Patent Publication Number: US-10318175-B2

Title: SSD with heterogeneous NVM types

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to and the benefit of U.S. Provisional Application No. 62/468,265, filed Mar. 7, 2017, entitled “SSDS WITH HETEROGENEOUS NVM TYPES”, the entire content of which is incorporated herein by reference. 
    
    
     FIELD 
     One or more aspects of embodiments according to the present invention relate to data storage, and more particularly to a storage device with heterogeneous storage. 
     BACKGROUND 
     Enterprise storage architectures may use a battery backed nonvolatile random access memory (NVRAM) or nonvolatile dual in-line memory module (NVDIMM) device for logging incoming write input/output requests (IO requests). The IO requests my eventually be flushed to lower performance nonvolative data storage media. NVRAM/NVDIMM solutions may be used as a staging area to reduce write latency. However, such solutions may have limited functionality due to having small capacity, increased complexity (e.g., related to central processing unit (CPU) asynchronous dynamic random access memory (DRAM) refresh (ADR) synchronization), and the need to include a battery in the system. Moreover, the performance of such a system may be limited by the number of NVRAMs and/or NVDIMMs that a system can include, and in some applications, the inclusion of batteries may be unacceptable, due to system environment constraints, rendering a battery backed solution unworkable. 
     Further complicating the use of battery backed NVRAM or NVDIMM is the possible need to size the NVDIMMs and/or NVRAMs to trade off cost against high write latency jitter. Thus, there is a need for an improved alternative to a storage system including battery backed NVRAM or NVDIMM. 
     SUMMARY 
     Aspects of embodiments of the present disclosure are directed toward a storage device. The device includes both low-latency persistent memory and higher-latency nonvolatile memory. The persistent memory may be used for write caching or for journaling. A B-tree may be used to maintain an index of write requests temporarily stored in the persistent memory. Garbage collection may be performed in the nonvolatile memory while write requests are being stored in the persistent memory. 
     According to an embodiment of the present invention there is provided a storage device including: a first controller; a first memory bank including a plurality of slow nonvolatile memory cells; a second memory bank including a plurality of fast persistent memory cells; and a storage interface, the first controller being configured to respond to input output requests received through the storage interface; each of the plurality of slow nonvolatile memory cells being: at least ten times greater in read latency than any of the fast persistent memory cells; and at least ten times greater in write latency than any of the fast persistent memory cells, the storage device including at least ten times as many of the slow nonvolatile memory cells as of the fast persistent memory cells. 
     In one embodiment, a page size of the second memory bank is equal to a block size of the second memory bank. 
     In one embodiment, each of the slow nonvolatile memory cells is: at least 100 times greater in read latency than any of the fast persistent memory cells; and at least 100 times greater in write latency than any of the fast persistent memory cells. 
     In one embodiment, the storage interface is selected from the group consisting of: Peripheral Component Interconnect Express (PCIe) interfaces, remote direct memory access (RDMA) over Ethernet interfaces, Serial Advanced Technology Attachment (SATA) interfaces, Fibre Channel interfaces, Serial Attached SCSI (SAS) interfaces, and Non Volatile Memory Express (NVMe) interfaces. 
     In one embodiment, the storage device has a form factor selected from the group consisting of a 3.5 inch hard drive form-factor, and a 2.5 inch hard drive form-factor. 
     In one embodiment, the storage device is configured, upon receiving a write request through the storage interface, to: first, store the write request in the second memory bank; second, execute the write request; and third, delete the write request from the second memory bank. 
     In one embodiment, a portion of the second memory bank is configured to operate as a circular buffer, and the storage device is configured, upon receiving a write request through the storage interface, to store the write request in the circular buffer. 
     In one embodiment, the storage device is further configured upon receiving a write request through the storage interface to store an index entry for the write request in a B-tree index of write requests. 
     In one embodiment, the storage device is configured to concurrently: perform garbage collection in the first memory bank; and store write requests in the circular buffer. 
