and computing the logical block indices by employing the permutation on the set of indices.
The selecting of a set of indices may include the selecting of a contiguous set of integers starting from the next index after the index which was last used for either merge or cleaning.
The selecting of a set of indices may include the selecting of a contiguous set of integers starting from the next index after the index which was last used for either merge or cleaning, wherein indices which correspond to logical block indices which were never merged since they were last cleaned are omitted.
There may be provided a method of managing a non-volatile memory device, the method may include: partitioning the buffer into several equal-size smaller scale buffers and associating each smaller scale buffer a disjoint user space subset; partitioning further each user space subset into several second order user space subsets; opening a data block for writing in each smaller scale buffer for every second order user space subset; receiving data sectors; writing each data sector into one of the open data blocks according to the logical address of the said data sector; creating pointers in a data management structure for each data sector, corresponding to an associated logical address and a storage location of the data sector in the buffer; detecting within a smaller scale buffer that a first predefined criterion may be fulfilled; wherein the first predefined criterion may be fulfilled before the smaller scale buffer becomes full; opening, in response to the detecting, a new data block and associating it with one of the user space second order subsets; detecting within a smaller scale buffer that a second predefined criterion may be fulfilled; wherein the second predefined criterion may be fulfilled before the smaller scale buffer becomes full; determining, in response to the detecting that the second predefined criterion may be fulfilled, a set of logical blocks to be merged; writing, in response to the detecting and to the determining, data sectors corresponding to the determined logical blocks, from the buffer to a primary non-volatile data storage memory of a non-volatile memory device; wherein the non-volatile memory device mandates erasing erase blocks of data.



BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:
FIG. 1 illustrates a system according to an embodiment of the invention;
FIG. 2 illustrates data structures according to an embodiment of the invention;
FIG. 3 illustrates the performance of a system according to an embodiment of the invention;
FIG. 4 illustrates a method according to an embodiment of the invention;
FIG. 5 illustrates a method according to an embodiment of the invention;
FIG. 6 illustrates the performance of a system according to an embodiment of the invention;
FIG. 7 illustrates a method according to an embodiment of the invention;
FIG. 8 illustrates the performance of a system according to an embodiment of the invention;
FIG. 9 illustrates cleaning and merging processes according to an embodiment of the invention;
FIG. 10 illustrates a method according to an embodiment of the invention;
FIG. 11 illustrates a method according to an embodiment of the invention;
FIG. 12 illustrates a method according to an embodiment of the invention; and
FIG. 13 illustrates the performance of a system according to an embodiment of the invention;
FIG. 14 illustrates various data blocks at various points of time and a multiple sub-buffers according to an embodiment of the invention;
FIG. 15 illustrates a method according to an embodiment of the invention; and
FIG. 16 illustrates a method according to an embodiment of the invention.



It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.
DETAILED DESCRIPTION OF THE DRAWINGS
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.
The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.
Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.
Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method and should be applied mutatis mutandis to a non-transitory computer readable medium that stores instructions that once executed by a computer result in the execution of the method.
Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system and should be applied mutatis mutandis to a non-transitory computer readable medium that stores instructions that may be executed by the system.
Any reference in the specification to a non-transitory computer readable medium should be applied mutatis mutandis to a system capable of executing the instructions stored in the non-transitory computer readable medium and should be applied mutatis mutandis to method that may be executed by a computer that reads the instructions stored in the non-transitory computer readable medium.
U.S. patent application Ser. No. 13/434,083, filing date Mar. 29, 2012, titled “SYSTEM AND METHOD FOR FLASH MEMORY MANAGEMENT”, which is incorporated herein by reference, teaches of an extended random sector buffer (ERSB) which is used in Flash storage systems employing block mapping to mitigate the need to perform block merge for every input sector, when writing a sequence of sectors to random logical addresses. The ERSB is a collection of auxiliary data blocks which hold short write transactions until a later stage when the data of these transactions are merged to their original logical blocks. When writing short random transactions, the transactions are written contiguously to some ERSB data block. A special data base maps each physical page in the ERSB to a corresponding logical page. When the ERSB becomes full, block merges are initiated for all logical blocks present in the ERSB until the data blocks are freed and new sectors can be written to the ERSB.
It is especially instructive to consider the IOPS behavior of the ERSB system when writing a sequence of write transactions where each write consists of one data sector (e.g., 512 B) or segment (e.g., 4 KB) and where the address of each write transaction is randomly chosen from some test area which is some subset of the user space. In this situation, all transactions are performed initially to the ERSB. Initially, the throughput achieved for these writes is relatively high and is determined primarily by the write speed which is supported by the flash array. As the ERSB becomes close to full, it is necessary to vacate some of its blocks. Since each data block in the ERSB can potentially have pages from all the logical blocks in the test area, in order to vacate even a single data block, it is required to merge a very large population of the logical blocks. The merges require a substantial amount of time, rendering the throughput to be much lower during this time. This situation continues until at least one data block is freed, and then high throughput is restored again.
FIG. 1 illustrates a system 10 according to embodiments of the invention. System 10 includes an interface 20 that may be linked to a memory controller 30 and may be also linked to a nonvolatile memory module 60 and a volatile memory module 80.
The nonvolatile memory module 60 may contain a random portion 54, a sequential portion 62, a data buffer 210 and a metadata buffer 70.
The random portion may refer to a logically allocated random portion memory, while the sequential portion may refer to a logically allocated sequential portion memory. The metadata buffer 70 and other management portions may be allocated within the nonvolatile memory module 60. In FIG. 1 some data structures such as the metadata buffer 70 may be illustrated as being contained outside the random portion 54 or sequential portion 62, although these structures may be contained within nonvolatile memory. It is noted that the data buffer 210 may be included in the random portion 54.
System 10 may store a management data structure that may store metadata about the content of the volatile memory module 80, the content of the nonvolatile memory module 60 or both memory modules. The management data structure can be stored at the volatile memory module 80 and, additionally or alternatively at the nonvolatile memory module 60.
FIG. 1 illustrates volatile memory module 80 and metadata buffer 70 as storing a management data structure 90 that includes location metadata (such as pointer) about a location of data sector in the data buffer 210.
FIG. 1 also illustrates a volatile merger portion 233 that can be used when data sectors are merged. Data sectors that were previously stored at the random portion can be merged before being written to the sequential portion. Additionally or alternatively, the merging can occur between data sectors from sequential and random portions. The merging may include copying data sectors to be merged to volatile merger portion 233 and then writing the merged data sectors to the sequential portion.
FIG. 2 depicts the content of data buffer 210. Data buffer 210 includes Ne data blocks 210(1)-210(Ne). Each data block may include multiple pages (data block 210(2) is illustrated as including Np data pages 210(2,1)-210(2,Np)). Each page may store one or more data sectors.
A logical memory space (such as a user space) 220 may include multiple (Nl) logical memory blocks (LBAs) 220(1)-220(NL) and may be partitioned to multiple (n) memory subsets—230(1)-230(n).
FIG. 2 illustrates a memory space and a buffer that are characterized by the following parameters:

