BACKGROUND OF THE INVENTION 
     The present invention generally relates to solid-state mass storage media and their operation. More particularly, the present invention relates to flash-based memory devices that comprise multiple volumes and adapted to operate by exposing volumes with different service levels for different types of data. 
     Non-volatile solid-state memory technologies used with computers and other processing apparatuses (host systems) are currently largely focused on NAND flash memory technologies, with other emerging non-volatile solid-state memory technologies including phase change memory (PCM), resistive random access memory (RRAM), magnetoresistive random access memory (MRAM), ferromagnetic random access memory (FRAM), organic memories, and nanotechnology-based storage media such as carbon nanofiber/nanotube-based substrates. These and other non-volatile solid-state memory technologies will be collectively referred to herein as solid-state mass storage media. Mainly for cost reasons, at present the most common solid-state memory technology used in solid-state drives (SSDs) are NAND flash memory components, commonly referred to as flash-based memory devices, flash-based storage devices, flash-based media, or raw flash. 
     Similar to rotating media-based hard disk drives (HDDs), SSDs utilize a type of non-volatile memory media and therefore provide persistent data storage (persistency) without application of power. In comparison to HDDs, SSDs can service a READ command in a quasi-immediate operation, yielding much higher performance especially in the case of small random access read commands. This is largely due to the fact that flash-based storage devices (as well as other non-volatile solid-state mass storage media) used in SSDs are purely electronic devices that do not contain any moving parts. In addition, multi-channel architectures of modern NAND flash-based SSDs result in sequential data transfers saturating most host interfaces. A specialized case is the integration of an SSD into a hard disk drive (HDD) to form what is typically referred to as a hybrid drive. However, even in the case of a hybrid drive, the integrated SSD is functionally equivalent to a stand-alone SSD. 
     Another difference between HDDs and flash-based SSDs relates to the write endurance of flash-based media. Briefly, flash-based memory components store information in an array of floating-gate transistors, referred to as cells. NAND flash memory cells are organized in what are commonly referred to as pages, which in turn are organized in predetermined sections of the component referred to as memory blocks (or sectors). Each cell of a NAND flash memory component has a top gate (TG) and a floating gate (FG), the latter being sandwiched between the top gate and the channel of the cell. The floating gate is separated from the channel by an oxide layer, often referred to as the tunnel oxide. Data are stored in a NAND flash memory cell in the form of a charge on the floating gate which, in turn, defines the channel properties of the NAND flash memory cell by either augmenting or opposing the charge of the top gate. This charge on the floating gate is achieved by applying a programming voltage to the top gate. The process of programming (writing 0&#39;s to) a NAND cell requires injection of electrons into the floating gate by quantum mechanical tunneling, whereas the process of erasing (writing 1&#39;s to) a NAND cell requires applying an erase voltage to the device substrate, which then pulls electrons from the floating gate. Programming and erasing NAND flash memory cells is an extremely harsh process utilizing strong electrical fields to move electrons through the oxide layer. After multiple writes to a flash memory cell, it will inadvertently suffer from write endurance problems caused by the breakdown of the oxide layer. With smaller process geometries becoming more prevalent, write endurance problems are becoming increasingly important. 
     Another difference between HDDs and NAND flash memory technology relates to data retention, that is, the maximum time after which data is written that the information is still guaranteed to be valid and correct. Whereas HDDs retain data for a practically unlimited period of time, NAND flash memory cells are subjected to leakage currents that cause the programming charge to dissipate and hence result in data loss. Retention time for NAND flash memory may vary between different levels of reliability, for example, about five years in an enterprise environment to about one to three years in consumer products. Retention problems are also becoming increasingly important with smaller process geometries. 
     Data access and reliability-related characteristics and requirements associated with volatile and non-volatile memory components are collectively referred to herein as service levels and encompass such requirements as persistence, validity, write endurance, retention, etc. In view of the above, to be considered as viable storage alternatives to HDDs, SSDs using flash-based solid-state mass storage devices are required to meet certain service levels that include write endurance and retention time. Write endurance can be addressed by, for example, wear leveling techniques based on the number of P/E (program/erase) cycles among memory blocks. The retention constraint has mandated various mechanisms. As an example, the number of P/E cycles may be limited to satisfy the service level probability of the retention requirement. Strong error correction, such as through the use of error checking and correction (ECC) algorithms, can also be applied to reduce errors over time. With decreasing process geometries, constant data scrubbing is required to counteract increasing failure rates associated with retention. As known in the art, scrubbing generally refers to refreshing data by reading data from a memory component, correcting any errors, then writing the data back, usually to a different physical location within the memory component. 
