Patent Publication Number: US-11023150-B2

Title: Block mode toggling using hybrid controllers

Description:
BACKGROUND 
     The present invention relates to data storage systems, and more particularly, this invention relates to block mode toggling using hybrid controllers. 
     Using Flash memory as an example, the performance characteristics of conventional NAND Flash-based solid state drives (SSDs) are fundamentally different from those of traditional hard disk drives (HDDs). Data in conventional SSDs is typically organized in pages of 4, 8, or 16 KB sizes. Moreover, page read operations in SSDs are typically one order of magnitude faster than write operations and latency neither depends on the current nor the previous location of operations. 
     However, in Flash-based SSDs, memory locations are erased in blocks prior to being written to. The size of an erase block unit is anywhere from 256, to 512, or even several thousands of pages and the erase operations takes approximately one order of magnitude more time than a page program operation. Due to the intrinsic properties of NAND Flash, Flash-based SSDs write data out-of-place whereby a mapping table maps logical addresses of the written data to physical ones. This mapping table is typically referred to as the Logical-to-Physical Table (LPT). 
     As Flash-based memory cells exhibit read errors and/or failures due to wear or other reasons, additional redundancy may be used within memory pages as well as across memory chips (e.g., RAID-5 and RAID-6 like schemes). The additional redundancy within memory pages may include error correction code (ECC) which, for example, may include BCH, LDPC, or other codes. While the addition of ECC in pages is relatively straightforward, the organization of memory blocks into RAID-like stripes is more complex. For instance, individual blocks are retired over time which requires either reorganization of the stripes, or capacity reduction of the stripe. As the organization of stripes together with the LPT defines the placement of data, SSDs typically utilize a Log-Structured Array (LSA) architecture, which combines these two methods. 
     The LSA architecture relies on out-of-place writes. In this approach, a memory page overwrite will result in writing the memory page data to a new location in memory, marking the old copy of the memory page data as invalid, and then updating the mapping information. Due to the limitations of current NAND memory technology, an invalidated data location cannot be reused until the entire block it belongs to has been erased. Before erasing, though, the block undergoes garbage collection, whereby any valid data in the block is relocated to a new block. Garbage collection of a block is typically deferred for as long as possible to maximize the amount of invalidated data in block, and thus reduce the number of valid pages that are relocated, as relocating data causes additional write operations, and thereby increases write amplification. 
     SUMMARY 
     A computer-implemented method, according to one embodiment, is used to toggle block modes in a storage system. The computer-implemented method includes: maintaining a block switching metric for each block of memory in the storage system. A determination is made as to whether a first block in a first pool should be transferred to a second pool according to a block switching metric which corresponds to the first block. In response to determining that the first block in the first pool should be transferred to the second pool according to the block switching metric which corresponds to the first block, the first block is erased. The first block is then transferred from the first pool to a second ready-to-use (RTU) queue which corresponds to the second pool. A second block in the second pool is also erased and transferred from the second pool to a first RTU queue which corresponds to the first pool. The blocks in the first pool are configured in single-level cell (SLC) mode, while the blocks in the second pool are configured in multi-bit-per-cell mode. 
     A computer program product, according to another embodiment, includes a computer readable storage medium having program instructions embodied therewith. The program instructions are readable and/or executable by a processor to cause the processor to: perform the foregoing method. 
     A system, according to yet another embodiment, includes: a plurality of non-volatile random access memory (NVRAM) blocks configured to store data. In some approaches, the system also includes a processor and logic integrated with and/or executable by the processor. The logic is configured to: perform the foregoing method. 
     Other aspects and embodiments of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a non-volatile memory card, in accordance with one embodiment. 
         FIG. 2  is a diagram of a data storage system architecture, in accordance with one embodiment. 
         FIG. 3  is a system diagram, in accordance with one embodiment. 
         FIG. 4  is a conceptual diagram which includes a block-stripe and page-stripe, in accordance with one embodiment. 
         FIG. 5  is a partial representational view of a non-volatile memory module, in accordance with one embodiment. 
         FIG. 6  is a partial representational view of a data and block flow overview, in accordance with one embodiment. 
         FIG. 7  is a flowchart of a method, in accordance with one embodiment. 
         FIG. 8  is a network architecture, in accordance with one embodiment. 
         FIG. 9  is a representative hardware environment that may be associated with the servers and/or clients of  FIG. 8 , in accordance with one embodiment. 
         FIG. 10  is a tiered data storage system in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations. 
     Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc. 
     It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The following description discloses several preferred embodiments of data storage systems, as well as operation and/or component parts thereof. It should be appreciated that various embodiments herein can be implemented with a wide range of memory mediums, including for example non-volatile random access memory (NVRAM) technologies such as NAND Flash memory, NOR Flash memory, phase-change memory (PCM), magnetoresistive RAM (MRAM) and resistive RAM (RRAM). To provide a context, and solely to assist the reader, various embodiments may be described with reference to a type of non-volatile memory. This has been done by way of example only, and should not be deemed limiting on the invention defined in the claims. 
     In one general embodiment, a computer-implemented method is used to toggle block modes in a storage system. The computer-implemented method includes: maintaining a block switching metric for each block of memory in the storage system. A determination is made as to whether a first block in a first pool should be transferred to a second pool according to a block switching metric which corresponds to the first block. In response to determining that the first block in the first pool should be transferred to the second pool according to the block switching metric which corresponds to the first block, the first block is erased. The first block is then transferred from the first pool to a second ready-to-use (RTU) queue which corresponds to the second pool. A second block in the second pool is also erased and transferred from the second pool to a first RTU queue which corresponds to the first pool. The blocks in the first pool are configured in single-level cell (SLC) mode, while the blocks in the second pool are configured in multi-bit-per-cell mode. 
     In another general embodiment, a computer program product includes a computer readable storage medium having program instructions embodied therewith. The program instructions are readable and/or executable by a processor to cause the processor to: perform the foregoing method. 
     In yet another general embodiment, a system includes: a plurality of non-volatile random access memory (NVRAM) blocks configured to store data. In some approaches, the system also includes a processor and logic integrated with and/or executable by the processor. The logic is configured to: perform the foregoing method. 
       FIG. 1  illustrates a memory card  100 , in accordance with one embodiment. It should be noted that although memory card  100  is depicted as an exemplary non-volatile data storage card in the present embodiment, various other types of non-volatile data storage cards may be used in a data storage system according to alternate embodiments. It follows that the architecture and/or components of memory card  100  are in no way intended to limit the invention, but rather have been presented as a non-limiting example. 
     Moreover, as an option, the present memory card  100  may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. However, such memory card  100  and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the memory card  100  presented herein may be used in any desired environment. 
     With continued reference to  FIG. 1 , memory card  100  includes a gateway  102 , a general purpose processor (GPP)  112  (such as an ASIC, FPGA, CPU, etc.) connected to a GPP memory  114  (which may comprise RAM, ROM, battery-backed DRAM, phase-change memory PC-RAM, MRAM, STT-MRAM, etc., or a combination thereof), and a number of memory controllers  108 , which include Flash controllers in the present example. Each memory controller  108  is connected to a plurality of NVRAM memory modules  104  (which may comprise NAND Flash or other non-volatile memory type(s) such as those listed above) via channels  106 . 
     According to various embodiments, one or more of the controllers  108  may be or include one or more processors, and/or any logic for controlling any subsystem of the memory card  100 . For example, the controllers  108  typically control the functions of NVRAM memory modules  104  such as, data writing, data recirculation, data reading, etc. The controllers  108  may operate using logic known in the art, as well as any logic disclosed herein, and thus may be considered as a processor for any of the descriptions of non-volatile memory included herein, in various embodiments. 
