Abstract:
A memory leveling system updates physical memory blocks, or blocks, to maintain generally even wear. The system maintains an update count for each block, incrementing a wear level count when the update count reaches a wear level threshold. The system compares a wear level of blocks to determine whether to update a block in place or move data on the block to a less-worn physical block. The system groups the blocks into wear level groups identified by a common wear level to identify blocks that are being worn at a faster or slower than average rate. If an empty block count of a least worn group drops below a threshold, the system moves data from one of the blocks in the least worn group to an empty block in a most worn group.

Description:
GOVERNMENT INTEREST LANGUAGE 
     “This invention was made with Government support under Agreement No. NBCH30390004 awarded by DARPA. The Government has certain rights in the invention.” 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to addressable memories and in particular to storage class memories with a finite allowable number of writes to the memory before the memory wears out. The present invention further relates to a method of updating a memory to level wear on the memory, maximizing the life of the memory. 
     BACKGROUND OF THE INVENTION 
     Storage Class Memories, SCM, are nonvolatile storage technologies using low cost materials such as chalcogenides, perovskites, phase change materials, or magnetic bubble technologies. Storage class memories exhibit DRAM-like performance at lower cost than DRAM. The extrapolated cost over time can be equivalent to or less than that of enterprise class disk drives. The cost performance of the storage class memories provides a level in the storage hierarchy between the DRAM main system memory and disk storage. This level of storage may be viewed as a very large disk cache in which data can be stored permanently due to the nonvolatile characteristics of storage class memories. 
     Many storage class memory technologies are physical block addressed; i.e., unlike DRAM, a block of data is read or written. The physical block sizes typically range from 512 bytes to 4K bytes. Hence, storage class memory is suitable as a replacement for block access disk drives. 
     However, unlike DRAM and disk drives, storage class memory technologies provide a finite number of write cycles. Flash memories also exhibit this characteristic. While flash memories provide 10 6  to 10 8  write cycles, storage class memory technologies support 10 10  to 10 12  write cycles. To optimize the life of a storage device, data are written so that the storage medium is used in a uniform manner even if the write accesses are skewed to use a small set of addresses. The physical device space is divided into physical blocks that are written and read. The number of write cycles is tracked for each physical block so that when a physical block is updated, the data in the physical block (further referenced as a block of data or data) may be written to another physical block with lower wear. Distributing the written block of data to level the write operations prevents the loss of the device due to wear in a subset of physical block address that has frequent updates. 
     Flash memories use algorithms and file management structures to level the wear in the device. These differ from the storage class memory technologies in that flash memories require data be written into a physical block that has been erased. The erase process takes significant time compared to the read and write cycle time. Flash memories are organized with erase zones containing multiple physical blocks to enable the erasing of a number of physical blocks in parallel so that physical blocks are available for updating physical blocks of data. Thus, in a flash memory, the data moves with each update. Journal file structures have been used for wear leveling; tracking the wear level for each physical block is not required since all physical blocks are uniformly written. While providing even wear, many journal file mechanisms move a significant portion of the data and are actually a major cause of wear. Further, a time delay of milliseconds may be required to erase the physical blocks prior to updating. While useful for current applications, this time delay is unacceptable for most system memory applications. 
     In contrast to memory management of flash memories, storage class memory technologies provide an update in place capability so data need not be moved for update. A conventional wear leveling mechanism used by storage class memory technologies prolongs the life of the storage class memory device and requires additional data structures and overhead in accessing the physical blocks compared to direct physical block addressing. 
     A conventional wear leveling mechanism comprises an address translation system and a method for tracking physical block wear and for identifying physical blocks with low usage. Although this technology has proven to be useful, it would be desirable to present additional improvements. 
     The address translation system uses an address to access a block of data stored in a physical block of memory; the address translation system expects that the address used to access the data is constant independent of the physical block in which the data is stored. The storage class memory device provides a mapping of the system address for a block of data to the physical location of the physical block. When a block of data is accessed, the address translation identifies the physical block. For rapid address translation, an index structure or hash tables may be constructed. When a block of data is written to another physical block location, the map is changed to reflect the address of the newly written physical block. For a hashed or indexed translation table, the hash tables or indices are also changed. However, changing location of the data to another physical block requires address translation updating overhead. 
     A file system may provide the address translation; in this case the file system directories are updated when a block of data is written in a physical block of lower wear rather than the original physical block for the data. 
     Conventional storage class memory systems comprise a method for tracking wear of a physical block such that the number of write cycles for each physical block is tracked. For each write operation that requires the data to be moved for even wear, a physical block with a low wear level is identified and used. 