    
    
        Np—Number of (4 KB) pages (segments) in a physical block
        NL—Number of logical blocks (LBAs)
        Ne—Number of ERSB data blocks
        ƒ—Number of merge fragments (or transaction timeout durations) required to complete a block merge
        tw—Page write duration [sec]
        tm—Merge duration [sec]

For the system in FIGS. 1 and 2, the average IOPS performance is given by

    
    






  
    
      
        
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FIG. 3 shows the IOPS performance of a representative ERSB system employing a prior art management scheme with Np=600, NL=240, Ne=60, ƒ=5, tw=0.001 [sec], and tm=1.2 [sec]. Instantaneous IOPS (curve 310) are the number of 4 KB pages written to the ERSB per second at each time instant (averaged over a 100 sec time window). Theoretical average IOPS (curve 320) are the average number of 4 KB pages written to the ERSB per second as predicted by Equation 1. Empirical average IOPS (curve 330) are the average number of 4 KB pages written to the ERSB per second over the entire time duration as measured in the representative ERSB system.
The suggested methods seek to increase the average IOPS performance of an ERSB system, and to maintain as much as possible a steady level of instantaneous IOPS.
Splitting the User Space into 2 User Space Subsets
One embodiment of the present invention seeks to increases the average IOPS performance of an ERSB system by partitioning the user space into 2 disjoint user subsets (E.g., user subset A and user subset B). A user subset is a collection of logical pages belonging to the user space. The user space is the collection of all the logical pages in the storage system. At each time instant, 2 ERSB data blocks may be open for writing, data block A and data block B, which are associated with user space subset A and user space subset B, respectively.
The partitioning can be done in an arbitrary manner or based upon expected or monitored memory access patterns. It may be beneficial that the user utilizes all memory space subsets at the same manner (or at the same quantity)—in order to benefit from the partitioning. The subsets can be of the same size or differ from each other by size. Each subset can include a continuous range or addresses or not. For example—the memory space subsets can include interleaving addresses.
FIG. 4 includes a flow chart implementing this procedure. Method 400 of FIG. 4 includes the following stages: Partitioning (410) a user space into 2 subsets—user space subset A and user space subset B; Opening (420) a data block and associate it with user space subset A; and Opening (430) a data block and associate it with user space subset B.
Short random transactions may then be written to the ERSB as follows. Each transaction may be written to the ERSB block corresponding to the user space subset to which the transaction belongs. A transaction destined to an address belonging to set A (respectively B) may be written to ERSB data block A (respectively B). Once an ERSB block associated to some user space subset becomes full, a free ERSB block may be allocated and associated with the said user space subset.
Once the ERSB becomes close to full, i.e. the number of free blocks becomes less than some threshold T, a sequence of block merges may be initiated as follows. The user space subset for which the number of associated ERSB blocks is maximal may be chosen, and all LBAs in that user space subset may then be merged. While the merges are performed, new incoming pages may be written to their corresponding data blocks. The buffer is organized as a collection of 4 KB segments (or pages). Data sectors which arrive from the host are grouped into pages before written to the buffer. Any reference to a “page” can be referred to a reference to a “data sector” and vice verse. The merges may continue until some or all of the ERSB data blocks are emptied. The threshold T may be chosen as some function of ƒ, NL, and Np. This function may chosen as




  T
  =
  
    
      ⌈
      
        
          f
          ⁢
          
            
          
          ⁢
          
            N
            L
          
        
        
          2
          ⁢
          
            N
            p
          
        
      
      ⌉
    
    +
    1.
  




The addition of “+1” is aimed to protect against the following worst case scenario. Assume that just after the number of free blocks becomes smaller than T, one of the remaining blocks is allocated to one of the sets, but all superseding writes are done to the other user space subset. In this case, even though there were T blocks left, one of the blocks is not being used because all the traffic is directed to the other set.

FIG. 5 includes a flow chart implementing the above procedure.
Method 500 of FIG. 5 includes the following stages:
For the ERSB system in FIG. 1, the average IOPS performance when performing LBA splitting to 2 user space subsets may approximately be given by




  
    
      
        
          Average
          ⁢
          
            
          
          ⁢
          IOPS
          ⁢
          
            
          
          ⁢
          when
          ⁢
          
            
          
          ⁢
          splitting
          ⁢
          
            
          
          ⁢
          user
          ⁢
          
            
          
          ⁢
          space
          ⁢
          
            
          
          ⁢
          to
          ⁢
          
            
          
          ⁢
          2
          ⁢
          
            
          
          ⁢
          subsets
        
        ⁢
        
          
        
        ⁢
        
          

        
        ⁢
        
          
            
              IOPS
              
                SPLT
                ⁡
                
                  (
                  2
                  )
                
              
            
            =
            
              
                
                  2
                  3
                
                ⁢
                
                  (
                  
                    
                      N
                      e
                    
                    -
                    G
                  
                  )
                
                ⁢
                
                  N
                  p
                
              
              
                
                  
                    2
                    3
                  
                  ⁢
                  
                    (
                    
                      
                        N
                        e
                      
                      -
                      G
                    
                    )
                  
                  ⁢
                  
                    N
                    p
                  
                  ⁢
                  
                    t
                    w
                  
                
                +
                
                  
                    1
                    2
                  
                  ⁢
                  
                    N
                    L
                  
                  ⁢
                  
                    t
                    m
                  
                
              
            
          
          ;
        
        ⁢
        
          

        
        ⁢
        
          G
          =
          
            
              ⌈
              
                
                  f
                  ⁢
                  
                    
                  
                  ⁢
                  
                    N
                    L
                  
                
                
                  2
                  ⁢
                  
                    N
                    p
                  
                
              
              ⌉
            
            +
            2
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        2
      
    
  