     Moreover, the controller  108  may be configured and/or programmable to perform or control some or all of the methodology presented herein. Thus, the controller  108  may be considered to be configured to perform various operations by way of logic programmed into one or more chips, modules, and/or blocks; software, firmware, and/or other instructions being available to one or more processors; etc., and combinations thereof. 
     Referring still to  FIG. 1 , each memory controller  108  is also connected to a controller memory  110  which preferably includes a cache which replicates a non-volatile memory structure according to the various embodiments described herein. However, depending on the desired embodiment, the controller memory  110  may be battery-backed DRAM, phase-change memory PC-RAM, MRAM, STT-MRAM, etc., or a combination thereof. 
     As previously mentioned, memory card  100  may be implemented in various types of data storage systems, depending on the desired embodiment.  FIG. 2  illustrates a data storage system architecture  200  according to an exemplary embodiment which is in no way intended to limit the invention. Moreover, it should be noted that the data storage system  220  of  FIG. 2  may include various components found in the embodiment of  FIG. 1 . 
     Looking to  FIG. 2 , the data storage system  220  comprises a number of interface cards  202  configured to communicate via I/O interconnections  204  to one or more processor systems  201 . The data storage system  220  may also comprise one or more RAID controllers  206  configured to control data storage in a plurality of non-volatile data storage cards  208 . The non-volatile data storage cards  208  may comprise NVRAM, Flash memory cards, RAM, ROM, and/or some other known type of non-volatile memory. 
     The I/O interconnections  204  may include any known communication protocols, such as Fiber Channel (FC), FC over Ethernet (FCoE), Infiniband, Internet Small Computer System Interface (iSCSI), Transport Control Protocol/Internet Protocol (TCP/IP), Peripheral Component Interconnect Express (PCIe), etc., and/or any combination thereof. 
     The RAID controller(s)  206  in the data storage system  220  may perform a parity scheme similar to that employed by RAID-5, RAID-10, or some other suitable parity scheme, as would be understood by one of skill in the art upon reading the present descriptions. 
     Each processor system  201  comprises one or more processors  210  (such as CPUs, microprocessors, etc.), local data storage  211  (e.g., such as RAM  914  of  FIG. 9 , ROM  916  of  FIG. 9 , etc.), and an I/O adapter  218  configured to communicate with the data storage system  220 . 
     Referring again to  FIG. 1 , memory controllers  108 , GPP  112  and/or other controllers described herein (e.g., RAID controllers  206  of  FIG. 2 ) may be able to perform various functions on stored data, depending on the desired embodiment. Specifically, memory controllers or GPP  112  may include logic configured to perform any one or more of the following functions, which are in no way intended to be an exclusive list. In other words, depending on the desired embodiment, logic of a storage system may be configured to perform additional or alternative functions, as would be appreciated by one skilled in the art upon reading the present description. 
     Garbage Collection 
     Garbage collection in the context of SSD memory controllers of the present description may include the process of identifying blocks of data to be reclaimed for future usage and relocating all pages that are still valid therein. Moreover, depending on the specific controller and/or the respective garbage collection unit of operation, logical erase blocks (LEBs) may be identified for being reclaimed and/or relocated. Typically, one LEB corresponds to one block stripe, but alternative implementations may consider a fixed number of block stripes or a single block building a LEB as well. 
     A physical “block” represents a minimal unit that may be erased on non-volatile memory, e.g., such as NAND Flash memory, and thereby prepared for writing data thereto. However, a typical garbage collection unit of operation is often a multiple of the physical blocks of non-volatile memory, and is also referred to herein as a LEB. This is due to the fact that typically RAID-like parity information is added in LEBs. Therefore, in case of a page or block failure data can only be rebuilt when all blocks in the LEB are still holding data. Accordingly, the individual blocks from the garbage collection unit can only be erased either individually or in a single unit once all still valid data from all blocks in the LEB has been relocated successfully to new locations. Hence, the full garbage collection units are garbage-collected as a single unit. Moreover, the size of the LEB directly affects the garbage collection induced write amplification. The larger the LEB, the more likely it becomes that unrelated data are stored together in the LEB, and therefore more of the LEB data may have to be relocated upon garbage collection selection. 
     Frequently, blocks from different dies and/or flash channels are grouped together, such that blocks from the same group can be read or written in parallel, thereby increasing overall bandwidth. It is also possible to combine the previous two methods, and to compose RAID stripes using blocks from different flash channels that can be accessed in parallel. 
     It should also be noted that an LEB may include any multiple of the physical memory block, which is a unit of physical erasure. Moreover, the organization of memory blocks into LEBs not only allows for adding RAID-like parity protection schemes among memory blocks from different memory chips, memory planes and/or channels but also allows for significantly enhancing performance through higher parallelism. For instance, multiple non-volatile memory blocks may be grouped together in a RAID stripe. As will be appreciated by one skilled in the art upon reading the present description, RAID schemes generally improve reliability and reduce the probability of data loss. 
     According to an exemplary embodiment, which is in no way intended to limit the invention, memory controllers (e.g., see  108  and/or GPP  112  of  FIG. 1 ) may internally perform a garbage collection. As previously mentioned, the garbage collection may include selecting a LEB to be relocated, after which all data that is still valid on the selected LEB may be relocated (e.g., moved). After the still valid data has been relocated, the LEB may be erased and thereafter, used for storing new data. The amount of data relocated from the garbage collected LEB determines the write amplification. Moreover, an efficient way to reduce the write amplification includes implementing write heat separation. 
     Write Heat Separation 
     In the present context, the “write heat” of data refers to the rate (e.g., frequency) at which the data is updated (e.g., rewritten with new data). Memory blocks that are considered “hot” tend to have a frequent updated rate, while memory blocks that are considered “cold” have an update rate slower than hot blocks. 
     Tracking the write heat of a logical page may involve, for instance, allocating a certain number of bits in the LPT mapping entry for the page to keep track of how many write operations the page has seen in a certain time period or window. Typically, host write operations increase the write heat whereas internal relocation writes decrease the write heat. The actual increments and/or decrements to the write heat may be deterministic or probabilistic. 
     Similarly, read heat may be tracked with a certain number of additional bits in the LPT for each logical page. To reduce meta-data, read heat can also be tracked at a physical block level where separate counters per block for straddling and non-straddling reads can be maintained. However, it should be noted that the number of read requests to and/or read operations performed on a memory block may not come into play for write heat separation when determining the write heat of the memory block for some embodiments. For example, if data is frequently read from a particular memory block, the high read frequency does not necessarily mean that memory block will also have a high update rate. Rather, a high frequency of read operations performed on a given memory block may denote an importance, value, etc. of the data stored in the memory block. 
     By grouping memory blocks of the same and/or similar write heat values, write heat separation may be achieved. In particular, heat segregating methods may group hot memory pages together in certain memory blocks while cold memory pages are grouped together in separate memory blocks. Thus, a heat segregated LEB tends to be occupied by either hot or cold data. 
     The merit of write heat separation is two-fold. First, performing a garbage collection process on a hot memory block will prevent triggering the relocation of cold data as well. In the absence of write heat separation, updates to hot data, which are performed frequently, also results in the undesirable relocations of all cold data collocated on the same LEB as the hot data being relocated. Therefore, the write amplification incurred by performing garbage collection is much lower for embodiments implementing write heat separation. 
     Secondly, the relative heat of data can be utilized for wear leveling purposes. For example, hot data may be placed in healthier (e.g., younger) memory blocks, while cold data may be placed on less healthy (e.g., older) memory blocks relative to those healthier memory blocks. Thus, the rate at which relatively older blocks are exposed to wear is effectively slowed, thereby improving the overall endurance of a given data storage system implementing heat separation. 