     One conventional mechanism for identification of empty physical blocks with low wear is an ordered list of physical blocks. The physical block at the end of the list has the lowest wear level. When the physical block with lowest wear is used, the wear level is incremented and the physical block is removed from the list. When the physical block is updated, the data in the physical block is moved to the physical block with least wear and the previously used physical block is inserted into the empty physical block list to maintain the ordered list. This conventional approach is useful for small devices. However, large devices pose a significant problem in maintaining the ordered list. For a device of 10 million physical blocks, the list requires a double linked list in which each link can address 10 million elements. An index structure can be defined for ease of insertion; the index can point to the boundaries between physical blocks with the same count. However, the number of wear level values can be very large since these technologies provide 10 10  to 10 12  write cycles. 
     Conventional storage class memory systems further comprise a method for identifying physical blocks with low usage. Some of the physical blocks are written infrequently or contain read-only data. These physical blocks have very low wear levels compared to physical blocks that have an average number of updates. If the percentage of physical blocks with infrequent write activity is low, having physical blocks with low usage does not affect the maximum lifetime of the storage class memory. However, if the percentage of physical blocks with infrequent write activity is significant, the remaining physical blocks experience significant wear compared to the physical blocks with infrequent write activity. For example, if 33% of the physical blocks in a storage class memory device are physical blocks with infrequent write activity, then the other 66% of the storage class memory device experiences 150% of the wear. Consequently, the life of the storage class memory device is 66% of a similar storage class memory device with even wear on most of the physical blocks. 
     What is therefore needed is a system, a computer program product, and an associated method for updating a memory to maintain even wear. A system is further needed to minimize overhead by minimizing a frequency of physical block moves and address table updates required to level wear on the memory. The need for such a solution has heretofore remained unsatisfied. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies this need, and presents a system, a service, a computer program product, and an associated method (collectively referred to herein as “the system” or “the present system”) of updating a memory that includes physical memory blocks to maintain generally even wear across the physical blocks. 
     The present system maintains an update count for each physical memory block. When the update count reaches a predetermined wear level threshold, the present system increments a wear level for the physical memory block. The present system further compares a current wear level of the physical memory block to current wear levels of other physical memory blocks to determine whether to update the physical memory block in place or move data on the physical memory block to a less-worn physical memory block to provide even wear on the physical memory blocks. Updating the memory comprises editing, deleting, or otherwise changing portions of the data on the selected physical memory block. The update count represents a count of data writing events addressed each of to the physical memory blocks. Updating in place comprises updating to the physical memory block in which the data to be updated resides. 
     The present system groups the physical memory blocks into a plurality of wear level groups; each of the wear level groups identified by a common wear level to identify physical memory blocks that are being worn at a faster than average rate or slower than average rate. The wear level groups comprise a least worn group with a lowest wear level, a least worn group plus one with a wear level equivalent to the least worn group plus one, and a least worn group plus two with a wear level equivalent to the least worn group plus two. 
     Moving the data comprises moving the data to a selected physical memory block that is empty in the least worn group and incrementing the wear level of the selected physical memory block. 
     The present system maintains an empty block count of empty physical blocks in the least worn group; and if the empty block count drops below a predetermined empty block count threshold, moving data from at least one of the physical memory blocks in the least worn group to a selected physical memory block that is empty in the least worn group plus two and incrementing the wear level of selected physical block. 
     The present system utilizes an address table that provides translation from an address memory location to a location of any of the physical memory blocks; the address table comprises a listing of at least some of the physical memory blocks. The address table comprises a double linked list to identify one or more of a plurality of empty physical memory blocks and one or more of a plurality of not-empty physical blocks. The address table further comprises a value of the wear level for at least some of the physical memory blocks represented in the address table. 
     In one embodiment, the present system identifies an empty physical memory block in which data can be written by scanning the address table to locate a selected physical memory block in the least worn group to receive data from a physical memory block in the least worn group plus two. The present system further identifying an empty physical memory block for receiving data by scanning the address table to locate a selected physical memory block in the least worn group plus two to receive data from a physical memory block in the least worn group. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein: 
         FIG. 1  is a schematic illustration of an exemplary operating environment in which a wear leveling system of the present invention can be used; 
         FIG. 2  is a block diagram of the high-level architecture of the wear leveling system of  FIG. 1 ; 
         FIG. 3  is a block diagram of a block manager of the wear leveling system of  FIGS. 1 and 2 ; 
         FIG. 4  is comprised of  FIGS. 4A ,  4 B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H,  4 I, and  4 J and represents a diagram illustrating an operation of the wear leveling system of  FIGS. 1 and 2 ; 
         FIG. 5  is a diagram representing a row in an address table of the wear leveling system of  FIGS. 1 and 2  in which a double linked list in the address table is used to level wear on physical blocks in the storage class memory; 
         FIG. 6  is a diagram representing a row in an address table of the wear leveling system of  FIGS. 1 and 2  in which the address table is scanned to level wear on physical blocks in the storage class memory; 
         FIG. 7  is a process flow chart illustrating an exemplary method of operation of the wear leveling system of  FIGS. 1 and 2  in updating data in physical blocks in a storage class memory; and 
         FIG. 8  is a process flow chart illustrating an exemplary method of operation of the wear leveling system of  FIGS. 1 and 2  in a wear-leveling background process. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  portrays an exemplary overall environment in which a system, a service, a computer program product, and an associated method (the wear leveling system  10  or the “system  10 ”) for distributing addressed write accesses to a memory to level wear on the memory according to the present invention may be used. System  10  comprises a software programming code or a computer program product that is typically embedded within, or installed on a host system  15  utilizing a storage class memory  20 . Alternatively, system  10  can be saved on a suitable storage medium such as a diskette, a CD, a hard drive, or like devices. 