FIG. 6 shows the IOPS performance of a representative ERSB system employing user space splitting to 2 subsets with Np=600, NL=240, Ne=60, ƒ=5, tw=0.001 [sec], and tm=1.2 [sec]. Instantaneous IOPS (curve 610) are the number of 4 KB pages written to the ERSB per second at each time instant. Theoretical average IOPS (curve 620) are the average number of 4 KB pages written to the ERSB per second as predicted by Equation 2. Empirical average IOPS (curve 630) are the average number of 4 KB pages written to the ERSB per second over the entire time duration as measured in the representative ERSB system.
Splitting the User Space into n User Space Subsets
Another embodiment of the present invention seeks to increases the average IOPS performance of an ERSB system even further by partitioning the user space into n disjoint, equal-sized user space subsets (E.g., subset A1, subset A2, . . . , subset An). At each time instant, n ERSB data blocks may be open for writing, data blocks A1, A2, . . . , and An, which are associated with user space subsets A1, A2, . . . , and An, respectively.
FIG. 7 includes a flow chart implementing this procedure. Method 700 of FIG. 7 includes stages 710, 720, 730, and 790.
Stage 710 of partitioning the user space into n sub sets, user space subset A1, A2, . . . , and An.
Stage 720 of opening a data block for writing and associate it with user space subset A1.
Stage 730 of opening a data block for writing and associate it with user space subset A2.
Stage 790 of Open a data block for writing and associate it with user space subset An.
Short random transactions may be then written to the ERSB as follows. Each transaction may be written to the ERSB block corresponding to the user space subset to which the transaction belongs. A transaction destined to a logical address belonging to user space subset Ak is written to ERSB data block Ak.
Once an ERSB block associated to some user space subset becomes full, a free ERSB block may be allocated and associated with the said user space subset. The data blocks associated with some user space subset (not including the block which is open for writing) are called an ERSB sub pool.
Once the ERSB becomes close to full, i.e. the number of free blocks becomes less than some threshold T, a sequence of block merges may be initiated as follows. The user space subset for which the number of associated ERSB blocks is maximal may be chosen, and all LBAs in that user space subset may then be merged. While the merges are performed, new incoming pages may be written to their corresponding data blocks. The merges may continue until some or all of the ERSB data blocks are emptied. The threshold T may be chosen as some function of ƒ, NL, and Np. This function may be chosen as




  T
  =
  
    
      ⌈
      
        
          f
          ⁢
          
            
          
          ⁢
          
            N
            L
          
        
        
          n
          ⁢
          
            
          
          ⁢
          
            N
            p
          
        
      
      ⌉
    
    +
    n
    -
    1.
  




The addition of “n−1” is aimed to protect against the following worst case scenario. Assume that just prior to the time when the number of free blocks becomes equal to T, all open ERSB blocks have all pages written in them except perhaps for a single page. At the next page write, one of the blocks may become full and a new block will be allocated to it, rendering the number of free blocks less than T, and causing the merges sequence of the corresponding user space subset to commence. Since n−1 ERSB blocks might cause a new block allocation during the following page writes, we must add these to the gap calculation. FIG. 4 includes a flow chart implementing the above procedure.

For the ERSB system in FIG. 1, the average IOPS performance when performing splitting of the user space into n user space subsets may be approximately given by




  
    
      
        
          Average
          ⁢
          
            
          
          ⁢
          IOPS
          ⁢
          
            
          
          ⁢
          when
          ⁢
          
            
          
          ⁢
          splitting
          ⁢
          
            
          
          ⁢
          user
          ⁢
          
            
          
          ⁢
          space
          ⁢
          
            
          
          ⁢
          to
          ⁢
          
            
          
          ⁢
          n
          ⁢
          
            
          
          ⁢
          subsets
        
        ⁢
        
          
        
        ⁢
        
          

        
        ⁢
        
          
            
              IOPS
              
                SPLT
                ⁡
                
                  (
                  n
                  )
                
              
            
            =
            
              
                
                  2
                  
                    n
                    +
                    1
                  
                
                ⁢
                
                  (
                  
                    
                      N
                      e
                    
                    -
                    G
                  
                  )
                
                ⁢
                
                  N
                  p
                
              
              
                
                  
                    2
                    
                      n
                      +
                      1
                    
                  
                  ⁢
                  
                    (
                    
                      
                        N
                        e
                      
                      -
                      G
                    
                    )
                  
                  ⁢
                  
                    N
                    p
                  
                  ⁢
                  
                    t
                    w
                  
                
                +
                
                  
                    1
                    n
                  
                  ⁢
                  
                    N
                    L
                  
                  ⁢
                  
                    t
                    m
                  
                
              
            
          
          ;
        
        ⁢
        
          

        
        ⁢
        
          G
          =
          
            
              ⌈
              
                
                  f
                  ⁢
                  
                    
                  
                  ⁢
                  
                    N
                    L
                  
                
                
                  n
                  ⁢
                  
                    
                  
                  ⁢
                  
                    N
                    p
                  
                
              
              ⌉
            
            +
            
              2
              ⁢
              
                (
                
                  n
                  -
                  1
                
                )
              
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        3
      
    
  




Equation 3 is explained by considering the steady state behavior of the ERSB system. Specifically, denote by K1, K2, . . . , Kn the number of data blocks allocated to each ERSB sub pool. Next, denote by N1, N2, . . . , Nn the ordering of the sequence K1, K2, . . . , Kn from largest to smallest. Finally, denote by




  
    
      a
      j
    
    =
    
      
        N
        j
      
      
        
          N
          e
        
        -
        G
      
    
  
  ,
  
    1
    ≤
    j
    ≤
    n
  




the fractions of (Ne−G) ERSB blocks allocated to each ERSB sub pool. Whenever the number of free ERSB blocks falls below the threshold, the LBAs belonging to the largest ERSB sub pool are merged, causing the freeing of a fraction a1 of the total (Ne−G) ERSB allocated data blocks. Following the merge, and until the next time that the free block threshold is reached, theses free blocks are evenly distributed among all ERSB sub pools. It follows that whenever the threshold is reached, each fraction is increased by





  
    
      a
      1
    
    n
  
  ,




except for the one which is vacated, and which becomes just





  
    
      a
      1
    
    n
  
  .




In other words





  
    
      
        
          Fractions
          ⁢
          
            
          
          ⁢
          of
          ⁢
          
            
          
          ⁢
          ERSB
          ⁢
          
            
          
          ⁢
          blocks
          ⁢
          
            
          
          ⁢
          in
          ⁢
          
            
          
          ⁢
          each
          ⁢
          
            
          
          ⁢
          ERSB
          ⁢
          
            
          
          ⁢
          pool
          ⁢
          
            
          
          ⁢
          in
          ⁢
          
            
          
          ⁢
          steady
          ⁢
          
            
          
          ⁢
          
            state
            ⁢
            
              

            
            (
            
              
                a
                1
              
              ,
              
                a
                2
              
              ,
              ...
              ⁢
              
                
              
              ,
              
                a
                n
              
            
            )
          
        
        =
        
          (
          
            
              n
              ⁢
              
                
                  a
                  1
                
                n
              
            
            ,
            
              
                (
                
                  n
                  -
                  1
                
                )
              
              ⁢
              
                
                  a
                  1
                
                n
              
            
            ,
            
              
                (
                
                  n
                  -
                  2
                
                )
              
              ⁢
              
                
                  a
                  1
                
                n
              
            
            ,
            ...
            ⁢
            
              
            
            ,
            
              
                a
                1
              
              n
            
          
          )
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        4
      
    
  




Finally, summing all terms to unity on the right hand side of Equation 4 yields





  
    a
    1
  
  =
  
    
      2
      
        n
        +
        1
      
    
    .
  