     Write Allocation 
     Write allocation includes placing data of write operations into free locations of open LEBs. As soon as all pages in a LEB have been written, the LEB is closed and placed in a pool holding occupied LEBs. Typically, LEBs in the occupied pool become eligible for garbage collection. The number of open LEBs is normally limited and any LEB being closed may be replaced, either immediately or after some delay, with a fresh LEB that is being opened by associating one or more erased blocks to the LEB. 
     During performance, garbage collection may take place concurrently with user write operations. For example, as a user (e.g., a host) writes data to a device, the device controller may continuously perform garbage collection on LEBs with invalid data to make space for the new incoming data pages. As mentioned above, the LEBs having the garbage collection being performed thereon will often have some pages that are still valid at the time of the garbage collection operation; thus, these pages are preferably relocated (e.g., written) to a new LEB. 
     Again, the foregoing functions are in no way intended to limit the capabilities of any of the storage systems described and/or suggested herein. Rather, the aforementioned functions are presented by way of example, and depending on the desired embodiment, logic of a storage system may be configured to perform additional or alternative functions, as would be appreciated by one skilled in the art upon reading the present description. 
     Referring now to  FIG. 3 , a system  300  is illustrated in accordance with one embodiment. As an option, the present system  300  may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. However, such system  300  and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the system  300  presented herein may be used in any desired environment, e.g., in combination with a controller. 
     As illustrated, system  300  includes a write cache  302  which is coupled to several other components, including garbage collector  304 . As previously mentioned, garbage collector  304  may be used to free LEB units by relocating valid data and providing non-volatile memory blocks to be erased for later reuse. Thus, the garbage collector  304  may reclaim blocks of consecutive physical space, depending on the desired embodiment. According to an exemplary embodiment, block erase units may be used to keep track of and/or complete the erase of non-volatile memory blocks handed over by the garbage collector  304 . 
     Write cache  302  is also coupled to free block manager  306  which may keep track of free non-volatile memory blocks after they have been erased. Moreover, as would be appreciated by one of ordinary skill in the art upon reading the present description, the free block manager  306  may build free stripes of non-volatile memory blocks from different lanes (e.g., block-stripes) using the erased free non-volatile memory blocks. 
     Referring still to  FIG. 3 , write cache  302  is coupled to LPT manager  308  and memory I/O unit  310 . The LPT manager  308  maintains the logical-to-physical mappings of logical addresses to physical pages in memory. According to an example, which is in no way intended to limit the invention, the LPT manager  308  may maintain the logical-to-physical mappings of 4KiB or 16KiB logical addresses. The memory I/O unit  310  communicates with the memory chips in order to perform low level operations, e.g., such as reading one or more non-volatile memory pages, writing a non-volatile memory page, erasing a non-volatile memory block, etc. 
     To better understand the distinction between block-stripes and page-stripes as used herein,  FIG. 4  is a conceptual diagram  400 , in accordance with one embodiment. LEBs are built from block stripes and typically a single block stripe is used to build a LEB. However, alternative embodiments may use multiple block stripes to form an LEB. As an option, the present conceptual diagram  400  may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. However, such conceptual diagram  400  and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the controller conceptual diagram  400  presented herein may be used in any desired environment. Thus, the exemplary non-volatile memory controller conceptual diagram  400  of  FIG. 4  may be implemented in a cache architecture. However, depending on the desired embodiment, the conceptual diagram  400  of  FIG. 4  may be implemented in defining the organization of data stored in non-volatile memory. Accordingly, both implementations are described in turn below. 
     Non-Volatile Memory 
     Looking now to  FIG. 4 , the conceptual diagram  400  includes a set of M+1 aggregated planes labeled “Plane 0” through “Plane M”. An aggregated plane consists of all physical planes with the same plane index on different channels. It should be noted that aggregated planes are also referred to herein simply as planes. 
     When implemented with data stored in non-volatile memory, each physical plane on a channel may include a large set of blocks, e.g., typically in the order of 1024, 2048 or more. Moreover, one or more physical planes may also include several additional blocks which may be used as replacement blocks for bad blocks (e.g., blocks performing poorly, blocks having undesirable characteristics, etc.). 
     In each plane of non-volatile memory, a single block from each channel may form a respective block-stripe. It follows that a number of block-stripes supported by a given embodiment of non-volatile memory may be determined by the number of blocks per plane and the number of planes. 
     In the exploded view of Plane 0, the conceptual diagram  400  further illustrates a single block-stripe (Block-stripe 0) out of the set of block-stripes supported in the remainder of the planes. Block-stripe 0 of plane 0 is shown as including 11 blocks, one block from each channel labeled “Channel 0” through “Channel 10”. It should be noted that the association of blocks to block-stripe can change over time as block-stripes are typically dissolved after they have been garbage collected. Erased blocks may be placed in free block pools, whereby new block-stripes are assembled from blocks in the free block pools when write allocation requests fresh block-stripes. For example, looking to conceptual diagram  400 , Block 10 from Channel 0 and Block 41 from Channel 4 are currently associated with the illustrated Block-stripe 0 of Plane 0. Furthermore, the illustrated Block-stripe 0 holds N+1 page-stripes and each block therefore holds N+1 pages labeled “Page 0” through “Page N”. 
     Cache Architecture 
     Referring still to  FIG. 4 , each block of pages illustrated in the exploded view of aggregated Plane 0 may constitute a unique block from one channel when implemented in a cache architecture. Similarly, each channel contributes a single, individual block which form a block-stripe. For example, looking to conceptual diagram  400 , Block 10 from Channel 0 includes all pages (Page 0 through Page N) therein, while Block 41 from Channel 4 corresponds to all pages therein, and so on. 
     In the context of a memory controller, e.g., which may be capable of implementing RAID at the channel level, a block-stripe is made up of multiple blocks which amount to a stripe of blocks. Looking still to  FIG. 4 , the multiple blocks of aggregated Plane 0 constitute Block-stripe 0. While all blocks in a block-stripe typically belong to the same aggregated plane, in some embodiments one or more blocks of a block-stripe may belong to different physical planes. It follows that each aggregated plane may include one or more block-stripe. Thus, according to an illustrative embodiment, Block 0 through Block 10 from different physical planes may constitute a block-stripe. 
     Regardless of whether the conceptual diagram  400  of  FIG. 4  is implemented with non-volatile memory and/or a cache architecture, in different embodiments, the number of pages in each block and/or the number of channels in each plane may vary depending on the desired embodiment. According to an exemplary embodiment, which is in no way intended to limit the invention, a block may include 256 pages, but could include more or less in various embodiments. Analogously, the number of channels per plane and/or the number of planes may vary depending on the desired embodiment. 
     Referring still to  FIG. 4 , all pages in a block-stripe with the same page index denote a page-stripe. For example, Page-stripe 0 includes the first page (Page 0) of each channel in Block-stripe 0 of Plane 0. Similarly, Page-stripe N includes the last page (Page N) of each channel in Block-stripe 0 of Plane 0. 
     As alluded to above, physical configurations of memory blocks in non-volatile memory have changed over time. For instance, in the past Flash blocks had a single-bit-per-cell configuration, but improvements in data storage have introduced Flash blocks that implement multi-bit-per-cell configurations. While blocks having multi-bit-per-cell configurations are able to store more information than blocks having a single-bit-per-cell configuration, this increase in storage capacity comes at a price in terms of performance latency. For example, memory blocks having a single-bit-per-cell configuration experience write latencies that are from about 2.5 times to about 10 times lower than experienced by memory blocks having multi-bit-per-cell configurations. Similarly, memory blocks having a single-bit-per-cell configuration experience lower read latencies than memory blocks having multi-bit-per-cell configurations. 