     Flash-based memory technologies are seldom used as a system memory replacement in host systems, as opposed to a mass storage replacement that takes advantage of the large capacity of flash-based memory components. An intuitive example is in the use of swap files. Modern computer system memory are typically made up of random access memory (RAM) integrated circuit (IC) components, often SDRAM (synchronous dynamic random access memory). As the RAM area of system memory may often be limited and insufficient, operating systems of computers often use the disk area of an HDD as a swap file to temporarily dump and retrieve memory. Another example of such usage is by applications, such as temporary space by databases (e.g., TempDB in SQL Server). 
     Although traditionally provided by HDDs, system memory replacement does not require the long retention or even persistency offered by HDDs. A typical retention period for a swap file is very short (typically a few minutes at most) and can be limited to a day. Hence, mechanisms that limit the P/E cycles can be relaxed for such files or data, as there is no requirement to be able to read the data very far into the future. 
     Other applications that have been introduced with flash-based media can further relax more constraints. For example, a flash read cache application can handle loss of data as it can use the production volume&#39;s data (the cache just holds a local copy of the same data). That is, the application can use error detection mechanisms to verify data correctness and can tolerate data errors returned from the flash-based media. Also, persistence is not required in this case as again, the reference copy of the data is always available. Hence, such applications can allow even lower levels of data assurance. 
     In view of the above, different applications require different service levels from flash-based media in terms of retention, persistence and validity, and the use of a device with the highest service level for all data places unnecessary constraints and reduces efficient utilization of flash-based media. 
     The concept of exposing different logical unit numbers (LUNs), representing multiple volumes, with different service levels is well known in the storage industry. For example, U.S. Patent Application Publication No. 2010/0169570 addresses the issue from a performance perspective, providing different quality of service (QoS) levels to each volume. That is, volumes are configured to provide different performance metrics (for example, input/output (IO) operations per second (IOPS), bandwidth, latency, etc.) and assigned to different applications according to their importance and requirements. 
     However, flash-based storage is by nature a high performance volume. Hence, the above service level characteristics do not apply for such volumes. Instead, and as mentioned above, service level characteristics for flash-based storage generally relate to the endurance and retention levels of the data. These characteristics have been addressed from different perspectives. For example, U.S. Pat. No. 8,621,328 discloses memory that is logically divided into regions, and in which data are stored applying different error correction for dynamic data and static data. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides methods for providing non-volatile solid-state mass storage media with different service levels for different types of data associated with different applications, and in doing so provides the opportunity for promoting more optimal usage of the storage media. 
     According to an aspect of the invention, a method of storing data on non-volatile solid-state mass storage media includes partitioning the non-volatile solid-state mass storage media into at least first and second volumes, individually assigning different service levels to the first and second volumes based on a type of data to be stored in the first and second volumes and based on the first and second volumes having different data retention requirements and/or data reliability requirements, and then performing service maintenance on data stored within at least the first volume according to the service level of the first volume. 
     Another aspect of the invention is a non-volatile solid-state mass storage media adapted to perform a method comprising the steps described above. 
     Technical effects of the method and non-volatile solid-state mass storage media described above preferably include the ability to use the media as replacement or complementary memory media for volatile system memory media and HDD storage media, and in particular to selectively utilize certain features and attributes of the media for such purposes, including the high storage capacity and random access performance of the non-volatile solid-state mass storage media, while also preferably accommodating and/or adjusting for data retention and write endurance limitations often associated with such media. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that represents DRAM memory media, flash-based memory storage media, and HDD storage media and properties thereof for use in a host system, as well as indications for the use of flash-based memory as replacement or complementary memory media for the DRAM memory media and HDD storage media. 
         FIG. 2  is block diagram representing usage in a host system of flash-based memory storage media that provides different volumes with different service levels for different data types used in different applications operating within the host system. 
         FIG. 3  is a block diagram of a wear-leveling process that can be performed with a flash-based memory storage media utilized, for example, as represented in  FIG. 2 . 
         FIG. 4  is a block diagram of a garbage collection process that can be performed with a flash-based memory storage media utilized, for example, as represented in  FIG. 2 . 
         FIG. 5  is a block diagram of a background erase process that can be performed with a flash-based memory storage media utilized, for example, as represented in  FIG. 2 . 