It follows that every time the ERSB becomes full,





  
    2
    
      n
      +
      1
    
  
  ⁢
  
    (
    
      
        N
        e
      
      -
      G
    
    )
  




data blocks are vacated by merging





  
    1
    n
  
  ⁢
  
    N
    L
  




LBAs. In other words, for each





  
    2
    
      n
      +
      1
    
  
  ⁢
  
    (
    
      
        N
        e
      
      -
      G
    
    )
  




data blocks which are written,





  
    1
    n
  
  ⁢
  
    N
    L
  




LBAs must be merged. This relation implies Equation 3

Optimizing the Value of n

FIG. 8 depicts the IOPS performance of the representative ERSB system (curve 800) employing splitting of the user space into n user space subsets as function of the splitting parameter n with Np=600, NL=240, Ne=60, ƒ=5, tW=0.001 [sec], and tm=1.2 [sec]. It can be seen that the average IOPS performance grows with n up to a certain point, and then begins to drop. The reason for this behavior is that there are two contradicting factors governing the IOPS expression. The first is the ratio between the amount of data written to the ERSB between two consecutive merge phases and between the time required to merge all the LBAs which belong to the appropriate LBA set, and the second is the amount of gap (G) required to support the ERSB from exhausting all of its free blocks and having nowhere to write incoming data. When n begins to grow, the first term is more dominant, hence the gain which is achieved grows with n. However, as n continues to grow, the second term becomes increasingly more dominant, and the increasing gap reduces the effective size of the ERSB, causing a decrease in IOPS performance. Per a given system and its parameters, it is possible to find numerically the optimal value of n, and optimize the IOPS gain by using this approach. This may be achieved by computing the expression in Equation 3 for all values of n≦Ne and choosing the value of n for which the result is maximum.
Achieving Increased Average IOPS Performance while Maintaining Constant Instantaneous IOPS Performance by Combining Merging and Cleaning
One embodiment of the present invention seeks to increase the average IOPS performance while maintaining a constant instantaneous IOPS performance, by employing a procedure called LBA cleaning and combining it with merging. Cleaning of an LBA in the ERSB may refer to a procedure in which all the pages belonging to the said LBA and residing in one or more data blocks in the ERSB are copied to the currently opened data block in the ERSB and marked as superseded in the data block from which they were copied. In this scheme, all data blocks may contain pages from all LBAs. At each time instant, however, only a single data block may be open for writing. Once a data block is complete (i.e., all of its data pages have been written), a quantity




  
    X
    n
  
  =
  
    
      (
      
        1
        -
        α
      
      )
    
    ⁢
    
      
        N
        L
      
      
        N
        e
      
    
  




and a quantity





  
    Y
    n
  
  =
  
    α
    ⁢
    
      
        N
        L
      
      
        N
        e
      
    
  




may be computed. Then, a quantity Mn of LBAs may be merged and a quantity Cn of LBAs may be cleaned, where Mn=ƒ(Xn, Xn-1, . . . , X1) and Cn=g(Yn, Yn-1, . . . , Y1).

In one embodiment of the invention, the functions ƒ and g are the same.
In one embodiment of the invention, the functions ƒ and g are the same and Mn=ƒ(Xn, Xn-1, . . . , X1) is defined as follows. Let Ψ0=0. For each n≧1, Mn=└Ψn11+Xn┘ and Ψn=Ψn-1+Xn−Mn.
In one embodiment of the invention, LBAs are chosen for merging and cleaning in a round-robin fashion. For example, assume the LBAs in the system are denoted by LBA0, LBA1, . . . , LBAN-1. Assume further that at some point in time immediately after a data block was complete, the last LBA which had been either merged or cleaned is LBAπ(x) and that Mn=m and Cn=c. Then, LBAπ((x+1)% N), LBAπ((x+2)% N), . . . , LBAπ((x+m) % N) may be chosen for merge and LBAπ((x+m+1)% N), LBAπ((x+m+2)% N), . . . , LBAπ((x+m+c) % N) may be chosen for cleaning, where π(•) is an arbitrary fixed permutation on the logical blocks. If one of the LBAs chosen for cleaning has never been merged since it had last been cleaned, then it is skipped. For example, if LBAπ((x+m+k) % N) has never been merged since it had last been cleaned, the LBAs chosen for cleaning may be LBAπ((x+m+1)% N), LBAπ((x+m+2)% N), . . . , LBAπ((x+m+k−1)% N), LBAπ((x+m+k+1)% N), . . . , LBAπ((x+m+k+2)% N), . . . , LBAπ((x+m+c+1)% N).
FIG. 9 shows schematically the procedure which may be employed whenever a new data block is opened for writing. Whenever a new data block (210(8)) is opened for writing, some pages (922, 924 and 926) from each completed data block (210(10), 210(4) and 210(2)) are merged (denoted by “merging 960”) to their corresponding LBAs, and other pages (932, 934 and 936) are copied during a cleaning process (970) to the new data block. New incoming data 940 is then written to the new data block until it is complete. The data blocks 210(10), 210(4) and 210(2) still maintain data sectors 942, 944 and 946 after the merging and cleaning—and these data sectors can be copied to the new data block in order to empty the data blocks.
FIG. 10 includes a flow chart implementing the above embodiment.
Method 1000 of FIG. 10 may include the following stages:
Stage 1010 of getting write transaction data and address.
Stage 1020 of writing transaction's data to currently open data block.
Stage 1030 of chacking if a data block full? if ‘No’ back to stage 1010.
Stage 1040 of allocating a free data block and open it for writing.
Stage 1050 of computing the set of LBAs which will be merged.
Stage 1060 of merging data sectors associated with LBAs which were computed in previous step.
Stage 1070 of computing the set of LBAs which will be cleaned.
Stage 1080 of cleaning data sectors associated with LBAs which were computed in previous step.
Stage 1090 of checking if a data block became empty? If ‘No’ back to stage 1010.
Stage 1095 of closing empty data block. Back to stage 1010.
FIG. 11 includes a flow chart for computing the set of LBAs which will be merged and the set of LBAs which will be cleaned.
Method 1100 of FIG. 11 may include the following stages:
Stage 1110 of getting ERSB parameters α, NL, Ne for the n-th transaction.
Stage 1120 of computing the quantity Xn=(1−α)*NL/Ne.
Stage 1130 of computing the quantity Yn=α*NL/Ne.
Stage 1140 of computing the number of LBAs to merge according to Mn=f(Xn, Xn−1, . . . , 1).
Stage 1150 of computing the number of LBAs to clean according to Cn=g(Yn, Yn−1, . . . , 1).
Stage 1160 of computing the set of LBAs to merge: LBA(π((x+1)% NL)), LBA(π((x+2)% NL)), . . . , LBA(π((x+Mn) % NL)).
Stage 1170 of computing the set of LBAs to clean: LBA(π((x+Mn+1)% NL)), LBA(π((x+Mn+2)% NL)), . . . , LBA(π((x+Mn+Cn) % NL)).
Stage 1180 of In the set of LBAs to clean, skipping LBAs which were not merged since they were last cleaned.
FIG. 12 includes a flow chart for implementing a building block required for implementing the flow chart in FIG. 11.
Method 1200 of FIG. 12 may include the following stages:
Stage 1210 of getting Xn.
Stage 1220 of Computing Mn=floor(Ψn−1+Xn); Ψ0=0.
Stage 1230 of Updating Ψn=Ψn−1+Xn−Mn.
For the ERSB system in FIG. 1, the average IOPS performance when combining merging and cleaning may be approximately given by