     This tradeoff between storage capacity and performance latency allows for storage systems to tailor the performance capabilities of memory to some extent. For instance, a storage system predicted to have a high amount of I/O traffic or program/erase (P/E) cycles is able to selectively include a greater number of memory blocks having a single-bit-per-cell configuration than those having multi-bit-per-cell configurations. Conversely, a storage system predicted to store a large amount of cold data is able to selectively include a greater number of memory blocks having multi-bit-per-cell configurations than those having a single-bit-per-cell configuration. However, blocks of non-volatile memory degrade with use over time, eventually becoming error prone and ultimately unusable. As a result, conventional storage systems implementing a static assignment of memory blocks to different pools have been unable to perform wear leveling and/or maintain efficient memory performance. 
     In sharp contrast to these shortcomings experienced by conventional systems, various ones of the embodiments included herein implement hybrid controllers which are able to dynamically toggle between different block configurations as well as adapt block pool sizes based on utilization and workload properties, e.g., as will be described in further detail below. 
     Looking now to  FIG. 5 , a non-volatile memory module  500  having a hybrid controller  502  is illustrated in accordance with one embodiment. As an option, the present memory module  500  may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. However, such memory module  500  and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, the memory module  500  presented herein may be used in any desired environment. Thus  FIG. 5  (and the other FIGS.) may be deemed to include any possible permutation. 
     As noted above, the memory module  500  includes a hybrid controller  502  which is capable of managing and/or adapting blocks which are configured in single-bit-per-cell mode (also referred to herein as “single-level cell mode”, or “SLC mode”) as well as multi-bit-per-cell mode. According to some approaches, the hybrid controller  502  manages the different block configurations by splitting them into two different pools  504 ,  506 . One of these pools  504  is designated as including those blocks which are configured in SLC mode, while the other pool  506  includes the blocks configured in multi-bit-per-cell mode. 
     Each of the pools  504 ,  506  extend across a number of data storage components (e.g., NVRAM memory modules such as NAND flash memory devices) which together serve as a memory array  508 . It follows that the various components illustrated in  FIG. 5  function as a memory card and may implement any of the approaches described above with respect to memory card  100  of  FIG. 1 . Referring still to  FIG. 5 , the hybrid controller  502  is also coupled to a cache memory  510  as well as a gateway  512  which receives data commands from one or more hosts, storage systems, running applications, etc. The gateway  512  is in turn coupled to a GPP  514  and GPP memory  516 . As mentioned above, the GPP  514  may be of any desired type, e.g., such as an ASIC, FPGA, CPU, etc. Similarly, the GPP memory  516  may be of any desired type, e.g., such as RAM, ROM, battery-backed DRAM, phase-change memory PC-RAM, MRAM, STT-MRAM, etc., and/or combinations thereof. 
     The hybrid controller  502  may store data that is frequently accessed (e.g., read, rewritten, appended to, etc.) in blocks that are configured in SLC mode and therefore included in the first pool  504 , while data that is accessed less frequently is stored in blocks that are configured in multi-bit-per-cell mode and therefore included in the second pool  506 . This storage scheme achieves an efficient use of the different blocks of memory and their respective performance characteristics. However, as memory blocks in each of the pools  504 ,  506  experience different amounts of wear as well as different numbers of P/E cycles over time, the effective health of each block diverges from the others. Even though wear of different blocks increases differently with increasing P/E cycles, for the same P/E cycles, the wear experienced in multi-bit-cells increases faster than that experienced in SLC cells on average. Moreover, as the ratio of hot and cold data stored in the memory module  500  fluctuates over time, each of the respective pools  504 ,  506  may be underutilized and/or overutilized. 
     Accordingly, the hybrid controller  502  is able to selectively reconfigure any of the blocks in the first and/or second pools  504 ,  506  to dynamically adapt the memory based on actual utilization and/or workload properties. This allows for the memory module  500  to regulate the effective health of each of the memory blocks, e.g., by performing wear leveling. Efficient performance of the overall module  500  is also maintained irrespective of the amount of hot or cold data stored therein. 
       FIG. 6  provides a detailed view of the first and second pools  504 ,  506  in  FIG. 5  and therefore various components of  FIG. 6  have common numbering with those of  FIG. 5 . As shown, the first and second pools  504 ,  506  each receive write operations from the hybrid controller  502 . Data in the write operations received by each of the first and second pools  504 ,  506  is used to fill blocks of memory which have accumulated in the first and second ready-to-use (RTU) queues  600 ,  602 . As blocks in the first and second RTU queues  600 ,  602  are filled, they are returned to the respective occupied block pools  604 ,  606  where they remain until they are identified as being ready to undergo a garbage collection procedure. 
     Upon reaching a low level of erased blocks in one of the RTU queues  600 ,  602 , the garbage collector selects a block from the corresponding occupied block pools  604 ,  606  which has a high number of invalidated pages. As noted above, data included in blocks of memory is invalidated over time as a result of write updates, deletion operations, etc. Once invalidated, the data in a given block of memory is effectively useless and may be deleted. However, selected portions of data cannot be deleted from a block of non-volatile memory while maintaining the remainder of the data in the block. Rather, valid data is first relocated before the whole block is erased. It follows that as the amount of invalidated data in a given block of memory increases, the block becomes an increasingly inefficient use of storage space, thereby increasing the incentive to perform a garbage collection operation on the block. Inversely, performing a garbage collection operation on a block of memory which has a low amount of invalidated data therein is an inefficient use of computing resources and actually decreases efficiency. This tradeoff between computing resource utilization and storage space reclamation may thereby be balanced as desired. The garbage collector then relocates all still valid data to new locations before the block is identified as ready to erase. 
     Blocks identified as ready to erase are transitioned to a ready to erase queue  608 ,  610  in each of the first and second pools  504 ,  506 , respectively. After entering the ready to erase queues  608 ,  610 , a block is eventually erased and returned to the respective RTU queue  600 ,  602 . Blocks of memory may also be transitioned between the first and second pools  504 ,  506 . However, it should be noted that erase operations are performed slightly differently for blocks configured in SLC mode compared to blocks that are configured in multi-bit-per-cell mode. Therefore, the target mode (e.g., the mode a given block is intended to be configured in) for a block is preferably specified when the block is being erased. This results in blocks being able to move from ready to erase queue  610  to RTU queue  602  or  600 , and similarly move from ready to erase queue  608  to RTU queue  600  or  602 , e.g., as indicated by the different arrowed lines in  FIG. 6 . 
     According to the present description, a block that is “transferred” between pools remains in a same plane and channel of memory. In other words, while the functional characteristics of the given block may change as a result of activating and/or deactivating certain bits in each of the cells therein, the blocks themselves do not physically move in the memory. Rather, the blocks are “transferred” between pools in a logical sense. The erase operation may involve specifying the target mode of the block. In some approaches these transfers are indicated in an LPT, an operations log, a block status table, etc. 
     Certain blocks may be identified as being eligible for transfer between the first and second pools  504 ,  506 . According to some approaches, this eligibility may be based on a number of P/E cycles the block has experienced since a last transfer between the pools, whether the block is ready to be erased, one or more credits which are associated to the pools or channels of each pool and correspond to the number of blocks that may be transferred in each direction among the pools, etc., e.g., as will be described in further detail below. 
     Looking now to  FIG. 7 , a method  700  for toggling block modes in a storage system is shown according to one embodiment. The method  700  may be performed in accordance with the present invention in any of the environments depicted in  FIGS. 1-6 , among others, in various embodiments. Of course, more or less operations than those specifically described in  FIG. 7  may be included in method  700 , as would be understood by one of skill in the art upon reading the present descriptions. For instance, the various processes included in method  700  are described below in the context of evaluating memory blocks in a first pool which are configured in SLC mode. However, any one or more of the various processes included in method  700  may be applied in the context of evaluating memory blocks in a second pool which are configured in multi-bit-per-cell mode, e.g., as will be described in further detail below. 