     System  10  can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment containing both hardware and software elements. In one embodiment, system  10  is implemented in software, which includes but is not limited to firmware, resident software, microcode, etc. 
     Furthermore, system  10  can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid-state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read-only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
     The storage class memory  20  comprises a set of physical blocks (interchangeably referenced herein as physical memory blocks or blocks) in which data is stored; the physical blocks are represented in  FIG. 1  as block  0 ,  25 , block  1 ,  30 , through block N,  35 , collectively represented as physical blocks  40  (interchangeably referenced as blocks  40 ). System  10  comprises a wear leveling controller  45  and a wear counter  50  for at least some of the physical blocks  40 . For an associated physical block, the wear counter  50  maintains a value of a wear level and a count of the number of updates (interchangeably referenced as data writing events) of an associated physical block. The host system  15  accesses the storage class memory  20  through a controller  55 . 
     Controller  55  receives the storage block address from the host system  15  and maps the storage address to the physical block address in the SCM  20  with a storage address to physical block address table. Controller  55  communicates with the wear leveling controller  45  on write operations to update the counters and when required, write data to a block with less wear and change the storage address to physical block address table. 
       FIG. 2  illustrates a high-level architecture of system  10 . The wear leveling controller  45  comprises a block update module  205 , a background process module  210 , and a block manager  215 . The block update module  205  monitors the wear level for each of the physical blocks  40  and maintains wear leveling of the physical blocks within a range of updates for the physical blocks  40 . To maintain wear leveling, the block update module  205  writes data that are updated frequently to any of the physical blocks  40  that are less used. The block update module  205  may also update data in place. Consequently, the block update module  205  minimizes the variances of a number of update writes for each of the physical blocks  40 . 
     One or more of the physical blocks  40  may contain data that is read-only or updated infrequently. The background process module  210  provides wear for these low-wear physical blocks by moving data in these low-wear physical blocks to physical blocks  40  with higher wear. The background process module  210  minimizes a frequency of moving data with low-update frequency while still using the physical blocks  40  for data with higher wear characteristics. System  10  minimizes the frequency of moving data with low-update frequency by updating data in place until a predetermined threshold is exceeded. In contrast, conventional storage class memory technology moves data from one physical block to another physical block at each update. 
       FIG. 3  illustrates an exemplary high-level hierarchy of the block manager  215  indicating wear level groups in which system  10  organizes the physical blocks  40 . The block manager  215  groups physical blocks  40  according to wear as indicated by the wear counter  50  for each of the physical blocks  40 . In one embodiment, the block manager  215  comprises at least three wear level groups based on the remaining higher order bits of the wear counter  50  associated with each of the physical blocks  40 . For example, the wear counter  50  comprises 32 bits; 20 low order bits of the 32 bits (for 1M update in place cycles) are used to count update cycles. System  10  uses the higher order 12 bits to determine the wear level group for the associated physical block  40 . 
     The block manager  215  utilizes a window of at least three wear level groups to implement wear leveling: a (current) group  302 , a (current+1) group  304 , and a (current+2) group  306 , collectively referenced as wear level groups  308 . The (current) group  302  is the least worn group as determined by a wear level of the physical blocks  40 . The (current+1) group  304  represents the a least worn group plus one wear level as determined by a wear level of the physical blocks  40 . The (current+2) group represents the least worn group plus two wear levels as determined by a wear level of the physical blocks  40 . 
     The (current) group  302  comprises a set of zero or more (current) empty physical block(s)  310  and a set of zero or more (current) not-empty physical block(s)  312 . The (current+1) group  304  comprises a set of zero or more (current+1) empty physical blocks  314  and a set of zero or more (current+1) not-empty physical blocks  316 . The (current+2) group comprises a set of zero or more (current+2) empty physical block(s)  318  and a set of (current+2) not-empty physical block(s)  320 . 
     System  10  does not require that the absolute least worn physical block in the (current) group  302  be used; any of the least worn physical blocks can be selected. Since storage class memory technologies support a large but not infinite number of write cycles, the range between wear level groups  308  in number of updates or write cycles can be large. For example, for a technology that supports 10 10  write cycles, the wear level groups  308  can be spaced approximately 10 5  or 10 6  write cycles apart. The wear characteristics are not precise. For example, the difference between 1,134,567 write cycles and 1,687,654 write cycles is difficult to discern when compared to a life of 10 10  write cycles. 