  
    
      
        
          Average
          ⁢
          
            
          
          ⁢
          IOPS
          ⁢
          
            
          
          ⁢
          when
          ⁢
          
            
          
          ⁢
          combining
          ⁢
          
            
          
          ⁢
          merging
          ⁢
          
            
          
          ⁢
          and
          ⁢
          
            
          
          ⁢
          cleaning
        
        ⁢
        
          
        
        ⁢
        
          

        
        ⁢
        
          
            
              IOPS
              
                SH
                ⁢
                
                  -
                
                ⁢
                CL
              
            
            =
            
              
                
                  1
                  
                    1
                    +
                    α
                  
                
                ⁢
                
                  (
                  
                    
                      N
                      e
                    
                    -
                    G
                  
                  )
                
                ⁢
                
                  N
                  p
                
              
              
                
                  
                    (
                    
                      
                        N
                        e
                      
                      -
                      G
                    
                    )
                  
                  ⁢
                  
                    N
                    p
                  
                  ⁢
                  
                    t
                    w
                  
                
                +
                
                  
                    (
                    
                      1
                      -
                      α
                    
                    )
                  
                  ⁢
                  
                    N
                    L
                  
                  ⁢
                  
                    t
                    m
                  
                
              
            
          
          ;
        
        ⁢
        
          

        
        ⁢
        
          G
          =
          
            ⌈
            
              
                f
                ⁢
                
                  
                
                ⁢
                
                  N
                  L
                
              
              
                N
                p
              
            
            ⌉
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        5
      
    
  




Equation 5 is explained by considering the steady state behavior of the ERSB system employing this embodiment. Denote by x the fraction of the data block which is written with pages from the remaining data blocks during the cleaning process. Since LBAs are chosen in a round robin fashion, and since no LBA is chosen for cleaning unless it has been merged after it had last been cleaned, it follows that at least a fraction of x from all other ERSB data blocks does not contain any pages from the LBAs currently chosen for cleaning. Therefore,




  α
  ⁢
  
    
      N
      p
    
    
      N
      e
    
  
  ⁢
  
    (
    
      1
      -
      x
    
    )
  




pages are copied from each ERSB data block to the new data block. Equating the number of cleaned pages in the new data block x Np to the aggregate number of pages copied from all ERSB data blocks yields the relation





  x
  =
  
    α
    
      1
      +
      α
    
  




Finally, the IOPS may be given by considering the number of newly written pages (1−x)Np, and the time required for writing these pages. This time includes writing the entire data block plus the time needed for LBA merge. By substituting α into the resulting expression Equation 5 is obtained.

FIG. 13 depicts the IOPS performance (curve 1310) of the representative ERSB system employing combining merging and cleaning according to a certain embodiment of the invention with Np=600, NL=240, Ne=60, ƒ=5, tw=0.001 [sec], tm=1.2 [sec], and α=0.5. The skilled reader may observe the gain of this scheme w.r.t. prior art in terms of increased average IOPS performance and constant instantaneous IOPS performance. FIG. 13 also illustrates theoretical average IOPS (curve 1320) and empirical average IOPS (curve 1330).
In one embodiment of the present invention, the LBAs are chosen for merge according to the number of pages from each LBA which reside in the ERSB and/or the time which passed since the first page of each LBA entered the ERSB. For example, LBAs which have more pages in the ERSB and which reside in the ERSB for a longer period of time are merged sooner than others.
In one embodiment of the present invention, LBAs are chosen for cleaning according to the number of pages from each LBA which reside in the ERSB and on the time which passed since the first page of each LBA entered the ERSB.
In one embodiment of the invention, LBAs are chosen for cleaning according to their previous merge/clean history. For example, an LBA may not be chosen for cleaning if it had been chosen for cleaning for more than K times consecutively without being chosen for merge.
An additional element which can generalized the above merge-clean process may include both the LBA merge and LBA cleaning as mentioned above, but also ERSB data block copy. The idea is that a certain portion of LBAs is merged, another portion of LBA is cleaned, and then we are left with a number of data segments still lying in the ERSB data block that are copied to the new location, where the cleaned LBAs reside. By optimizing the ratios between the LBAs that are merged and cleaned we can further optimize performance.
Achieving Increased Average IOPS Performance while Maintaining Constant Instantaneous IOPS Performance by Partitioning the ERSB to a Plurality of ERSBs
Another embodiment of the present invention seeks to achieve an increased average IOPS performance while maintaining constant instantaneous IOPS performance. In one embodiment of the invention, the ERSB blocks are divided among J mini ERSBs (MERSBs) comprising each K data blocks (e.g., K=3). The entire user space may be divided among the J MERSBs, and in each MERSB, the allotted user space may further be divided into L (e.g., 2) LBA sets. Each MERSB may be managed by LBA splitting as is described above.
In one embodiment of the invention, in each MERSB, at each time instant, there may be two data blocks open for writing, corresponding to two user space subsets dividing the user space allotted to the said MERSB. When one of the data blocks which belongs to a first user space subset becomes full, the third data block is allocated to the same user space subset and further writes to this user space subset resume to this data block. Next, if the newly allocated data block becomes full, or if the data block of the second user space subset becomes full, merges of all LBAs corresponding to the user space subset already occupying two data blocks may then be performed causing two data blocks to be freed.
FIG. 14 schematically shows this embodiment. Each column represents the fullness state of each data block in each of the MERSBs at some time instant. For example, MERSB 2 at time instant 1 has one data block open for writing allocated for user space subset A(2) and two data blocks (one open for writing and the other complete) allocated for user space subset B(2). Of the two data blocks allocated to user space subset B(2), one data block is complete and closed and the other is open for writing. At time instant 1, the two open data blocks have little space left in them. At time instant 2, the data block allocated to user space subset A(2) become full. At this point, the LBAs belonging to user space subset B(2) are merged, and a new data block is allocated to user space subset B(2). At time instant 3, the data block allocated to user space subset A(2) become full, and another data block is allocated to this user space subset.
In one embodiment of the present invention, the write pointers of the open data blocks are randomized at the beginning of life in order to spread the merge times as much as possible. In other embodiments, other forms of synchronization between the MERSBS may be employed to spread the merge events across time.
FIG. 14 depicts the IOPS performance of the representative ERSB system employing partitioning of the ERSB to a plurality of ERSBs with Np=600, NL=240, Ne=60, f=5, tw=0.001 [sec], tm=1.2 [sec], K=3, and J=18. The skilled reader may observe the gain of this scheme w.r.t. prior art in terms of increased average IOPS performance and constant instantaneous IOPS performance.
FIG. 15 illustrates method 1500 according to an embodiment of the invention.
Method 1500 may start by initialization stage 1510. Stage 1510 may include determining the number (n) of memory space subsets, determining the size of each memory space subset, partitioning the memory space to multiple memory space subsets and the like.
Stage 1510 may include at least one of the following stages:

    
    
        i. Allocating data blocks to the multiple memory space subsets while maintaining at least one non-full data block to each memory space subset.
        ii. Partitioning a memory space to the multiple memory space subsets.
        iii. Partitioning the memory space to uneven sized memory space subsets.
        iv. Partitioning the memory space to evenly sized memory space subsets.
        v. Determining a number (n) of the multiple memory space subsets.
        vi. Determining n in response to at least one out of: (a) an aggregate amount of data sectors received between consecutive merging operations; (b) a time required to write to the sequential portion all data blocks stored in the buffer, and (c) an amount of gap required to prevent a loss of data sectors due to a temporary absence of data blocks ready to receive new data sectors.
        vii. Computing, for each suggested number of memory space subsets out of multiple suggested numbers, a quantity in response to: (a) a number of transaction timeout durations required to perform a merge of a logical memory block; (b) an instantaneous number of logical memory blocks which have at least one associated data sectors in the buffer; (c) a number of data sectors in a data block of the buffer; (d) the suggested number of memory space subsets; (e) a time duration for writing a data sector; (f) a number of data blocks in the buffer, and (g) a time duration for performing a merge of a logical memory block; and selecting, out of the multiple suggested numbers of memory space subsets, a number that yields a highest value quantity.
        viii. Partitioning the memory space so that each memory space subset may include an integral number of logical memory blocks.
    
    


Stage 1510 may be followed by stage 1520 of receiving data sectors. Each data sector belongs to a memory space subset out of multiple memory space subsets. The multiple memory space subsets comprise a plurality of logical memory blocks.
Stage 1520 may be followed by stage 1530 of writing each data sector into a data block that is allocated to a memory space subset that is associated with the data sector. The data block belongs to a buffer of the non-volatile memory device.
Stage 1530 may also include maintaining a management data structure that may include location metadata about a location of each data sector in the buffer.
Stage 1530 may be followed by stage 1540 of determining to perform an operation such as a merging operation, a cleaning operation a copy operation or a combination of these operations. The determining may include checking whether one or more criteria are fulfilled. The one or more criteria may be fulfilled before the buffer becomes full.
A criterion may be fulfilled when a number of free data blocks of the buffer falls below a predetermined threshold. A non-limiting example of such a predetermined threshold is T. T may be regarded as a minimal value of free data blocks that can still facilitate merging without loss of data sectors.
Stage 1510 may include receiving the predetermined threshold or determining the predetermined threshold. The predetermined threshold can be determined in response to: (a) a number of transaction timeout durations required to perform a merge of data sectors that belong to a certain logical memory block; (b) an instantaneous number of logical memory blocks which have at least one associated data sector in the buffer; (c) a number of memory sectors in a data block of the buffer, and (d) a number of memory space subsets.
If it is determined to perform a merge operation then stage 1540 may be followed by stage 1550 of merging data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device. The sequential portion differs from the buffer.
Stage 1540 may include selecting at least one logical memory block from the set of logical memory blocks (to be merged).
Stage 1540 may include selecting the set so that the set may include at least one logical memory blocks that is associated with a maximum amount of data blocks.
Stage 1540 may include selecting the set by determining a permutation transformation on a set of logical block indices; selecting a set of indices; and computing the logical block indices by employing the permutation on the set of indices.
Stage 1550 may include merging of the data sectors into the sequential portion by: merging data sectors into a plurality of erase block sections of data; writing the plurality of erase block sections of data to a plurality of erase blocks of the sequential portion; and creating for each data sector a pointer in the data management structure, the pointer corresponds to an associated logical address and to a location of an erase block in which the data sector is stored.
If it is determined to perform a cleaning operation then stage 1540 may be followed by stage 1560 of performing a cleaning process that may include writing data sectors that are associated with a group of one or more logical memory blocks and are stored at one or more data blocks to one or more other data blocks.
Stage 1560 may include determining the logical data blocks that should be cleaned. The determining may include at least one out of:

    
    
        i. Computing a quantity in response to (a) an instantaneous number of logical memory blocks which have at least one associated data sector in the buffer; (b) a number of data blocks in the buffer, and (c) a predefined constant; computing, based on said quantity and on at least one previously computed quantities, an integer number of logical memory blocks to be cleaned. This may also include selecting, based on said number of logical data blocks to be cleaned, the one or more other data blocks.
        ii. Determining a permutation transformation on a set of logical memory block indices; selecting a set of indices; and computing logical memory block indices by employing the permutation on the set of indices.
        iii. Selecting of a contiguous set of integers starting from a next index after an index which was last used for either merging or cleaning
        iv. Selecting of a contiguous set of integers starting from the next index after the index which was last used for either merging or cleaning, wherein indices which correspond to logical block indices which were never merged since they were last cleaned are omitted.
    