     Each of the steps of the method  700  may be performed by any suitable component of the operating environment. For example, in various embodiments, the method  700  may be partially or entirely performed by a hybrid controller, a controller, a processor, a computer, etc., or some other device having one or more processors therein. Thus, in some embodiments, method  700  may be a computer-implemented method. In such embodiments, the computer used to implement the method may include a memory module itself or a portion thereof, e.g., such as the hybrid controller. Moreover, the terms computer, processor and controller may be used interchangeably with regards to any of the embodiments herein, such components being considered equivalents in the many various permutations of the present invention. 
     Moreover, for those embodiments having a processor, the processor, e.g., processing circuit(s), chip(s), and/or module(s) implemented in hardware and/or software, and preferably having at least one hardware component may be utilized in any device to perform one or more steps of the method  700 . Illustrative processors include, but are not limited to, a central processing unit (CPU), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), etc., combinations thereof, or any other suitable computing device known in the art. 
     As shown in  FIG. 7 , operation  702  of method  700  includes maintaining a block switching metric for each block of memory in the storage system. The block switching metric is used in some approaches to determine whether a given block of memory may and/or should (e.g., “must”) be reconfigured and switched to the other pool. It follows that the block switching metric maintained for a given block is preferably capable of tracking the amount of use the given block has experienced. It should be noted that in some approaches, block switching metrics are only maintained for blocks that have a predetermined number of programmed pages included therein. For example, block switching metrics are maintained for blocks for which at least half of the pages therein have been programmed before the block switch operation is initiated. 
     Depending on the approach, different types of block switching metrics may be maintained for each of the blocks of memory. For instance, in some approaches the block switching metric includes a P/E cycle count for the respective block. Moreover, the P/E cycle count may be specific to a certain configuration for the respective block. For instance, the P/E cycle count may only track the number of P/E cycles that have been experienced by the respective block while configured in multi-bit-per-cell mode or SLC mode. However, endurance of blocks configured in multi-bit-cell mode is generally lower than endurance of blocks configured in SLC mode, and therefore it is advantageous to track P/E cycles in multi-bit-mode if tracked in only one mode. Moreover, in a preferred embodiment, the P/E cycle count in the switching metrics may be reset when switching the block mode such that only the P/E cycles in the current mode since the last switch are used. This desirably reduces the amount of metadata stored for each block of memory, thereby increasing the efficiency of the overall system. This P/E cycle count may be used to extrapolate the P/E cycle count for the respective block while configured in other modes, e.g., as would be appreciated by one skilled in the art after reading the present description. 
     In other approaches, the block switching metrics includes a raw bit error rate (RBER) for the respective block. The RBER may further be maintained as an average value for all pages in the given block, the highest (e.g., worst) value of any of the pages or codewords in the given block, the median value of all pages in the given block, etc. A block switching metric based on the RBER of a block preferably reflects the contributions to the RBER from permanent changes in the underlying threshold voltage distributions from program-erase cycling. In still further approaches, the block switching metrics includes a data access times for the respective block, e.g., such as read performance times, write performance times, erase times, etc. Combinations of different block switching metrics may even be implemented in some approaches. Although not depicted in  FIG. 7 , method  700  may also include initializing one or more credits which correspond to the number of blocks that can be transferred in each direction among the pools. With respect to the present description, a “credit” is used in some approaches to ensure that the memory pools remain relatively equal in terms of their respective size, e.g., as will be described in further detail below. 
     The method  700  proceeds with operation  703  where a block is selected to be erased from the ready to erase queue  608  in the first pool before proceeding to decision  704 . In an alternative embodiment operation  703  may also select a block from the occupied block pool  606 . However, as all still valid data must be relocated prior to erasing the block, it is advantageous to select a block from the ready to erase queue  608 . Decision  704  includes determining whether an amount of programmed pages in the first block is in a predetermined range. Some approaches specify that at least half of the pages are to be programmed in order to change the mode of the block while others may not have such a restriction. 
     In response to determining that the amount of programmed pages in the first block is not in the predetermined range, method  700  proceeds with operation  724  which is described below. However, in response to determining that the amount of programmed pages in the first block is sufficient and hence in the predetermined range, method  700  proceeds to decision  705 . There, decision  705  includes determining whether a first block in a first pool must be transferred to a second pool according to the block switching metric which corresponds to the first block. As mentioned above, blocks of memory may be divided into two different pools, where each pool includes blocks of memory that are configured in the same mode. For instance, the blocks in the first pool are configured in SLC mode while the blocks in the second pool are configured in multi-bit-per-cell mode (e.g., see pools  504 ,  506  of  FIG. 5  above). However, the number of pools and the types of block configurations included therein may vary depending on the desired approach. 
     It should also be noted that “the first block” is in no way intended to be limited to a specific block of memory. Rather, the term “first block” has been used to reference any one of the blocks in the first pool which is currently being evaluated by method  700 . Method  700  inspects blocks in the ready to erase queues (e.g., see  608 ,  610 ), but in an alternative approach, method  700  may also inspect all blocks of memory in each pool in a sweeping fashion and therefore decision  705  and/or any of the other processes included herein may be repeated in an iterative fashion. Clearly, any block selected by method  700  that is currently in the occupied block pool (e.g., see  606 ,  604 ) preferably has all valid data therein relocated first by a garbage collection procedure before erasing the block, e.g., as would be appreciated by one skilled in the art after reading the present description. 
     Again, the block switching metric which corresponds to the first block is used to determine whether the first block must be transferred to the second pool. Accordingly, in some approaches decision  705  includes determining whether the block switching metric is in a predetermined range. Different types of block switching metrics may be maintained for each pool or each of the blocks of memory and therefore the process of determining whether the first block should be transferred to the second pool may vary depending on what type of metric is available. For instance, in some approaches the block switching metric includes a P/E cycle count for the first block which may be used to determine if the first block has experienced a predetermined number of P/E cycles since being switched to the current mode or globally (i.e., P/E cycles in either mode). In other approaches, a RBER value reflecting permanent changes in the underlying threshold voltage distributions from program-erase cycling and/or data access times associated with the first block may be used to determine whether the first block is performing undesirably. It follows that the use of thresholds is a desirable way of implementing the block switching metrics, as the thresholds in each mode can be different. The thresholds may also be dynamically adapted based on workload properties (e.g., when pools are being resized). 
     In response to determining that the first block must be transferred to the second pool according to the block switching metric corresponding thereto, method  700  proceeds to operation  708  which includes erasing the first block. 
     Method  700  further includes transferring the first block from the first pool to the second RTU queue which corresponds to the second pool. See operation  710 . According to the present description, a block that is “transferred” between RTU queues and/or pools remains in a same plane and channel of memory (e.g., see  FIG. 4 ). In other words, while the functional characteristics of the given block may change as a result of activating and/or deactivating certain bits in each of the cells therein, the blocks themselves do not physically move in the memory. Rather, the blocks are “transferred” between RTU queues and/or pools in a logical sense. In some approaches these transfers are indicated in an LPT, an operations log, etc. 
     Again, the first pool (along with the first RTU queue) includes memory blocks configured in SLC mode while the second pool (along with the second RTU queue) includes memory blocks configured in multi-bit-per-cell mode according to the present embodiment. Depending on the approach, the blocks in the second pool that are configured in multi-bit-per-cell mode may have a number of different configurations. For instance, in some approaches the blocks in the second pool that are configured in multi-bit-per-cell mode are configured in multi-level cell (MLC) mode, while in other approaches the blocks in the second pool that are configured in multi-bit-per-cell mode are configured in triple-level cell (TLC) mode. In still other approaches, the blocks in the second pool that are configured in multi-bit-per-cell mode are configured in quad-level cell (QLC) mode, and each of the blocks configured in QLC mode include lower pages, upper pages, extra pages, and top pages. 