     Any of the physical blocks  40  may be updated in place for a number of cycles (i.e., a predetermined update threshold) before moving a data block stored on one of the physical blocks  40  to a less worn physical block. For example, system  10  may update in place a data block in block  0 ,  25 , until the low order bits of the wear counter  50  associated with block  0 ,  25 , are all zero. System  10  then increments the higher order bits of the associated wear counter  50 . For instance, a wear counter  50  with low order 10 bits at zero indicates that the associated physical block (e.g., block  0 ,  25 ) was updated 1024 times, etc. System  10  may use a higher update threshold, for example, 15 bits (32K or 2 15 ) or 20 bits (1 M or 2 20 ). Increasing the update threshold reduces the address directory updates and the level of garbage collection. Essentially, the frequency of overhead operations associated with moving a data block from one of the physical blocks  40  to another of the physical blocks  40  is reduced by the reciprocal of the update threshold. 
     A (current) group manager  322  manages the (current) group  302 . A (current+1) group manager  324  manages the (current+1) group  304 . A (current+2) group manager  326  manages the (current+2) group  306 . The (current) group manager  322 , the (current+1) group manager  324 , and the (current+2) group manager  326  are collectively referenced as the group managers  328 . 
     The (current) group manager  322  comprises a (current) empty block counter  330 , a (current) empty block list  332 , a (current) not-empty block counter  334 , and a (current) not-empty block list  336 . The (current+1) group manager  310  comprises a (current+1) empty block counter  338 , a (current+1) empty block list  340 , a (current+1) not-empty block counter  342 , and a (current+1) not-empty block list  344 . The (current+2) group manager  315  comprises a (current+2) empty block counter  346 , a (current+2) empty block list  348 , a (current+2) not-empty block counter  350 , and a (current+2) not-empty block list  352 . An address table  354  comprises the (current) empty block counter  330 , (current) empty block list  332 , the (current) not-empty block counter  334 , the (current) not-empty block list  336 , the (current+1) empty block counter  338 , the (current+1) empty block list  340 , the (current+1) not-empty block counter  342 , the (current+1) not-empty block list  344 , the (current+2) empty block counter  346 , the (current+2) empty block list  348 , the (current+2) not-empty block counter  350 , and the (current+2) not-empty block list  352 . 
     System  10  maintains a tight distribution of high order block counter values by grouping the physical blocks  40  into wear level groups  308 . The (current) group manager  322  tracks the physical blocks  40  that are least worn and thus are candidate physical blocks to be filled. The (current+1) group manager  324  tracks the physical blocks  40  with wear levels in the least worn group plus one. The (current+2) group manager  326  tracks the blocks with wear levels in the least worn group plus two. For example, the update threshold is 1 M. When physical blocks  40  in the least worn group (the (current) group  302 ) have a wear level of 125, each of the physical blocks  40  in the (current) group  302  has been updated 125×1 M times. Consequently, the (current+1) group  304  has a wear level of 126 (126 M updates) and the (current+2) group  306  has a wear level of 127 (127 M updates). 
     When system  10  selects one of the physical blocks  40  for updating, the block update module  205  examines the wear value for the selected physical block as maintained by the wear counter  50  for the selected physical block. If the wear counter  50  comprises all zeros in the lower 20 bits (for 1 M update writes per block before moving), then the higher order bits are compared with the value of the wear level for the (current+1) group  304 ; i.e., 126 for the previously discussed example. If the wear level of the selected physical block is the same as the wear level of the (current+1) group  304 , then the selected physical block is updated in place. Updating in place avoids changing the address table  354  and indices. If the value of the high order bits is that of the (current+2) group  306  (with a wear level of 127 in the previously discussed example), then the data in the physical block is moved to any of the (current) empty blocks  310  in the (current) group  302  (with a wear level of 125 in the previously discussed example). 
     When the value of the (current) empty block counter  330  drops below a predetermined empty-block threshold (near zero), data in physical blocks  40  associated with the (current) group  302  are moved to physical blocks  40  associated with the (current+2) group  306  (with a wear level of 127 in the previously discussed example). The block manager  215  increments by one the wear counter  50  of the physical blocks  40  to which the data is moved, indicating an update in the physical block. This moves the low update activity and read-only data as far forward in wear as possible to minimize the number of times this low update activity and read-only data is moved. If the data block is not read-only or low update frequency, the data block is moved to a block in the (current) group  302  when the data block is updated. A penalty for incorrectly estimating the wear of a physical block is one extra data move and one extra write cycle. If the number of updates-in-place is 1 M or even as low as 1024, one extra write cycle for an incorrect estimate of wear is insignificant. 
     When some or all of the physical blocks  40  in the (current) group  302  are used and assigned to the (current+1) group  204  (with a wear level of 125 in the previous example), the (current) group  302  is empty. The (current+1) group  304  becomes the (current) group  302  (with a wear level of 126); the (current+2) group  306  becomes the (current+1) group  304  (with a wear level of 127). The wear counter  50  for the physical blocks  40  in the previous (current) group  302  (with a wear level of 125) are zero and used for the (current+2) group  306  (with a wear level of 128). At the start of this cycle, the (current+2) group  306  has no members so the (current+2) not-empty block counter  350  and the (current+2) empty block counter  346  are zero. 