     In one embodiment, the storage device includes a second controller configured to control operations in the first memory bank, and wherein the storage device is configured to: calculate a future buffering period, as the ratio of: an amount of free space in the circular buffer, and a maximum write rate for the second memory bank; and instruct the second controller to perform garbage collection operations during an interval of time having a length equal to the future buffering period. 
     In one embodiment, the storage device includes a second controller configured to control operations in the first memory bank, and wherein the storage device is configured to: calculate a future buffering period, as the ratio of: an amount of free space in the circular buffer, and a maximum write rate for the second memory bank; and when a dirty block ratio of the first memory bank exceeds a first threshold, instruct the second controller to perform garbage collection operations during an interval of time having a length equal to the future buffering period. 
     In one embodiment, the storage device is further configured to: recalculate the future buffering period during the garbage collection operations, and when the recalculated future buffering period exceeds the previously calculated future buffering period, notify the second controller of the recalculated future buffering period. 
     In one embodiment, the storage device is configured, upon receipt of a write request for a location in the first memory bank, when the B-tree contains a previously made index entry, and the second memory bank contains a corresponding write request, for the same location in the first memory bank: to store the write request in the circular buffer; to store an index entry for the write request in the B-tree index; to delete the previously made index entry from the circular buffer; and to delete the write request corresponding to the previously made index entry in the circular buffer. 
     In one embodiment, the storage device is configured, upon receipt of a read request for a location in the first memory bank, when the B-tree contains a previously made index entry for the same location in the first memory bank: to respond to the read request with data from the circular buffer. 
     In one embodiment, the storage device is configured, when an empty fraction of the circular buffer falls below a threshold, to delay sending a command completion through the storage interface. 
     According to an embodiment of the present invention there is provided a method for operating a storage system, the storage system including a storage device including: a first memory bank including a plurality of slow nonvolatile memory cells; a second memory bank including a plurality of fast persistent memory cells; and a storage interface, each of the plurality of slow nonvolatile memory cells being: at least ten times greater in read latency than any of the fast persistent memory cells; and at least ten times greater in write latency than any of the fast persistent memory cells, the storage device including at least ten times as many of the slow nonvolatile memory cells as of the fast persistent memory cells, the method including: receiving a write request through the storage interface; and storing the write request in a circular buffer in the second memory bank. 
     In one embodiment, the method includes: performing garbage collection in the first memory bank while storing the write request in the circular buffer. 
     In one embodiment, the method includes: storing an index entry for the write request in a B-tree index of write requests. 
     In one embodiment, the method includes: before storing the write request in the circular buffer, calculating a future buffering period, as the ratio of: an amount of free space in the circular buffer, and a maximum write rate for the second memory bank, wherein the performing of the garbage collection includes performing garbage collection during an interval of time having a length equal to the future buffering period. 
     According to an embodiment of the present invention there is provided a storage system, including: a plurality of storage devices, each including: a first memory bank including a plurality of slow nonvolatile memory cells; a second memory bank including a plurality of fast persistent memory cells; and a storage interface, each of the plurality of slow nonvolatile memory cells being: at least ten times greater in read latency than any of the fast persistent memory cells; and at least ten times greater in write latency than any of the fast persistent memory cells, the storage device including at least ten times as many of the slow nonvolatile memory cells as of the fast persistent memory cells, the system being configured to pool the respective second memory banks to form a virtual second memory bank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be appreciated and understood with reference to the specification, claims, and appended drawings wherein: 
         FIG. 1  is a block diagram of a host and a storage device, according to an embodiment of the present invention; 
         FIG. 2  is memory diagram, according to an embodiment of the present invention; 
         FIG. 3  is a block diagram of a storage device, according to an embodiment of the present invention; 
         FIG. 4  is a block diagram of a storage device, according to an embodiment of the present invention; 
         FIG. 5  is a block diagram of a storage device, according to an embodiment of the present invention; 
         FIG. 6  is a block diagram of a storage system, according to an embodiment of the present invention; and 
         FIG. 7  is a block diagram of a storage system, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of an SSD with heterogeneous NVM types provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. 