     Therefore, because each of the first and second pools include memory blocks which are configured in different modes, the process of actually transferring the first block in operation  710  involves reconfiguring the block. Depending on whether the selected block is being transferred from the first pool to the second RTU queue or the second pool to the first RTU queue, the process of reconfiguring the block of memory may be performed differently. For instance, blocks being transferred from the first pool to the second RTU queue are preferably reconfigured from SLC mode to multi-bit-per-cell mode. However, blocks which are being transferred from the second pool to the first RTU queue are preferably reconfigured from multi-bit-per-cell mode to SLC mode. The process of reconfiguring a given block between SLC and multi-bit-per-cell mode may be performed by deactivating and/or reactivating certain ones of the bits included in the block, logically redefining the given block, etc., or using any processes which would be apparent to one skilled in the art after reading the present description. 
     Blocks are able to be reconfigured in a different mode in response to experiencing changes in the capacity used and/or changes in the workload properties experienced (e.g., skew changes). However, some approaches may not track workload properties. Moreover, when utilization and workload properties remain unchanged, but there are still write operations being performed (e.g., host and/or relocation writes), swapping blocks between pools is performed. 
     Normally, recalibrating a block of memory has an effect on the performance of the block itself. For instance, the read threshold voltage shift values associated with accessing the given block may change in some situations as a result of the recalibration thereof. However, because method  700  causes memory blocks to be reconfigured periodically over time independent of utilization and workload properties, previous read threshold voltage shift values that compensate changes in the underlying threshold voltage distributions from P/E cycling (i.e., read threshold voltages shift values from base calibrations) are often accurate and may be reused in such situations. These previous read threshold voltage shift values may be stored in memory (e.g., a lookup table) and accessed as needed, thereby decoupling base calibration from the block mode toggling process. Moreover, the processes included herein do not rely on write heat separation. This desirably reduces performance delays and conserves computing power, thereby increasing efficiency of the system. However, in some situations a new threshold voltage shift value may be recalibrated for a given block using any processes which would be apparent to one skilled in the art after reading the present description. Further, the periodically swapping of blocks between the pools is advantageous for wear leveling. Performing wear leveling within a pool only, reduces the overall endurance to be dictated by the pool that wears out first. Periodical swapping of blocks allows for performing wear leveling such as health binning in each of the pools independently while the wear between the pools is equalized through the periodical block swapping at the same time such that overall endurance is enhanced to the average endurance of all blocks. 
     From operation  710 , method  700  proceeds to operation  712  which includes selecting a second block from the second pool to be transferred to the first pool. Again, it is preferred that the first and second pools maintain their respective sizes over time (e.g., when the utilization and workload properties do not change or change only marginally). Transferring the second block to the first pool thereby ensures that the second pool does not increase in size as well as ensuring that the first pool does not decrease in size. 
     Typically, the selected first block was previously eligible to be transferred to the second pool according to the block switching metric in the past but there were not enough credits to transfer the block from the first to the second pool as will be further explained below. The method  700  as shown in  FIG. 7  assumes that the credits did not change in the meantime, hence a second block is selected from the second pool in operation  712 . However, in an alternative embodiment, the credits may have been replenished in the meantime (e.g., as a result of another block being moved from the second to the first pool in the meantime or the necessity to shrink the first pool and enlarge the second pool due to changes in the utilization and/or workload properties) as would be appreciated by one skilled in the art upon reading the present description. Consequently, method  700  may decrement a counter corresponding to the switching direction, incrementing a counter corresponding to the opposite switching direction, and directly return (not shown in  FIG. 7 ) to operation  702  (i.e., skipping operations  712 ,  714 ,  716 , and  718  in this case). 
     The block selected in operation  712  may be any of the blocks in the second pool. However, in some approaches a block from the ready to erase queue  610  is selected. In other approaches a block with the highest ongoing P/E cycle count is selected to be transferred to the first pool. In yet other approaches, the block identified as having the highest health metric is selected to be transferred to the first pool. In still other approaches, certain blocks that are marked (e.g., flagged) to indicate that they should be transferred to the other pool may be selected from in operation  712 . 
     Operation  714  includes relocating valid data from the selected second block when the selected second block is from the occupied block pool  606  and not from the ready to erase queue  610 , e.g., according to any of the approaches included herein. If the second selected block is from the ready to erase queue  610 , all valid data has already been relocated and operation  714  is skipped. Relocating the valid data from a block may be performed in any desired manner depending on the approach. For instance, the valid data may be temporarily stored in a cache (e.g., see cache memory  510  in  FIG. 5 ) before being stored in a different block of memory in some approaches. In other approaches, the valid data may be transferred to another block of memory directly. 
     Referring still to  FIG. 7 , operation  716  includes erasing the second block. Furthermore, operation  718  includes actually transferring the second block from the second pool to the first RTU queue which corresponds to the first pool. 
     While this process of transferring blocks between the first and second pools is performed such that swapping the first and second blocks is performed substantially at the same time, the process of balancing the size of the first and second pools may be somewhat delayed in other approaches. For instance, it may be desirable to limit the number of concurrent block switches that occur in normal operation of the system. It may thereby make sense to allow only a limited number of blocks to be switched in each direction between the pools at any point in time using a credit-based scheme. 
     For instance, this credit-based scheme may involve maintaining two counters, each of which corresponds to the number of blocks which can be transferred between the pools in one of the directions (e.g., first to second pool and second to first pool). Initially, n1 and n2 blocks may be switched in each direction, where the values of n1 and n2 may be equal or different. According to an example, which is in no way intended to limit the invention, [n1, n2]=[1, 0], [1, 1], [5,5], etc. According to some approaches, blocks may only be transferred between pools if the counter for the corresponding direction is non-zero. In the situation that a counter is non-zero, the counter is preferably decremented after a block is transferred between the pools in the corresponding direction, while the counter which corresponds to the other direction is incremented. In some approaches, one or both of the counters may be saturating counters. However, in other approaches the pools may even be resized by intentionally not transferring a second block in response to the first block being transferred. The pools may thereby be adjusted in size as desired. 
     Referring still to  FIG. 7 , method  700  returns to operation  702  from operation  718 , e.g., such that the block switching metrics may be updated and/or continue to be monitored. It follows that method  700  is able to successfully regulate the wear of various blocks in memory while also maintaining block pool sizes in real time, thereby ensuring memory longevity and efficient performance and of the overarching storage system. 
     However, returning to decision  705 , method  700  advances to decision  720  in response to determining that the first block in the first pool does not have to be transferred to the second pool according to the block switching metric corresponding thereto. In other words, method  700  advances to decision  720  in response to determining that the block switching metric does not indicate that it is critical that the given block is transferred to the other pool. There, decision  720  includes determining whether the first block is eligible to be transferred to the second pool. 
     As previously mentioned, as the amount of invalidated data in a given block of memory increases, the block becomes an increasingly inefficient use of storage space, thereby increasing the incentive to perform a garbage collection operation on the block. Inversely, performing a garbage collection operation on a block of memory which has a low amount of invalidated data therein is an inefficient use of computing resources and actually decreases inefficiency. This tradeoff between computing resource utilization and storage space reclamation may thereby be implemented in performing decision  720 . 