     Physical blocks  40  assigned to any of the wear level groups  308  are classified as empty physical blocks (further referenced as empty blocks) or not-empty physical blocks (further referenced as not-empty blocks). Empty blocks may be used for data from more worn physical blocks  40 . As previously described, the block manager  215  maintains an empty block counter and a not-empty block counter for each of the wear level groups  308 . When the block manager  215  increments the wear counter  50  associated with one of the physical blocks  40 , the wear counter  50  is checked to determine whether the associated physical block has to move to the next of the wear level groups  308 . Associated group counters (i.e., the (current) empty block counter, the (current) not-empty block counter, etc.) are updated when any of the physical blocks  40  moves from one of the wear level groups  308  to the next of the wear level groups  308 . The physical block that previously held the moved data is marked as empty or available for update in the (current+1) group  304 . 
     In one embodiment, the block manager  215  associates linked lists with each of the wear level groups  308 . One linked list references the empty physical blocks  40  for a specific group in the associated one of the wear level groups  308 ; that is, the data in the empty physical block has been moved to a physical block with less wear. Another linked list references physical blocks  40  in a specific group of the wear level groups  308  with active data. In  FIG. 3 , the linked lists for the (current) group  302  comprise the (current) empty block linked list  332  and the (current) not-empty block linked list  336 . The linked lists for the (current+1) group  304  comprise the (current+1) empty block linked list  340  and the (current+1) not-empty block linked list  344 . The linked lists for the (current+2) group  306  comprise the (current+2) empty block linked list  348  and the (current+2) not-empty block linked list  352 . 
     Order in the linked lists is not critical since all blocks in a linked list are treated similarly. Any of the empty blocks in the (current) group  302 , the group with lowest wear, is used for the next data move. To smooth the workload, the block data migration of read-only, low update usage data can be triggered when the number of available blocks falls below a threshold; the data movement can be a background task. If the wear is set so that a block is changed after a large number of write cycles (1024, 1 M, etc.), then the address directory change frequency can be reduced by dividing the update write frequency by this large factor. 
     Another embodiment does not require linked lists and uses the physical blocks  40  in sequence by scanning through the physical blocks  40 . When an empty physical block has a wear level in the (current) group  302 , the empty physical block may be used for data from a block with higher wear. When the number of available blocks in the (current) group  302  is below a low level number, then not-empty physical blocks  40  with a wear level corresponding to the value of the (current) group  302  are moved to the (current+2) group  308 . 
     The background process module  210  scans physical blocks  40  and moves data in a background task that maintains a list of least worn candidate blocks for the updated physical blocks and most worn candidate blocks for the low update frequency data blocks. The scan need only find a number of candidates and not be exhaustive. As previously described, if the wear is set so that a physical block is changed after a large number of write cycles (1024, 1 M, etc.), the address directory change frequency can be reduced by dividing the update write frequency by the large factor. When the count of available physical blocks  40  drops below a target number, an address table may be scanned to construct a linked list of available empty physical blocks and a linked list of physical blocks that were in a group when the group was made the least worn group. These lists can be significantly shorter than linked lists of all of the blocks. Physical blocks  40  cannot reduce their wear level so the address table need only be scanned once per group cycle. 
     In yet another embodiment, system  10  can be used in a storage device without extra physical blocks  40  for “empty” blocks. Data that causes high wear is exchanged with data that causes lower wear to effect wear leveling. 
       FIG. 4  ( FIGS. 4A ,  4 B,  4 C,  4 D,  4 E,  4 F,  4 G,  4 H,  4 I, and  4 J) illustrates an exemplary storage class memory  405  comprising block  0 ,  410 , block  1 ,  415 , block  2 ,  420 , block  3 ,  425 , block  4 ,  430 , and block  5 ,  435  (collectively referenced as physical blocks  440 ). Data is written to the physical blocks  440  in the form of data blocks comprising data block A,  445 , data block B,  450 , data block C,  455 , and data block D,  460  (collectively referenced as data blocks  465 ). In this example, the number of updates before a block move in the illustration is one. As disclosed, the number of updates before a block move may be 1024, 1M, etc. In the illustration of  FIG. 4 , “update C” may indicate a block update decision after 1 M updates to block C,  455 , rather than one update, as illustrated. 
     As illustrated in  FIG. 4A , data block A,  445 , is written to block  0 ,  410 ; data block B,  450 , is written to block  1 ,  415 ; data block C,  455 , is written to block  2 ,  420 ; and data block D,  460 , is written to block  3 ,  425 . Block  0 ,  410 , block  1 ,  415 , block  2 ,  420 , and block  3 ,  425 , have each experienced one write or update, as indicated by the wear level=1 for each of these blocks. The (current) group  302  (the least worn group) comprises empty blocks block  4 ,  430 , and block  5 ,  435 ; each of which has a wear level=0. The (current+1) group  304  (the least worn group plus one) comprises block  0 ,  410 , block  1 ,  415 , block  2 ,  420 , and block  3 ,  425 . 