     Referring to  FIG. 1 , in some embodiments, a host  100  is connected to a storage device (e.g., a heterogeneous SSD) including two banks of memory: a bank of relatively slow nonvolatile memory  110  (which may be NAND flash memory configured to act as a main backing store) and a bank of relatively fast persistent memory  120 . The persistent memory  120  may also be nonvolatile memory and is referred to herein as “persistent memory” primarily to distinguish it from the relatively slow nonvolatile memory  110 . As used herein, the terms “nonvolatile memory”, “persistent storage” and “persistent memory” all refer to memory capable of retaining stored data without having power supplied to it. 
     Persistent memories, such as phase change memory, Z-NAND™ (available from Samsung), 3D X-POINT™ (available from Intel and Micron Technology), Phase Change Memory, Resistive RAM, STTRAM, and the like, are used to provide significantly faster access and low IO latencies. A NAND controller  115  provides an interface to, and controls, the nonvolatile memory  110 , and a persistent memory controller  125  provides an interface to, and controls, the persistent memory  120 . A storage device controller  130 , connected to a storage interface  140  responds to IO requests received from the host  100  through the storage interface  140 . 
     A “memory cell”, as used herein, refers to the smallest set of elements capable of storing data. For example, for NAND flash, a storage cell may be a single floating gate transistor, and it may store one bit of data (if it is configured as a single-level cell) or several bits of data if it is configured as a multi-level cell. 
     The persistent memory  120  may be ten times or 100 times or more faster than the nonvolatile memory  110 , e.g., (i) the read latency of the nonvolatile memory  110  may be ten times or 100 times or more greater than the read latency of the persistent memory  120  and/or (ii) the write latency of the nonvolatile memory  110  may be ten times or 100 times or more greater than the write latency of the persistent memory  120 . The block size of the persistent memory  120  may be the same as the page size of the persistent memory  120 , making garbage collection unnecessary in the persistent memory  120 . As used herein, a “page” is a quantity of memory corresponding to the smallest number of cells that may be written (or programmed) in one operation. If each cell stores one bit (as in the case of NAND flash using single-level cells), a page size of 2048 bytes corresponds to 16,384 cells (2048×8 being 16,384). A “block” is a quantity of memory corresponding to the smallest number of cells that may be erased in one operation. 
     In some embodiments, the storage device conforms to a 3.5 inch hard drive form-factor (or large form factor (LFF)) standard. In other embodiments it conforms to a 2.5 inch hard drive form-factor (or small form factor (SFF)) standard. In other embodiments, the storage device conforms to an M.2 form factor. In some embodiments, derivatives of these form factors are used, e.g., SFF contemplates hot-plug/hot-swap by specification, but LFF does not (accordingly, in some embodiments the storage device conforms to a hot-swappable derivative of the LFF standard). The storage interface  140  may include a connector and a protocol customarily used by a host and storage device to exchange storage requests and responses. The connector and protocol may conform to, for example, Peripheral Component Interconnect Express (PCIe), remote direct memory access (RDMA) over Ethernet, Serial Advanced Technology Attachment (SATA), Fibre Channel, Serial Attached SCSI (SAS), Non Volatile Memory Express (NVMe), or to a more general-purpose interface such as Ethernet or Universal Serial Bus (USB). The storage interface  140  is used by the host  100  to communicate with the storage device. Input output (IO) requests such as data read and data write requests, as well as various media management requests (e.g., Identify, Get Log, etc.) may be sent to the storage device by the host over the storage interface  140 . The same interface may be used by the storage device to perform data transfers to and from internal memory in the host  100 . 