     Therefore, in preferred approaches decision  720  is determined according to the block switching metric corresponding to the first block as well as according to a saturating counter which corresponds to the switching direction. In other words, as the selected block is preferably from the ready to erase queue and the number of blocks in the ready to erase queue is typically low compared to the total number of blocks in the pool, there is a high chance that the selected block is not yet eligible according to the block switching metric while other blocks may already be eligible. As garbage collection is performed in the background, at some point selected blocks will eventually reach a block switching metric that makes them eligible for being reconfigured. The block switching metric that indicates that the block may be reconfigured and switched to the other pool is reached before the block switching metrics indicate that the block must be reconfigured. Hence, the probability that a block switches mode before it must be switched exist and is relatively high, thereby reducing the probability to perform a forced swapping which may result in operation  712  selecting a second block from the occupied block pool that involves additional overhead to relocate still valid data. For example, decision  720  may involve block switching metrics that determining whether the P/E cycle count since being switched to the current mode is in a predetermined range and/or the RBER for the given block is in a predetermined range which signifies that the block may be transferred if desired, but it is not critical in terms of wear leveling that the transfer occurs. 
     Decision  720  may also incorporate one or more credits which correspond to the amount of blocks that can be transferred between the pools in determining whether the block is eligible to be transferred to the second pool. In other words, the credits and corresponding counters correspond to a specific direction in which the block configuration modes are switched. For example, saturating counters may be assigned to each of the blocks for keeping track of how many times the given block has been transferred from one pool to the other. Again, in some approaches a block may only be transferred between pools if the counter for the intended transfer direction is non-zero. In the situation that a counter is non-zero, the counter is preferably decremented after a block is transferred between the pools in the given direction, while the counter which corresponds to the other direction is incremented. 
     In response to determining the first block is not eligible to be transferred to the second pool according to the block switching metric which corresponds thereto, method  700  proceeds to operation  724  which includes erasing the first block, while operation  726  includes returning the first block to the first RTU queue. From operation  726 , method  700  returns to operation  702  as depicted, e.g., such that the block switching metrics may be updated and/or continue to be monitored. 
     However, in response to determining that the first block is eligible to be transferred to the second pool according to the block switching metric which corresponds thereto, method  700  proceeds to operation  730  which includes erasing the first block. Operation  732  further includes transferring the first block from the first pool to the second RTU queue according to any of the approaches described herein. For approaches implementing counters for each switching direction, a counter which corresponds to the switching direction from the first to the second pool may also be decremented, and a counter which corresponds to the opposite switching direction may be incremented, in response to finally transferring the first block from the first pool to the second RTU queue. From operation  732 , method returns to operation  702 . 
     As previously mentioned, one or more of the various processes included in method  700  may be applied in the context of evaluating memory blocks in a second pool which are configured in multi-bit-per-cell mode, and potentially transitioning one or more of them to being configured in SLC mode. Accordingly, any one or more of the processes included in method  700  and/or any of the approaches described in conjunction therewith, may be implemented in the context of evaluating blocks of memory included in the second pool and determining whether they should be transferred to the first pool. In other words, the processes included in method  700  may be used to monitor any block of memory in a storage system, e.g., as would be appreciated by one skilled in the art after reading the present description. 
     According to an example, which is in no way intended to limit the invention, the processes included in method  700  may be used to determining whether a block in the second pool must be transferred to the first pool according to a block switching metric which corresponds to the given block. In response to determining that the block in the second pool must be transferred to the first pool according to the block switching metric which corresponds thereto, valid data is relocated from the block if the block is not from the ready to erase queue before the block is erased. The block is further transferred from the second pool to the first RTU queue. A second block in the first pool is also selected to replace the block transferred from the second pool. Valid data in the selected second block is relocated if any before the selected second block is erased and transferred from the first pool to the second RTU queue. Further, the processes included in method  700  may be used to determining whether a block in the second pool is eligible to be transferred to the first pool according to a block switching metric which corresponds to the given block. In response to determining that the block in the second pool is eligible to be transferred to the first pool according to the block switching metric which corresponds thereto, valid data is relocated from the block if the block is not from the ready to erase queue before the block is erased. The block is further transferred from the second pool to the first RTU queue. 
     It follows that the various approaches described herein are able to use characterizations to determine the average block endurance for blocks which are configured in different modes. The average block endurance in each mode can also be used to determine a target ratio of P/E cycles the blocks are intended to experience in each mode. For example, a ratio of 1:9 P/E cycles in each mode may be set as a target, as blocks in SLC mode typically have nine times the endurance of blocks in QLC mode on average. Simple thresholds are also used to control toggling between the different block modes as well as reduce the block-mode switching frequency and/or the associated computing overhead. For instance, a first threshold may be used to indicate when a given block becomes eligible for switching, while a second threshold may be used to determine when a given block must be switched. As a result, the blocks of memory are switched between pools frequently enough to reduce variance of block wear between the pools, and combined with wear leveling, to reduce variance of block wear among all blocks, enable threshold voltage shift value updates in either mode, and accurate health tracking of the block. Health binning and/or write-stream mapping may further be implemented with any of the approaches included herein, e.g., in order to further improve wear leveling performance. 
     It should be noted that although various ones of the approaches have been described herein in the context of two memory block pools, any of these approaches may be extended to embodiments having more than two block pools. In such embodiments, the target pool can be determined using priorities, based on user input, by comparing the RTU block levels of each respective pool and selecting the pool that has the lowest number of RTU blocks, etc. It should also be noted that terms like “in a range” and “above a threshold” are in no way intended to limit the invention. Rather than determining whether a value is in a given range or above a given threshold, equivalent determinations may be made, e.g., as to whether a value is outside a predetermined range, whether an absolute value is above a threshold, whether a value is below a threshold, etc., depending on the desired approach. 
     The present invention may be a system, a method, and/or a computer program product at any possible technical detail level of integration. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
     The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
     Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, configuration data for integrated circuitry, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++, or the like, and procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
     These computer readable program instructions may be provided to a processor of a computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be accomplished as one step, executed concurrently, substantially concurrently, in a partially or wholly temporally overlapping manner, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions. 
     Moreover, a system according to various embodiments may include a processor and logic integrated with and/or executable by the processor, the logic being configured to perform one or more of the process steps recited herein. By integrated with, what is meant is that the processor has logic embedded therewith as hardware logic, such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), etc. By executable by the processor, what is meant is that the logic is hardware logic; software logic such as firmware, part of an operating system, part of an application program; etc., or some combination of hardware and software logic that is accessible by the processor and configured to cause the processor to perform some functionality upon execution by the processor. Software logic may be stored on local and/or remote memory of any memory type, as known in the art. Any processor known in the art may be used, such as a software processor module and/or a hardware processor such as an ASIC, a FPGA, a central processing unit (CPU), an integrated circuit (IC), a graphics processing unit (GPU), etc. 
       FIG. 8  illustrates a network architecture  800 , in accordance with one embodiment. As shown in  FIG. 8 , a plurality of remote networks  802  are provided including a first remote network  804  and a second remote network  806 . A gateway  801  may be coupled between the remote networks  802  and a proximate network  808 . In the context of the present network architecture  800 , the networks  804 ,  806  may each take any form including, but not limited to a LAN, a WAN such as the Internet, public switched telephone network (PSTN), internal telephone network, etc. 
     In use, the gateway  801  serves as an entrance point from the remote networks  802  to the proximate network  808 . As such, the gateway  801  may function as a router, which is capable of directing a given packet of data that arrives at the gateway  801 , and a switch, which furnishes the actual path in and out of the gateway  801  for a given packet. 
     Further included is at least one data server  814  coupled to the proximate network  808 , and which is accessible from the remote networks  802  via the gateway  801 . It should be noted that the data server(s)  814  may include any type of computing device/groupware. Coupled to each data server  814  is a plurality of user devices  816 . Such user devices  816  may include a desktop computer, laptop computer, handheld computer, printer, and/or any other type of logic-containing device. It should be noted that a user device  811  may also be directly coupled to any of the networks, in some embodiments. 