     As illustrated in  FIG. 4A  and  FIG. 4B , data block C,  455 , is updated. Since data block C,  455 , is already associated with the (current+1) group  304  and wear level=1, system  10  moves the data block C,  455 , to any of the physical blocks  440  with a wear level lower than that of block  2 ,  420 . In this case, system  10  moves the data block C,  455 , to block  4 ,  430 , in the (current) group  302 . The block manager  215  increments the wear level of block  4 ,  430 , by one (wear level=1). Block  4 ,  430 , is removed from the (current) group  302  and added to the (current+1) group  304 . 
     As illustrated in  FIG. 4B  and  FIG. 4C , data block C,  455 , is updated again. Since data block C,  455 , is associated with the (current+1) group  304  and wear level=1, system  10  moves the data block C,  455 , to any of the blocks  440  with a wear level=0; e.g., block  5 ,  435 , in the (current) group  302 . The block manager  215  increments the wear level of block  5 ,  435 , by one (wear level=1). Block  5 ,  435 , is removed from the (current) group  302  and added to the (current+1) group  304 . 
     As illustrated in  FIG. 4C  and  FIG. 4D , data block A,  445 , is updated. All physical blocks  440  are in the (current+1) group  304 . Consequently, data block A,  445 , is updated in place. The block manager  215  increments the wear level of block  0 ,  410 , by one (wear level=2). Block  0 ,  410 , is removed from the (current+1) group  304  and added to the (current+2) group  306 . 
     As illustrated in  FIG. 4D  and  FIG. 4E , data block D,  460 , is updated. No physical blocks  440  are available with a lower wear level than block  3 ,  425 , the physical block in which data block D,  460 , resides. Consequently, data block D,  460 , is updated in place. The block manager  215  increments the wear level of block  3 ,  425 , by one (wear level=2). Block  3 ,  425 , is removed from the (current+1) group  304  and added to the (current+2) group  306 . 
     As illustrated in  FIG. 4E  and  FIG. 4F , data block A,  445 , is updated. Block  0 ,  410 , is currently in the (current+1) group  304  with a wear level=2. The block manager  215  moves the data block A,  445 , to any of the blocks  440  with a wear level=1; e.g., block  4 ,  430 . The block manager  215  increments the wear level of block  4 ,  430 , by one (wear level=2). Block  4 ,  430 , is removed from the (current+1) group  304  and added to the (current+2) group  306 . 
     As illustrated in  FIG. 4F  and  FIG. 4G , data block C,  455 , is updated. No physical blocks  440  are available with a lower wear level than block  5 ,  435 , in which data block C,  455 , resides. Consequently, data block C,  455 , is updated in place. The block manager  215  increments the wear level of block  5 ,  455 , by one (wear level=2). Block  5 ,  455 , is removed from the (current+1) group  304  and added to the (current+2) group  306 . 
     As illustrated in  FIG. 4G  and  FIG. 4H , data block C,  455 , is updated again. The block manager  215  moves the data block C,  455 , to any of the blocks  440  with a wear level=1; e.g., block  2 ,  420 . The block manager  215  increments the wear level of block  2 ,  420 , by one (wear level=2). Block  2 ,  420 , is removed from the (current+1) group  304  and added to the (current+2) group  306 . 
     As illustrated in  FIG. 4H  and  FIG. 4I , no empty blocks remain in the (current+1) group  304  with a wear level=1. In a background task illustrated by  FIG. 4I , the background process module  210  moves the data block B,  450 , to any of the empty blocks  440  with a wear level=2 in the (current+2) group  304 ; e.g., block  0 ,  410 . The block manager  215  increments the wear level of block  0 ,  410 , by one (wear level=3). The data blocks  440  with a wear level=1 are placed in the (current) group  302 . The data blocks  440  with a wear level=2 are placed in the (current+1) group  304 . Block  0 ,  410 , is added to the (current+2) group  306 . 
     As illustrated in  FIG. 4J , data block D,  460 , is updated. The data block D,  460 , resides in block  3 ,  425 , in (current+1) group  304 . The block manager  215  moves the data block D,  460 , to one of the data blocks  440  in the (current) group  302 ; i.e., block  1 ,  415 . The block manager  215  increments the wear level of block  1 ,  415 , by one (wear level=2). Block  1 ,  415 , is removed from the (current) group  302  and added to the (current+1) group  304 . 