         FIG. 6  is a block diagram of a garbage collection process that can be performed with a flash-based memory storage media utilized, for example, as represented in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As represented in  FIG. 1 , current host systems typically utilize DRAM  100  as volatile system memory media and HDD storage  120  as non-volatile mass storage media. In a typical host system, the DRAM area is a relatively small area with very high random access but no persistency, whereas the HDD storage area is relatively large with high persistency but relatively poor random access. As also represented in  FIG. 1 , non-volatile solid-state mass storage media  110 , which in the particular nonlimiting example of  FIG. 1  is identified as flash-based memory storage (flash) media  110 , resides between the current computing media represented by the DRAM  100  and HDD storage  120 . The flash media  110  has certain features and attributes of both DRAM  100  and HDD storage  120 . For example, the capacity (volume) of flash media  110  is generally between those of DRAM  100  and HDD storage  120 , and flash media  110  provide better random access performance than HDD storage  120 , though less than the DRAM  100 . In addition, whereas DRAM  100  are volatile memory devices and therefore do not retain data in the absence of power, flash media  110  provides data persistency though for a more limited duration as compared to HDD storage  120 . 
     As a result, it would be desirable to utilize flash media  110  as replacement or complementary media in a manner that addresses weaknesses in certain features of DRAM  100  and HDD storage  120 . In the following discussion, the utilization of flash-based media as replacement and/or complementary memory media for volatile system memory media or non-volatile mass storage media will simply be referred to as “replacement” memory media as a matter of convenience, unless indicated otherwise. As indicated in  FIG. 1 , as a replacement or complementary memory media for DRAM  100  (referred to as “DRAM Replacement” in  FIG. 1 ) or other volatile system memory media, flash media  110  would provide complementary memory space and data persistence. As a replacement or complementary memory media for HDD storage (referred to as “Storage Replacement” in  FIG. 1 ) or other non-volatile mass storage media, flash media  110  provides a faster tier of data storage as a result of having higher random access performance, for example, based on IOPS. 
       FIG. 2  schematically represents a block diagram of a nonlimiting embodiment of the present invention, in which a database management system (DBMS)  200  utilizes flash media  260  through a memory controller  250 , all of which are functionally connected to a host system (not shown), typically as a result of the memory controller  250  being connected to the host system via a computer bus interface. As known in the art, firmware is executed on the memory controller  250  to provide software primitive functions and provide a software protocol interface and application programming interface (API) to the host system. The flash media  260  is represented in  FIG. 2  as having been partitioned to provide multiple different volumes  252 ,  254  and  256 . Furthermore,  FIG. 2  indicates different service levels as having been assigned to the different volumes  252 ,  254  and  256  for different data types of different applications operating within the host system. The different data types include (but are not limited to) data that may be stored in volatile and non-volatile memory spaces of a conventional host system, and the different applications include (but are not limited to) log files, databases, temporary databases, and indexes associated with the DBMS  200 . As represented, the DBMS  200  is configured to open a Write Ahead Log file (Log)  210  to rapidly record all changes to a database (DB)  214 . Those changes are later inserted in the database  214  itself (for example, by a background process).  FIG. 2  represents information relating to the Log  210  (log information) as being stored in a log volume  252  within memory space of the flash media  260 . As the log information is relatively small and can be limited (for example, by setting a size threshold for eviction from the Log  210  to the database  214 ), it can be placed on the flash media  260  to provide fast commit times and minimal latency. Since the log information is vital, the log volume  252  must provide long retention (preferably, for example, at least five years) and maximal reliability, in which case the log volume  252  can be considered to be a long-term storage area of the flash media  260  and preferably utilizes error correction as part of an error checking and correction (ECC) algorithm that can be performed by the controller  250 . In combination, the data retention (long-term storage) and data reliability (error treatment using error detection and correction) associated with the log volume  252  constitutes a type of service level. 
     The DBMS  200  is represented as using a temporary database space (TempDB)  212  for maintaining calculations and other temporary information created during analysis processes. The temporary space  212  is desirable if the DRAM (e.g., DRAM  100  of  FIG. 1  or other volatile system memory media) of the host system is not sufficiently large for the desired operation of the host system. Usually the DRAM would be used as a first tier of temporary data and processed information would be placed on a dedicated temporary location (e.g., SQL Server&#39;s TempDB) of an HDD.  FIG. 2  represents the temporary data as being stored in a swap volume  254  within memory space of the flash media  260 . As swap information life expectancy is relatively short, the retention of this volume  254  may be in a range of days at most (for example, one to two days, though longer retention is possible), in which case the swap volume  254  can be viewed as a short-term storage area of the flash media  260 . Also, as the swap volume  254  serves as replacement memory media for volatile system memory media (e.g., DRAM  100 ), there is no need for persistency in this volume  254  since the data stored in volatile memory media are not persistent. However, as indicated in  FIG. 2 , the swap volume  254  preferably utilizes error correction as part of an ECC algorithm that can be performed by the controller  250 . In combination, the data retention (short-term storage) and data reliability (error treatment using error detection and correction) associated with the swap volume  254  constitutes a type of service level that is different from the service level of the log volume  252 . 