     A peripheral  820  or series of peripherals  820 , e.g., facsimile machines, printers, scanners, hard disk drives, networked and/or local data storage units or systems, etc., may be coupled to one or more of the networks  804 ,  806 ,  808 . It should be noted that databases and/or additional components may be utilized with, or integrated into, any type of network element coupled to the networks  804 ,  806 ,  808 . In the context of the present description, a network element may refer to any component of a network. 
     According to some embodiments, methods and systems described herein may be implemented with and/or on virtual systems and/or systems which emulate one or more other systems, such as a UNIX system which virtually hosts a MICROSOFT WINDOWS environment, etc. This virtualization and/or emulation may be enhanced through the use of VMWARE software, in some embodiments. 
     In other embodiments, one or more networks  804 ,  806 ,  808 , may represent a cluster of systems commonly referred to as a “cloud.” In cloud computing, shared resources, such as processing power, peripherals, software, data, servers, etc., are provided to any system in the cloud in an on-demand relationship, thereby allowing access and distribution of services across many computing systems. Cloud computing typically involves an Internet connection between the systems operating in the cloud, but other techniques of connecting the systems may also be used, as known in the art. 
       FIG. 9  shows a representative hardware environment associated with a user device  816  and/or server  814  of  FIG. 8 , in accordance with one embodiment.  FIG. 9  illustrates a typical hardware configuration of a processor system  900  having a central processing unit  910 , such as a microprocessor, and a number of other units interconnected via a system bus  912 , according to one embodiment. In some embodiments, central processing unit  910  may include any of the approaches described above with reference to the one or more processors  210  of  FIG. 2 . 
     The processor system  900  shown in  FIG. 9  includes a Random Access Memory (RAM)  914 , Read Only Memory (ROM)  916 , and an I/O adapter  918 . According to some embodiments, which are in no way intended to limit the invention, I/O adapter  918  may include any of the approaches described above with reference to I/O adapter  218  of  FIG. 2 . Referring still to processor system  900  of  FIG. 9 , the aforementioned components  914 ,  916 ,  918  may be used for connecting peripheral devices such as storage subsystem  920  to the bus  912 . In some embodiments, storage subsystem  920  may include a similar and/or the same configuration as data storage system  220  of  FIG. 2 . According to an example, which is in no way intended to limit the invention, storage subsystem  920  may include non-volatile data storage cards, e.g., having NVRAM memory cards, RAM, ROM, and/or some other known type of non-volatile memory, in addition to RAID controllers as illustrated in  FIG. 2 . 
     With continued reference to  FIG. 9 , a user interface adapter  922  for connecting a keyboard  924 , a mouse  926 , a speaker  928 , a microphone  932 , and/or other user interface devices such as a touch screen, a digital camera (not shown), etc., to the bus  912 . 
     Processor system  900  further includes a communication adapter  934  which connects the processor system  900  to a communication network  935  (e.g., a data processing network) and a display adapter  936  which connects the bus  912  to a display device  938 . 
     The processor system  900  may have resident thereon an operating system such as the MICROSOFT WINDOWS Operating System (OS), a MAC OS, a UNIX OS, etc. It will be appreciated that a preferred embodiment may also be implemented on platforms and operating systems other than those mentioned. A preferred embodiment may be written using JAVA, XML, C, and/or C++ language, or other programming languages, along with an object oriented programming methodology. Object oriented programming (OOP), which has become increasingly used to develop complex applications, may be used. 
     Moreover,  FIG. 10  illustrates a storage system  1000  which implements high level (e.g., SSD) storage tiers in combination with lower level (e.g., magnetic tape) storage tiers, according to one embodiment. Note that some of the elements shown in  FIG. 10  may be implemented as hardware and/or software, according to various embodiments. The storage system  1000  may include a storage system manager  1012  for communicating with a plurality of media on at least one higher storage tier  1002  and at least one lower storage tier  1006 . However, in other approaches, a storage system manager  1012  may communicate with a plurality of media on at least one higher storage tier  1002 , but no lower storage tier. The higher storage tier(s)  1002  preferably may include one or more random access and/or direct access media  1004 , such as hard disks, nonvolatile memory (NVM), NVRAM), solid state memory in SSDs, flash memory, SSD arrays, flash memory arrays, etc., and/or others noted herein or known in the art. According to illustrative examples,  FIGS. 3-4  show exemplary architectures of SSD systems which may be used as a higher storage tier  1002  depending on the desired embodiment. 
     Referring still to  FIG. 10 , the lower storage tier(s)  1006  preferably includes one or more lower performing storage media  1008 , including sequential access media such as magnetic tape in tape drives and/or optical media, slower accessing HDDs, slower accessing SSDs, etc., and/or others noted herein or known in the art. One or more additional storage tiers  1016  may include any combination of storage memory media as desired by a designer of the system  1000 . Thus, the one or more additional storage tiers  1016  may, in some approaches, include a SSD system architecture similar or the same as those illustrated in  FIGS. 1-2 . Also, any of the higher storage tiers  1002  and/or the lower storage tiers  1006  may include any combination of storage devices and/or storage media. 
     The storage system manager  1012  may communicate with the storage media  1004 ,  1008  on the higher storage tier(s)  1002  and lower storage tier(s)  1006  through a network  1010 , such as a storage area network (SAN), as shown in  FIG. 10 , or some other suitable network type. The storage system manager  1012  may also communicate with one or more host systems (not shown) through a host interface  1014 , which may or may not be a part of the storage system manager  1012 . The storage system manager  1012  and/or any other component of the storage system  1000  may be implemented in hardware and/or software, and may make use of a processor (not shown) for executing commands of a type known in the art, such as a central processing unit (CPU), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc. Of course, any arrangement of a storage system may be used, as will be apparent to those of skill in the art upon reading the present description. 
     In more embodiments, the storage system  1000  may include any number of data storage tiers, and may include the same or different storage memory media within each storage tier. For example, each data storage tier may include the same type of storage memory media, such as HDDs, SSDs, sequential access media (tape in tape drives, optical disk in optical disk drives, etc.), direct access media (CD-ROM, DVD-ROM, etc.), or any combination of media storage types. In one such configuration, a higher storage tier  1002 , may include a majority of SSD storage media for storing data in a higher performing storage environment, and remaining storage tiers, including lower storage tier  1006  and additional storage tiers  1016  may include any combination of SSDs, HDDs, tape drives, etc., for storing data in a lower performing storage environment. In this way, more frequently accessed data, data having a higher priority, data needing to be accessed more quickly, etc., may be stored to the higher storage tier  1002 , while data not having one of these attributes may be stored to the additional storage tiers  1016 , including lower storage tier  1006 . Of course, one of skill in the art, upon reading the present descriptions, may devise many other combinations of storage media types to implement into different storage schemes, according to the embodiments presented herein. 
     According to some embodiments, the storage system (such as  1000 ) may include logic configured to receive a request to open a data set, logic configured to determine if the requested data set is stored to a lower storage tier  1006  of a tiered data storage system  1000  in multiple associated portions, logic configured to move each associated portion of the requested data set to a higher storage tier  1002  of the tiered data storage system  1000 , and logic configured to assemble the requested data set on the higher storage tier  1002  of the tiered data storage system  1000  from the associated portions. 
     Of course, this logic may be implemented as a method on any device and/or system or as a computer program product, according to various embodiments. 
     It will be clear that the various features of the foregoing systems and/or methodologies may be combined in any way, creating a plurality of combinations from the descriptions presented above. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.