     The wear counter  50  for each of the physical blocks  40  increments the wear level each time the associated physical block is updated. The wear counter  50  is logically divided into segments: a wear level group segment (interchangeably referenced herein as a wear level), a higher count segment, and a lower count segment (interchangeably referenced as an update count). The lower count segment counts the number of updates before the block manager  215  makes a block data move decision. The block manager  215  increments the wear level group segment when the lower count segment high order bit carries into the higher count segment when the wear counter  50  is incremented. When the high order bits of the wear level group segment are incremented, the block manager  215  makes a block data move decision. 
     If the incremented wear level of a physical block in the (current) group  302  is greater than the wear level for the (current+1) group  304 , the block manager  215  moves the data in the physical block to an empty block with the wear value of the (current) group. The block manager  215  marks as used the physical block to which the data is moved and the address table  354  is updated to point to the physical block as the address of the data. The previous physical block is marked as empty and added to the (current+2) group  306 . 
     If the incremented block wear level is less than or equal to the wear level of the (current+1) group  304 , then the data in the block is not moved. Instead, the data in the block is updated in place. 
     When the number of empty blocks with the value of the wear level group segment of the (current) group  302  drops below a target value, the background process module  210  moves the data in blocks that have the wear level of the (current) group  302  to blocks with the most wear, i.e., physical blocks  40  in the (current+2) group  306 . The data in these least-read blocks are read-only or have very low update frequency and are moved to maintain consistent wear level over some or all of the physical blocks  40 . Data in these least-read blocks are moved to physical blocks  40  with high wear to minimize the frequency of moving data blocks exhibiting low update frequency. When the data are moved, the background process module  210  adds the physical blocks  40  to which the data are moved to the (current+2) group  306  and marks as empty the physical blocks  40  from which the data is moved. These newly emptied physical blocks  40  have wear levels of the (current) group  302  and are now available for use. 
     When the low update activity blocks have been moved and the (current) empty block counter  320  is zero, the block manager  215  converts the (current+1) group  304  to the (current) group  302  and the (current+2) group  306  to the (current+1) group  304 . The (current) empty block counter  320  of the previous (current) group  302  is used for the (current+2) group  306 . 
     When data in one of the physical blocks with low update frequency is moved, the wear value may be very low and the block of data moved into the physical block may not have the activity to move the block of data into the (current) group  302 . The (current) not-empty physical block counter  334  includes counts of physical blocks  40  that have the wear value of the (current) group  302  or lower. When the (current) empty physical block linked list  332  is below the target for moving data, data in a physical block with the lower wear value are moved and another block of data moved into the physical block with low wear value. The replacement block of data may have higher update activity and wear the physical block. 
     The address table  354  associates an address table entry with each of the physical blocks  40  in the storage class memory  20 . The address table entry comprises an indication if the associated block is empty or contains data (is not empty). The address table entry further comprises the address of the data. Each address table entry comprises an update count that is the count of write cycles executed on the block. 
       FIG. 5  illustrates a row  505  of one embodiment of the address table  354  utilizing a double linked list that permits insertion and deletion of an element in the list. The double linked list references one or more of the physical blocks  40 , with one entry per physical block in the address table  354  indicated at link up  510  and link down  515 . Link up  510  and link down  515  are the entries in the row  505  for the double linked list. The empty physical blocks  40  for a group are linked, for example, in a “link up” list via link up  510 ; not-empty physical blocks  40  for a group are linked, for example, in a “link down” list via link down  515 . Alternatively, the empty physical blocks may be linked in the link down list and the not-empty physical blocks may be linked in the link up list. The empty and full counters are located in the block manager  215  and indicate the number of physical blocks in each list. The physical blocks  40  in either the link up  510  or the link down  515  may be located by traversing the appropriate list. The update count for a physical block represented by row  505  is indicated by an update count  520 . The address of the physical block represented by row  505  is indicated by an address  525 . 
       FIG. 6  illustrates a row  605  of another embodiment of a minimized address table. In this embodiment, empty physical blocks  40  and not-empty physical blocks  40  are located by sequential scan of entries in the minimized address table. The block manager  215  creates a list of empty physical blocks; a minimum number are kept in the list. The block manager  215  removes an empty physical block from the list of empty physical blocks when an empty physical block is needed to move data from a physical block with higher wear. When the number of empty physical blocks drops below the minimum number, the address block scan resumes and replenishes the list to a maximum number. When the total number of empty physical blocks drops below a target, the balance of the minimized address table is scanned. All of the remaining empty physical blocks and all not-empty physical blocks with the wear value of the (current) group  302  are found. 
     In this embodiment, the scans of the minimized address table need only be performed when a physical block reaches the number of updates in the lower count and the physical block count in the least worn empty physical block is below the minimum. A block of data is considered for a move when the lower count is completed. This may divide the update count by a million for a single physical block. The frequency of full address table scanning is very low. 
       FIG. 7  illustrates an exemplary method  700  of operation of the block update module  205  in updating data in the physical blocks  40  of the storage class memory  20 . The block update module  205  initiates a block update (step  705 ) in a selected physical block. The block manager  215  accesses the wear counter  50  for the selected physical block (interchangeably referenced as the selected block) (step  710 ). The block manager  215  increments the wear level for the selected physical block (step  715 ). 