    


Stage 1560 may include updating pointers associated with the data sectors to reflect the cleaning process.
If it is determined to perform a copying operation then stage 1540 may be followed by stage 1570 of copying a content of a first data block to a second data block. The copying may be executed in response to a fullness level of the first data block—in order to assist in emptying the first data block. Referring to the example set forth in FIG. 9—copying data sector 946 from data block 210(2) to new data block 210(8) can empty data block 210(2).
FIG. 16 illustrates method 1600 according to an embodiment of the invention.
Method 1600 differs from method 1500 by performing further partitions of the memory space subsets to memory space fragments, by partitioning the buffer to sub-buffers and by performing decisions based upon the state of the sub-buffers. For convenience of explanation the major differences between the two methods are illustrated below. Any stage of method 1500 can be included within method 1600 while applying the required changes.
Method 1600 may start by initialization stage 1610. Stage 1610 may include determining the number (n) of memory space subsets, determining the number of memory space fragments (larger than n), determining the size of each memory space subset, determining the size of each memory space fragment, partitioning the memory space to multiple memory space subsets and partitioning each memory space subset to multiple memory space fragments, and the like. FIG. 14 illustrates multiple data blocks that belong to multiple sub-buffers, each data block is allocated to a memory space fragment.
Stage 1610 may be followed by stage 1620 of receiving data sectors. Each data sector belongs to a memory space fragment.
Stage 1620 may be followed by stage 1630 of writing each data sector into a data block that is allocated to a memory space fragment that is associated with the data sector. That data block belongs to a sub-buffer. Multiple sub-buffers form a buffer of the non-volatile memory device. Stage 1630 may also include maintaining a management data structure that comprises location metadata about a location of each data sector in the buffer.
Stage 1630 may be followed by stage 1640 of determining to perform an operation such as a merging operation, a cleaning operation a copy operation or a combination of these operations. The determining may include checking whether one or more criteria are fulfilled. The one or more criteria may be fulfilled before a certain sub-buffer becomes full. The certain sub-buffer is a sub-buffer that its content is being merged.
If it is determined to perform a merge operation then stage 1640 may be followed by stage 1650 of merging data sectors stored at different data blocks of the certain sub-buffer and belong to a same set of logical memory blocks. The merging is done into a sequential portion of the non-volatile memory device. The sequential portion differs from the sub-buffer.
If it is determined to perform a cleaning operation then stage 1640 may be followed by stage 1660 of performing a cleaning process that may include writing data sectors that are associated with a group of one or more logical memory blocks and are stored at one or more data blocks (of the certain sub-buffer) to one or more other data blocks (of the certain sub-buffer).
If it is determined to perform a copying operation then stage 1640 may be followed by stage 1670 of copying a content of a first data block to a second data block. The first and second data blocks can belong to the certain sub-buffer.
The invention may also be implemented in a computer program for running on a computer system, at least including code portions for performing steps of a method according to the invention when run on a programmable apparatus, such as a computer system or enabling a programmable apparatus to perform functions of a device or system according to the invention. The computer program may cause the storage system to allocate disk drives to disk drive groups.
A computer program is a list of instructions such as a particular application program and/or an operating system. The computer program may for instance include one or more of: a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.
The computer program may be stored internally on a non-transitory computer readable medium. All or some of the computer program may be provided on computer readable media permanently, removably or remotely coupled to an information processing system. The computer readable media may include, for example and without limitation, any number of the following: magnetic storage media including disk and tape storage media; optical storage media such as compact disk media (e.g., CD-ROM, CD-R, etc.) and digital video disk storage media; nonvolatile memory storage media including semiconductor-based memory units such as FLASH memory, EEPROM, EPROM, ROM; ferromagnetic digital memories; MRAM; volatile storage media including registers, buffers or caches, main memory, RAM, etc.
A computer process typically includes an executing (running) program or portion of a program, current program values and state information, and the resources used by the operating system to manage the execution of the process. An operating system (OS) is the software that manages the sharing of the resources of a computer and provides programmers with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system.
The computer system may for instance include at least one processing unit, associated memory and a number of input/output (I/O) devices. When executing the computer program, the computer system processes information according to the computer program and produces resultant output information via I/O devices.
In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims.
Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.
The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals.
Although specific conductivity types or polarity of potentials have been described in the examples, it will be appreciated that conductivity types and polarities of potentials may be reversed.
Each signal described herein may be designed as positive or negative logic. In the case of a negative logic signal, the signal is active low where the logically true state corresponds to a logic level zero. In the case of a positive logic signal, the signal is active high where the logically true state corresponds to a logic level one. Note that any of the signals described herein may be designed as either negative or positive logic signals. Therefore, in alternate embodiments, those signals described as positive logic signals may be implemented as negative logic signals, and those signals described as negative logic signals may be implemented as positive logic signals.
Furthermore, the terms “assert” or “set” and “negate” (or “deassert” or “clear”) are used herein when referring to the rendering of a signal, status bit, or similar apparatus into its logically true or logically false state, respectively. If the logically true state is a logic level one, the logically false state is a logic level zero. And if the logically true state is a logic level zero, the logically false state is a logic level one.
Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures may be implemented which achieve the same functionality.
Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality.
Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.
Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.
Also for example, the examples, or portions thereof, may implemented as soft or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language of any appropriate type.
Also, the invention is not limited to physical devices or units implemented in non-programmable hardware but can also be applied in programmable devices or units able to perform the desired device functions by operating in accordance with suitable program code, such as mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, electronic games, automotive and other embedded systems, cell phones and various other wireless devices, commonly denoted in this application as ‘computer systems’.
However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.
While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

## Claims

1. A method of managing a non-volatile memory device, the method comprising:
computing, for each suggested number of memory space subsets out of multiple suggested numbers, a quantity thereby providing multiple quantities to the multiple suggested numbers; wherein each quantity is calculated in response to: (a) a number of transaction timeout durations required to perform a merge of a logical memory block; (b) an instantaneous number of logical memory blocks which have at least one associated data sectors in a buffer; (c) a number of data sectors in a data block of the buffer; (d) the suggested number of memory space subsets; (e) a time duration for writing a data sector; (f) a number of data blocks in the buffer, and (g) a time duration for performing a merge of a logical memory block;
selecting, out of the multiple suggested numbers of memory space subsets, a selected suggested number that yields a highest value quantity out of the multiple quantities;
partitioning, in response to the selected suggested number, the memory space to multiple memory space subsets;
receiving data sectors; wherein each data sector belongs to a memory space subset out of the multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks;
writing each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device;
maintaining a management data structure that comprises location metadata about a location of each data sector in the buffer; and
merging, if a criterion is fulfilled and before the buffer becomes full, data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer.

2. The method according to claim 1, wherein the set of logical memory blocks comprises a single logical memory block.

3. The method according to claim 1, comprising allocating data blocks to the multiple memory space subsets; wherein the allocating comprises maintaining at least one non-full data block to each memory space subset.

4. The method according to claim 1, wherein the criterion is fulfilled when a number of free data block of the buffer falls below a predetermined threshold.