     The persistent memory  120  may be used for write caching to accelerate access to the nonvolatile memory  110  and to reduce overall IO latency. The capacity of the persistent memory  120  in the heterogeneous SSD may be smaller (e.g., 16 GB) than the capacity of the nonvolatile memory  110  (e.g., 480 GB) in a heterogeneous SSD. In some embodiments, the capacity of the nonvolatile memory  110  (e.g., of the NAND flash memory) is presented to the host software as the capacity of the heterogeneous SSD. The low-latency/high-performance persistent memory  120  is not included in the SSD capacity exposed to the host software. 
     In the heterogeneous SSD, NAND Flash memory is configured to act as the main backing store which is responsible for storing the data from SSD users (e.g., from the host  100 ). The capacity of this SSD is the same as the available memory space in the nonvolatile memory  110  (e.g., the NAND flash memory space), and the logical block address (LBA) mapping is simply linear (or managed by the flash translation layer (FTL)). The persistent memory  120  may function as a write-ahead log (WAL); e.g., it may buffer all the coming writes temporary and write them back to backing store (e.g., to the nonvolatile memory  110 ) in the background. 
       FIG. 2  illustrates, in some embodiments, the internal data organization in the persistent memory  120  and how it buffers and writes back incoming writes. The persistent memory  120  is used as a circular buffer; each new write request is stored in the work-in-progress log record which is either a new log record or the current partially full log record. Log records are fixed size data chunks (e.g. 256 kB) each of which includes (e.g., consists of) a plurality of smaller write IO requests. The requests are filled into persistent memory LBA space sequentially; when the end of the persistent memory  120  is reached, the location being written (which may be identified by a write pointer) wraps around to the beginning of the circular buffer, which, if the buffer is not full, has free LBA space. The persistent memory super block  210  logs the start and end of valid log records. A background flushing process writes back valid and complete log records into the backing store (i.e., into the nonvolatile memory  110 ), and, when it does so, frees the corresponding persistent memory space. In this manner the persistent memory  120  embedded in a heterogeneous SSD may be used for journaling or write-ahead-logging (WAL) functionality, and these operations may be transparent to host software. 
     In some embodiments, the heterogeneous SSD also maintains an in-memory B-tree index (e.g., i the persistent memory  120 ) of all the buffered write requests. This index records the mapping of the LBA of each original write request to the specific log records which store this write request. The index serves two purposes, one of which is to remove earlier received, and not yet executed write requests to the same LBA address, to avoid superfluous writes to the main backing store (e.g., to the nonvolatile memory  110 ). Another purpose of the index is to serve those read requests which should be served using the data from persistent memory, for example, in situations in which a write request to the same LBA is pending and has not yet been executed on the nonvolatile memory  110 . As used herein, a B-tree refers to a self-balancing tree data structure that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time. As used herein, “executing” (or “flushing”) a write request (including data to be written and a logical block address (LBA) identifying the location to which the data are to be written) refers to storing data to be written at the physical address, in nonvolatile memory  110 , corresponding (according to one or more tables maintained by the flash translation layer) to the LBA. 
     Referring to  FIG. 3 , in some embodiments, the heterogeneous SSD performs coordinated background operations such as NAND flash garbage collection (which, when performed as a coordinated background operation may be referred to as “smart garbage collection”). The write-ahead log may at some times write back buffered write requests to the main backing store, and at other times it may buffer incoming write requests from the host. While the write-ahead log is buffering incoming write requests from the host, the heterogeneous SSD may issue a smart garbage collection request to the NAND controller. As used herein, garbage collection refers to one or more operations of moving one or more valid pages from a block containing one or more invalid pages to one or more other blocks, and erasing the block. 