     The DBMS  200  stores information in large files containing a plurality of records within the database  214  and preferably utilizes indexes  216  to improve performance. Although small databases can be placed entirely on a flash device, typical DBMS  200  would require a back-end storage system to store all the database information. The flash media  260  represented in  FIG. 2  can be used to accelerate the access to this information via a read cache application within an acceleration layer  230  and a read cache volume  256  within memory space of the flash media  260 . Because the read cache volume  256  serves as replacement memory media for volatile system memory media (effectively enlarging the DRAM cache), this volume  256  does not require persistency or long retention and therefore can be viewed as a short-term storage area of the flash media  260 . Furthermore, as the data reside in the back-end storage, loss of data is tolerated as long as it can be detected. Hence, the read cache volume  256  preferably utilizes error detection, in other words, the read cache application is made aware that errors have occurred, but that the data remain in error, not corrected, and not to be used. Error detection utilized by the read cache volume  256  can be performed by the controller  250 , but does not require error correction using an ECC algorithm. In combination, the data retention (short-term storage) and data reliability (error treatment using error detection) associated with the read cache volume  256  constitutes a type of service level that is different from the service levels of the log and swap volumes  252  and  254 . 
     Though three different volumes  252 ,  254  and  256  are represented in  FIG. 2 , fewer and greater numbers of volumes could be partitioned on the flash media  260 . According to particular but nonlimiting embodiments of the invention, each of the volumes  252 ,  254  and  256  can be provided by the same flash component through its controller  250 . A user can be permitted to configure the size of each volume  252 ,  254  and  256  and its type (for example, log, swap, or read cache). As known in the art, hardware and firmware elements in the controller  250  or otherwise associated with the flash media  260  can be used to partition the different volumes  252 ,  254  and  256  and assign their desired different service levels relating to data retention and write endurance, and/or internal components such as a flash management system can be used to relax P/E cycle count limitations and/or switch between error detection and error correction for the different volumes  252 ,  254  and  256  to assign their desired different service levels. For example, a user can configure the flash media  260  via an API and management software such that the flash media  260  exposes the volumes  252 ,  254  and  256  to the host system. Standard storage API commands for this purpose include, but are not limited to, SCSI Inquiry, SCSI Report LUN, SCSI Get Capacity, etc. The flash media  260  may, but is not required, to use all flash-based media, in other words, flash memory blocks (sectors) as a single pool to provide the desired volumes  252 ,  254  and  256  and their different service levels. 
       FIGS. 3 through 6  represent different manners by which service maintenance can be individually performed on data of different volumes partitioned on non-volatile solid-state mass storage media, such as the volumes  252 ,  254 , and  256  on the flash media  260 , according to different service levels that have been assigned to the volumes.  FIG. 3  schematically represents flash blocks  342 ,  344  and  346  (for example, of the flash media  260  of  FIG. 2 ) arranged in a single pool  330 . Each block  342 ,  344  and  346  (each made up of multiple pages comprising multiple memory cells) has a corresponding erase count  352 ,  354  and  356 , which indicates the number of times the block  342 ,  344 , or  346  was erased (and programmed). According to a nonlimiting embodiment of the invention, a wear-leveling process can be performed on the blocks  342 ,  344  and  346  to provide different erase levels to blocks associated with different volumes within the flash media, for example, the volumes  252 ,  254 , and  256  of  FIG. 2 . When a volume needs one or more new blocks to write incoming data, it receives such blocks  320  from a wear level allocator  300  according to a data type associated with a service level  310  associated with that volume. As a new erased block becomes available to the volume, the blocks  320  will have an erase count that is less than that required by the service level  310  specified for the particular data type to be stored in the volume, for example, the log, TempDB, DB or index data of  FIG. 2 . 