5. The method according to claim 4, comprising determining the predetermined threshold in response to: (a) the number of transaction timeout durations required to perform the merge of data sectors that belong to the certain logical memory block; (b) the instantaneous number of logical memory blocks which have the at least one associated data sector in the buffer; (c) the number of memory sectors in the data block of the buffer, and (d) a number of memory space subsets.

6. The method according to claim 1, comprising selecting at least one logical memory blocks that form the set of logical memory blocks.

7. The method according to claim 6, comprises selecting the set so that the set comprises at least one logical memory blocks that is associated with a maximum amount of data blocks.

8. The method according to claim 1, wherein the merging of the data sectors into the sequential portion comprises: merging data sectors into a plurality of erase block sections of data; writing the plurality of erase block sections of data to a plurality of erase blocks of the sequential portion; and creating for each data sector a pointer in the data management structure, the pointer corresponds to an associated logical address and to a location of an erase block in which the data sector is stored.

9. The method according to claim 1, further comprising performing a cleaning process that comprises writing data sectors that are associated with a group of one or more logical memory blocks and are stored at one or more data blocks to one or more other data blocks.

10. The method according to claim 1, comprising copying content of a first data block to a second data block.

11. A method of managing a non-volatile memory device, the method comprising:
receiving data sectors; wherein each data sector belongs to a memory space subset out of multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks;
writing each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device;
maintaining a management data structure that comprises location metadata about a location of each data sector in the buffer;
if a criterion is fulfilled and before the buffer becomes full, then selecting a set of logical blocks to be merged and merging data sectors stored at different data blocks and belong to the selected set of logical memory blocks of logical blocks to be merged into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer; and
wherein the selecting of the set of logical blocks to be merged comprises:
determining a permutation transformation on a set of logical block indices;
selecting a set of indices; and
computing the logical block indices by employing the permutation on the set of indices.

12. The method according to claim 11, comprising partitioning a memory space to the multiple memory space subsets; and determining a number (n) of the multiple memory space subsets in response to at least one out of: (a) an aggregate amount of data sectors received between consecutive merging operations; (b) a time required to write to the sequential portion all data blocks stored in the buffer, and (c) an amount of gap required to prevent a loss of data sectors due to a temporary absence of data blocks ready to receive new data sectors.

13. A method of managing a non-volatile memory device, the method comprising:
receiving data sectors; wherein each data sector belongs to a memory space subset out of multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks;
writing each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device;
maintaining a management data structure that comprises location metadata about a location of each data sector in the buffer;
merging, if a criterion is fulfilled and before the buffer becomes full, data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer;
performing a cleaning process that comprises writing data sectors that are associated with a group of one or more logical memory blocks and are stored at one or more data blocks to one or more currently open data blocks of the buffer;
determining the group of one or more logical memory blocks by: computing a quantity in response to an instantaneous number of logical memory blocks which have at least one associated data sector in the buffer in response to a number of data blocks in the buffer, and in response to a predefined constant; and
computing, based on said quantity and on at least one previously computed quantity that was computed during at least one previous cleaning process that preceded the current cleaning process, an integer number of logical memory blocks to be cleaned.

14. The method according to claim 13, comprising selecting of the one or more currently open data blocks of the buffer comprises:
determining a permutation transformation on a set of logical memory block indices;
selecting a set of indices; and
computing logical memory block indices by employing the permutation on the set of indices.

15. The method according to claim 14, wherein the selecting of a set of indices comprises the selecting of a contiguous set of integers starting from a next index after an index which was last used for either merging or cleaning.

16. The method according to claim 15, wherein the selecting of a set of indices comprises the selecting of a contiguous set of integers starting from the next index after the index which was last used for either merging or cleaning, wherein indices which correspond to logical block indices which were never merged since they were last cleaned are omitted.

17. A non-transitory computer readable medium that stores instructions that once executed by a memory controller cause the memory controller to:
compute, for each suggested number of memory space subsets out of multiple suggested numbers, a quantity thereby providing multiple quantities to the multiple suggested numbers; wherein each quantity is calculated in response to: (a) a number of transaction timeout durations required to perform a merge of a logical memory block; (b) an instantaneous number of logical memory blocks which have at least one associated data sectors in a buffer; (c) a number of data sectors in a data block of the buffer; (d) the suggested number of memory space subsets; (e) a time duration for writing a data sector; (f) a number of data blocks in the buffer, and (g) a time duration for performing a merge of a logical memory block;
select, out of the multiple suggested numbers of memory space subsets, a selected suggested number that yields a highest value quantity out of the multiple quantities;
partition, in response to the selected suggested number, the memory space to multiple memory space subsets;
receive data sectors; wherein each data sector belongs to a memory space subset out of the multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks;
write each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device;
maintain a management data structure that comprises location metadata about a location of each data sector in the buffer; and
merge, if a criterion is fulfilled and before the buffer becomes full, data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer.

18. A non-transitory computer readable medium that stores instructions that once executed by a memory controller cause the memory controller to:
receive data sectors; wherein each data sector belongs to a memory space subset out of multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks;
write each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device;
maintain a management data structure that comprises location metadata about a location of each data sector in the buffer;
if a criterion is fulfilled and before the buffer becomes full, then select a set of logical blocks to be merged and merge data sectors stored at different data blocks and belong to the selected set of logical memory blocks of logical blocks to be merged into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer; and
wherein a selecting of the set of logical blocks to be merged comprises:
determining a permutation transformation on a set of logical block indices; selecting a set of indices; and computing the logical block indices by employing the permutation on the set of indices.

19. A non-transitory computer readable medium that stores instructions that once executed by a memory controller cause the memory controller to:
receive data sectors; wherein each data sector belongs to a memory space subset out of multiple memory space subsets; wherein the multiple memory space subsets comprise a plurality of logical memory blocks;
write each data sector into a data block that is allocated to a memory space subset that is associated with the data sector; wherein the data block belongs to a buffer of the non-volatile memory device;
maintain a management data structure that comprises location metadata about a location of each data sector in the buffer;
merge, if a criterion is fulfilled and before the buffer becomes full, data sectors stored at different data blocks and belong to a same set of logical memory blocks into a sequential portion of the non-volatile memory device, wherein the sequential portion differs from the buffer;
perform a cleaning process that comprises writing data sectors that are associated with a group of one or more logical memory blocks and are stored at one or more data blocks to one or more currently open data blocks of the buffer;
determine the group of one or more logical memory blocks by: computing a quantity in response to an instantaneous number of logical memory blocks which have at least one associated data sector in the buffer in response to a number of data blocks in the buffer, and in response to a predefined constant; and
compute, based on said quantity and on at least one previously computed quantity that was computed during at least one previous cleaning process that preceded the current cleaning process, an integer number of logical memory blocks to be cleaned.