     The operation may proceed as follows. The persistent memory controller  125  may estimate the future buffering period (FBP) without data write-back using the following equation:
 
FBP=Available_PM_SPACE/PM_MAX_THROUGHPUT
 
     where Available_PM_SPACE is the amount of persistent memory  120  that is available for buffering, and PM_MAX_THROUGHPUT is the maximum rate at which the persistent memory  120  may be filled by write requests from the host. The storage device controller  130  then checks the NAND Flash memory dirty block ratio (i.e., the dirty block ratio in the nonvolatile memory  110 ) and if the NAND Flash memory dirty block ratio is higher than a configurable threshold, the storage device controller  130  then issues (as indicated by arrow  310 ) a smart garbage collection request to the NAND controller  115 , instructing the NAND controller  115  to perform garbage collection to the extent possible within an initial period of time having a length equal to the FBP. During the garbage collection operation, the storage device controller  130  may periodically recalculate the FBP (which may change if the actual rate at which write requests received from the host are filling the persistent memory  120  is less than the maximum possible rate), and notify the NAND controller  115  of any such increase in the FBP during the garbage collection operation, enabling the NAND controller  115  to make full use of the opportunity to perform garbage collection. The NAND controller  115  may notify (as indicated by arrow  320 ) the persistent memory controller  125  when the garbage collection operation has terminated (either at the end of the FBP, or, as a result of being completed earlier, before the end of the FBP), and write-back of data from the persistent memory  120  to the nonvolatile memory  110  may then resume (as indicated by arrow  330 ), under the control of the storage device controller  130 , the persistent memory controller  125 , and the NAND controller  115 . As used herein, the “dirty block ratio” is the ratio of (i) the number of blocks in the nonvolatile memory  110  each containing more than a set number of dirty pages, to (ii) the number of blocks in the nonvolatile memory  110 . 
       FIG. 4  illustrates how a heterogeneous SSD with a write-ahead log implemented in persistent memory  120  may handle incoming write IO requests along with coordinated NAND flash garbage collection. When a new write request is received, e.g., through the storage interface  140  (as indicated by arrow  410 ), the storage device controller  130  submits (as indicated by arrow  420 ) the write request to the persistent memory controller  125 , which stores a new log record in the persistent memory  120 , and returns (as indicated by arrow  430 ) an acknowledgement to the storage device controller  130 . The storage device controller  130  then returns (as indicated by arrow  440 ) an acknowledgement to the upper layer (e.g., to the host software, through the storage interface  140 ). Simultaneously, smart garbage collection operations may be conducted by the NAND controller  115  in the nonvolatile memory  110 . Background write-back operations of valid blocks (as indicated by arrow  450 ) may also take place, resulting in write data being stored in the nonvolatile memory  110 , and each write-back being acknowledged (as indicated by arrow  460 ). 
     In some embodiments, when a new log record updates the current block entry in the B-tree, the storage device controller  130  also marks the old block in its log record invalid, to prevent the old block from subsequently being written back to the nonvolatile memory  110  by the background write back process. In some embodiments, the storage device controller  130  uses a write throttling mechanism (e.g., it may delay sending a command completion to the host after receiving a write request from the host) if and when the write back process to the nonvolatile memory  110  is unable to keep up with user writes (e.g., with write requests received from the host). This condition may be detected, for example, by determining that the empty fraction of the circular buffer has fallen below a threshold. 
     For large sequential IO operations that are not latency sensitive, the persistent memory controller  125  may choose to redirect IOs directly to the NAND controller  115  without staging the data in its local buffers. 
     Referring to  FIG. 5  when a read request is received (as indicated by arrow  510 ), the storage device controller  130  looks up the logical block address of the read request in the B-tree (as indicated by arrow  520 ), to determine whether the data to be read are in the persistent memory  120 . If the data to be read are in the persistent memory  120 , then the persistent memory controller  125  reads the data from the persistent memory  120  and returns the data (as indicated by arrow  530 ) to the storage device controller  130 . Otherwise, the data are read (as indicated by arrow  535 ) from the nonvolatile memory  110  and returned to the storage device controller  130 . In either case, the storage device controller  130  then sends (as indicated by arrow  540 ) a response (e.g., to the host) including the data requested in the read request. 