     The block manager  215  determines whether the update count for the selected physical block is less than a predetermined maximum threshold (a maximum) (decision step  720 ). If no, the block manager  215  resets the update count for the selected physical block (step  725 ). The block update module  205  determines whether the wear level for the selected physical block is less than or equal to the value of the (current) group (the current value) (decision step  730 ). The wear level indicates to which of the wear level groups  308  the selected physical block is currently assigned. If yes, the block update module  205  increments the wear level for the selected physical block, moving the selected physical block from one of the wear level groups  308  to another of the wear level groups  308  (step  735 ). 
     The block update module  205  writes the update data in the selected physical block (step  740 ) and exits block update (step  745 ). If at decision step  720  the update count of the wear counter  50  of the selected physical block is less than a maximum or predetermined threshold, the block update module  205  writes the update data in the selected physical block (step  740 ) and exits block update (step  745 ). 
     If at decision step  730  the wear level is greater than the current value, the block update module  205  selects an empty physical block (further referenced as the selected empty block) with a group count less than or equal to the current value (step  750 ). The block update module  205  writes the update data in the selected empty physical block (step  755 ). The block update module  205  increments the wear level of the selected empty physical block (step  760 ), moving the selected physical block from one of the wear level groups  308  to another of the wear level groups  308 . The block update module adds the selected physical block as an empty physical block with group count=current value+1 (step  765 ) and exits block update (step  745 ). 
       FIG. 8  illustrates an exemplary method  800  of the background process module  210  in moving read only or low update frequency data from one of the physical blocks  40  to a physical block with higher wear, leveling wear of the physical blocks  40  of the storage class memory  20 . Method  800  is described for three wear level groups  308 ; additional wear level groups  308  may be used. 
     The background process module  210  initiates a background process (step  805 ). If the number of empty physical blocks in the (current) group=0 (decision step  815 ), the background process module  210  determines whether the number of used physical blocks in the (current) group=0 (decision step  815 ). If yes, the background process module  210  sets the wear level of the (current) group to the current value plus one (step  820 ). The background process module  210  sets the wear level of the (current+1) group to the current value plus 2 (step  825 ). The background process module  210  sets wear level of the (current+2) group to the current value plus three (step  830 ). The background process module  210  exits the background process (step  835 ). If at decision step  810  the number of empty physical blocks in the (current) group is greater than zero, the background process module  210  exits the background process (step  835 ). 
     If at decision step  815  the number of used physical blocks in the (current) group is greater than zero, the background process module  210  selects a not-empty physical block in the (current) group (step  840 ). The background process module  210  selects an empty physical block with a wear level greater than the (current) group (step  845 ). The background process module  210  moves data from the selected not-empty physical block to the selected empty physical block (step  850 ). The background process module  210  increments the wear level of the selected empty physical block in which data was written (step  855 ). The background process module  210  designates the selected not-empty physical block as an empty physical block and sets the associated wear level to the current value (step  860 ). The background process module  210  exits the background process (step  835 ). 
     In one embodiment, system  10  maintains more than three active wear level groups  308  so that the range between the least worn group and most worn group is greater than two. In this embodiment, the read-only or low update activity data moves at a lower rate. For example, if four wear level groups were used the wear level groups comprise a least worn group, a least worn+1 group, a least worn+2 group, and a least worn+3 group. System  10  moves the read-only and low update data when some or all of the least worn group and the empty least worn+1 group are used. System  10  moves the read-only and low update data to the least worn+3 group. With four wear level groups, these blocks of data bypass the least worn+1 group and least worn+2 group and do not move until the third cycle. With three wear level groups, these blocks of data bypass the lease worn+1 group and do not move until the second cycle. 
     System  10  uses an update-in-place property of the storage class memory to reduce the frequency of block address changes and address table updates to block update frequency divided by the number of update-in-place cycles. The number of update-in-place cycles before changing address may be in the range of 10 5  to 10 7  and is a significant reduction of the time and processing overhead for address changes in a conventional storage class memory. 
     System  10  maintains wear leveling within a narrow band of update writes for some or all physical blocks  40 . The band is approximately 3 times the number of update-in-place cycles. For example, if 10 6  updates are performed before a block address change is performed, most the physical blocks are within 3×10 6  update cycles of each other. 
     System  10  manages physical blocks  40  that are read-only or very low update usage. Low update usage physical blocks are exposed to updates so that the maximum number of updates can be supported in a storage class memory. When physical blocks with lowest usage are used, the data in physical blocks  40  that have low usage are moved to one of the physical blocks  40  that have had highest usage. By moving to physical blocks  40  of highest usage, the data need not be moved until the second cycle after the current cycle, 
     It is to be understood that the specific embodiments of the invention that have been described are merely illustrative of certain applications of the principle of the present invention. Numerous modifications may be made to the system and method of updating memory to maintain even wear described herein without departing from the spirit and scope of the present invention.