       FIG. 4  schematically represents flash blocks  442 ,  444  and  446  (for example, of the flash media  260  of  FIG. 2 ) arranged in a single pool  430 , and each block  442 ,  444  and  446  having a corresponding erase count  451 ,  455  or  457 , and a dirty level mark  452 ,  454 , or  456  (denoted the number of dirty pages in the block  442 ,  444  or  446 ). According to a nonlimiting embodiment of the invention, a garbage collection process  400  can be performed on the blocks  442 ,  444  and  446  in the background to erase dirty blocks or merge blocks with high levels of dirty data. A dirty block, in other words, a block in which all pages therein are dirty, is erased and sent to a free pages pool. Also according to a nonlimiting embodiment of the invention, following a request for one or more free blocks for a volume (and, therefore, requiring a particular data type and service level), one or more blocks  410  can be selected as candidates for merger and erase according to their dirty level, erase count and data type  420  associated with a service level consistent with the volume, for example, the log, TempDB, DB or index data of  FIG. 2 . 
       FIG. 5  schematically represents flash blocks  542 ,  544  and  546  (for example, of the flash media  260  of  FIG. 2 ) arranged in a single pool  540 , and each block  542 ,  544  and  546  having a corresponding erase count  522 ,  524  or  526 . According to a nonlimiting embodiment of the invention, a block allocation algorithm can be performed on the blocks  442 ,  444  and  446  in which the free blocks  542 ,  544  and  546  within the pool  540  are arranged by a sort process  530  into a list  505  according to their erase counts  522 ,  524  and  526 , ranging from relatively “low” to “high” erase counts. With nonlimiting reference to the log, swap, and read cache volumes  252 ,  254 , and  256  of  FIG. 2 , free blocks within the pool  540  can be allocated to the volumes  252 ,  254  and  256  based on the erase counts  522 ,  524  and  526  of the blocks. For example, if the log volume  252  requires a free block, the sort process  530  can provide a block with the smallest erase count (i.e., the first in the sorted list  505 ) to the log volume  252 . Likewise, if the cache volume  256  requires a free block, the sort process  530  will allocate a block with a high erase count, e.g., a block from the highest 10% of the free blocks. As a corollary, the log volume  252  can be provided with blocks having relatively lower P/E cycles than the cache volume  256  to meet the higher data retention reliability required of the log volume  252 . 
     As an alternative to the garbage collection scheme of  FIG. 4 ,  FIG. 6  represents a garbage collection process that can be performed on different volumes on flash media, for example, the log, swap, and read cache volumes  252 ,  254 , and  256  of the flash media  260  of  FIG. 2 . Each volume  252 ,  254 , and  256  maintains a set of values  641 ,  642  and  643  which include dirty level indicators  661 ,  662 ,  663 ,  664 ,  665  and  666  and erase counts  671 ,  672 ,  673 ,  674 ,  675  and  676  for each of individual data block  652 ,  654 ,  656 ,  658 ,  655  and  657  within the volumes  252 ,  254 , and  256 . 
     According to a nonlimiting embodiment of the invention, if the number of available blocks for the log volume  252  (i.e., free blocks with low erase counts) is below a threshold  622  and the number of partially dirty blocks is above a second threshold  631 , the garbage collection process starts merging blocks from the log volume  252 , and preferred candidates for merging are blocks with the highest dirty levels  661  and  662  within the log volume  252 . Also according to a nonlimiting embodiment of the invention, if the number of available blocks for the swap volume  254  is below a threshold  624  and the number of partially dirty blocks is above a second threshold  632 , the garbage collection process starts merging blocks from the swap volume  254 , and preferred candidates for merging are blocks with the highest dirty level  663  and  664  within the swap volume  254 . Still further according to a nonlimiting embodiment of the invention, if the number of available blocks for the cache volume  256  is below a threshold  626  and the number of partially dirty blocks is above a second threshold  633 , the garbage collection process starts merging blocks from the cache volume  256 , and preferred candidates for merging are blocks with the highest dirty level  665  and  666  within the cache volume  256 . Hence, as a corollary of these actions, each volume  252 ,  254  and  256  can be provided with a pool of blocks for a write peak. The decision criteria for a merge in the garbage collection process can be chosen to provide different levels of reliability. In addition, the criteria for the swap volume  254  can be relaxed to reduce the chance of block shortage in a write peak. 
     While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.
Methods for providing non-volatile solid-state mass storage media with different service levels for different types of data associated with different applications. The method includes partitioning the non-volatile solid-state mass storage media into at least first and second volumes, individually assigning different service levels to the first and second volumes based on a type of data to be stored in the first and second volumes and based on the first and second volumes having different data retention requirements and/or data reliability requirements, and then performing service maintenance on data stored within at least the first volume according to the service level of the first volume.