     In some embodiments, as mentioned above, a heterogeneous SSD has a smaller capacity of persistent memory  120  and a larger capacity of nonvolatile memory  110 . In applications in which the smaller capacity of persistent memory  120  may become a limitation, a larger virtual block device (which operates as a virtual bank of low-latency persistent memory) may be created by pooling the respective persistent memory banks of a plurality of heterogeneous SSDs, as illustrated in  FIG. 6 , using persistent memory  120  from each of the plurality of heterogeneous SSDs  620 . In some embodiments, the virtual log device  610  presents a logical device to the host software. The virtual log device  610  may be an instantiation of a software module or a logical representation of heterogeneous SSDs  620  that can communicate with each other using peering capabilities. 
       FIG. 7  illustrates the IO flow for a write request that is targeted at a specific heterogeneous SSD and that goes through a virtual log device  610 , the virtual log device  610  being distributed across multiple heterogeneous SSDs  620 . The flow for a write operation is illustrated. When a new write request arrives (as indicated by arrow  630 ) at a first virtual log device  610  (labelled VLD  1 ) of a heterogeneous SSD, the first virtual log device  610  submits (as indicated by arrow  635 ) the write request to the virtual log device manager  710 . The virtual log device manager  710  then selects a location in persistent memory  120  on one or more of the heterogeneous SSDs  620 , based on bandwidth and capacity requirements of the VLD  610  as specified by the application  105 , stores (as indicated by arrow  640 ) a new record in the selected location in persistent memory  120 , and returns (as indicated by arrow  645 ) an acknowledgement to the first virtual log device  610  (labelled VLD  1 ) which then returns (as indicated by arrow  650 ) the acknowledgement to the upper layer (e.g., to the host application). Subsequently, or concurrently, the virtual log device manager  710  flushes (as indicated by arrow  655 ) the write request to the nonvolatile memory  110  of the target heterogeneous SSD, which returns (as indicated by arrow  660 ) an acknowledgement to the virtual log device manager  710 . Finally the virtual log device manager  710  removes (as indicated by arrow  665 ) the flushed log record. 
     The location in persistent memory  120  selected by the virtual log device  610  may or may not be in the persistent memory  120  of the specific heterogeneous SSD targeted for the write request. The factors that the virtual log device  610  may take into account in making the selection may include, for example (i) the current workload on the targeted SSD, (ii) the amount of free space left in the persistent memory  120  of each of the heterogeneous SSDs  620 , and (iii) the state of any background operations on the persistent memory  120  of each of the heterogeneous SSDs  620 , that could impact the IO latency. 
     For read operations, the virtual log device  610  may perform a lookup of the requested data in the persistent memory  120 ; if the data are present in the persistent memory  120 , the virtual log device  610  returns the requested data from the persistent memory  120  on one of the heterogeneous SSDs  620 . If the requested data are not present in the persistent memory  120 , the requested data are directly read from the nonvolatile memory  110  of the heterogeneous SSD on which they are stored. Although  FIGS. 6 and 7  show only one virtual log device, the invention is not limited to such embodiments and in some embodiments multiple virtual log devices are formed from a group of heterogeneous SSDs. 
     Some applications, such as certain file systems, certain databases, or volume managers use a conventional, host-visible log device to implement a journal or write-ahead log. These applications may either not execute at all, or not perform optimally if such a log device is not provided or is provided in a manner that is transparent to the storage software stack on systems running these applications. Accordingly, some embodiments provide the following methods to support this class of applications. 
     A Log Device Map method may allow the low-latency persistent memory  120  of a heterogeneous SSD to be directly addressable in the IO path by the application and/or operating system running on the host connected to the heterogeneous SSD. The Map function may cause the low-latency device to be detected by the host operating system storage software stack as a separate storage device suitable for use as a journal or log device. The Map function may present either the entire addressable space of the persistent memory  120  of the heterogeneous SSD, or a portion thereof. 
     In another embodiment, a Map function is not used; instead the host is allowed the ability to selectively place data blocks either in the low-latency persistent memory  120 , or in the nonvolatile memory  110  (i.e., the backing storage) of a heterogeneous SSD, while still continuing to access and use a single storage device corresponding to the consolidated, heterogeneous SSD. In such an embodiment, a Log Device Partition method may allow only data blocks with offsets (or LBAs) less than a designated offset (a “Partition offset”) to be placed in the low-latency persistent memory  120  of the heterogeneous SSD. To write to its write-ahead log, the application may then simply assume that its log size is less than or equal to the size implied by the Partition offset and starts at offset 0. Data blocks that are intended for the main backing store (i.e., for the nonvolatile memory  110  of the heterogeneous SSD) may accordingly be sent using an offset exceeding the Partition offset, which may cause the heterogeneous SSD to dispatch those blocks directly to the nonvolatile memory  110  of the heterogeneous SSD. 
     Each of the NAND controller  115 , the storage device controller  130 , and the persistent memory controller  125  may be, or be part of, a processing circuit. The term “processing circuit” is used herein to mean any combination of hardware, firmware, and software, employed to process data or digital signals. Processing circuit hardware may include, for example, application specific integrated circuits (ASICs), general purpose or special purpose central processing units (CPUs), digital signal processors (DSPs), graphics processing units (GPUs), and programmable logic devices such as field programmable gate arrays (FPGAs). In a processing circuit, as used herein, each function is performed either by hardware configured, i.e., hard-wired, to perform that function, or by more general purpose hardware, such as a CPU, configured to execute instructions stored in a non-transitory storage medium. A processing circuit may be fabricated on a single printed circuit board (PCB) or distributed over several interconnected PCBs. A processing circuit may contain other processing circuits; for example a processing circuit may include two processing circuits, an FPGA and a CPU, interconnected on a PCB. In some embodiments at least one controller of the NAND controller  115 , the storage device controller  130 , and the persistent memory controller  125  is implemented in software executing on a processing circuit, which may be the same processing circuit, or a separate processing circuit from that implementing (or executing) the other controllers of the NAND controller  115 , the storage device controller  130 , and the persistent memory controller  125 . 
     It will be understood that, although the terms “first”, “second”, “third”, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section, without departing from the spirit and scope of the inventive concept. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “under”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that such spatially relative terms are intended to encompass different orientations of the device in use or in operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” or “under” other elements or features would then be oriented “above” the other elements or features. Thus, the example terms “below” and “under” can encompass both an orientation of above and below. The device may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein should be interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive concept. As used herein, the terms “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those of ordinary skill in the art. As used herein, the term “major component” refers to a component that is present in a composition, polymer, or product in an amount greater than an amount of any other single component in the composition or product. In contrast, the term “primary component” refers to a component that makes up at least 50% by weight or more of the composition, polymer, or product. As used herein, the term “major portion”, when applied to a plurality of items, means at least half of the items. 
     As used herein, the singular forms “a” and “an” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. Further, the use of “may” when describing embodiments of the inventive concept refers to “one or more embodiments of the present invention”. Also, the term “exemplary” is intended to refer to an example or illustration. As used herein, the terms “use,” “using,” and “used” may be considered synonymous with the terms “utilize,” “utilizing,” and “utilized,” respectively. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to”, “coupled to”, or “adjacent to” another element or layer, it may be directly on, connected to, coupled to, or adjacent to the other element or layer, or one or more intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on”, “directly connected to”, “directly coupled to”, or “immediately adjacent to” another element or layer, there are no intervening elements or layers present. 
     Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” is intended to include all subranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. 
     Although exemplary embodiments of an SSD with heterogeneous NVM types have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that an SSD with heterogeneous NVM types constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.