Abstract:
A storage device having prioritized-erasure capabilities including: a memory for storing data, the memory having at least one flash unit, wherein each flash unit has a plurality of blocks; and a controller configured: to write the data into the plurality of blocks; to assign an erasure-priority to each block, wherein the erasure-priority correlates with an erasure-priority of the data; and to erase the data in each block according to the erasure-priority of each block upon receiving an emergency-erase command. Preferably, the controller is configured to perform the writing of the data into the plurality of blocks in an arbitrary order in a first flash unit, and the writing into subsequent flash units is performed in correlation with the order in the first flash unit. Preferably, the erasing includes aborting erasure, before completing the erasure, for at least some of the plurality of blocks.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/824,452, filed Sep. 4, 2006, which is hereby incorporated by reference in its entirety. 
         [0002]    This patent application is related to U.S. patent application Ser. No. ______ of the same inventors, which is entitled “METHOD FOR PRIORITIZED ERASURE OF FLASH MEMORY” and filed on the same day as the present application. This patent application, also claiming priority to U.S. Provisional Application No. 60/824,452, is incorporated in its entirety as if fully set forth herein. 
     
     FIELD AND BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to devices for managing the storage and erasure of data in a storage device in such a way that more critical data is erased before less critical data is erased. 
         [0004]    The need to erase data in a storage device is well-known in the art of computer engineering, and is generally motivated by two alternative needs:
       (1) the need to clear space for new data; and   (2) the need to destroy confidential information for a storage device having a secure area.       
 
         [0007]    While the first reason has no urgency, the second reason is vital in critical situations. The “window of time” between the decision to erase the data and the loss of control over the storage device can be very short. Often, the time available for erasing a storage device is shorter than the time required for complete erasure of the entire storage device. Unfortunately, the prior art does not teach methods for organizing the erasure procedure so that more critical data is erased before less critical data is erased. 
         [0008]    It would be desirable to have devices for ensuring that when a critical erasure is needed, the most critical data is erased first. Such a prioritized-erasure procedure would provide best-choice solutions in time-critical situations. 
       SUMMARY OF THE INVENTION 
       [0009]    It is the purpose of the present invention to provide devices for managing the storage and erasure of data in a storage device in such a way that more critical data is erased before less critical data is erased. 
         [0010]    For the purpose of clarity, several terms which follow are specifically defined for use herein. The term “erasure procedure” is used herein to refer to an electronic process by which the content of a block is rendered useless by either: (1) setting all cells of the block to the same logical value, or (2) randomizing the content of all the cells of the block. The term “prioritized erasure” is used herein to refer to an erasure procedure in which parts of a memory are erased according to an erasure-priority protocol. The term “block” is used herein to refer to a physical part of a flash-memory storage device. The term “sanitized erasure” is used herein to refer to an erasure procedure by which the content of a digital memory is destroyed so that the content cannot be restored by any forensic means. Such sanitized erasure is in contrast to an ordinary erasure procedure that makes the content useless for ordinary reading commands, but may not prevent restoration by advanced technological means. The term “flash unit” is used herein to refer to a portion of flash-memory in a flash-memory device. 
         [0011]    The present invention applies to both single-level-cell (SLC) flash memories and multi-level-cell (MLC) flash memories. While the subsequent discussion focuses primarily on SLC cells, it will be clear to those skilled in the art how the present invention applies to MLC cells (and to other non-volatile storage devices in general). The terms “erasing”, “erasure”, and “writing” are used herein to refer to setting threshold voltages of a memory cell, where erasing and erasure typically set the voltages to correspond to one-logic values, and writing typically sets the voltages to correspond to zero-logic values for SLC cells. The terms “writing” and “programming” are used interchangeably herein. The present invention is particularly applicable to NAND-type flash memories that are read and programmed a page at a time. 
         [0012]    Table 1 shows three alternate embodiments of the present invention. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Three alternate embodiments of the present invention. 
               
             
          
           
               
                   
                 Writing 
                 Writing 
                   
                   
                   
               
               
                   
                 first flash 
                 other 
                   
                 Emergency 
               
               
                 Embodiment 
                 unit 
                 flash units 
                 Logging 
                 erasure 
                 Comments 
               
               
                   
               
               
                 1 
                 Arbitrary 
                 Arbitrary 
                 All high 
                 According to 
                 Simplest block 
               
               
                   
                   
                   
                 erasure- 
                 the log 
                 allocation in writing 
               
               
                   
                   
                   
                 priority 
               
               
                   
                   
                   
                 locations 
               
               
                 2 
                 Prescribed 
                 Prescribed 
                 N/A 
                 According to 
                 Most effective 
               
               
                   
                   
                   
                   
                 prescription 
                 emergency erasure 
               
               
                 3 
                 Arbitrary 
                 Aligned 
                 Only first 
                 According to 
                 Optimized performance 
               
               
                   
                   
                 with first 
                 flash unit 
                 log 
                 between Embodiments 
               
               
                   
                   
                 flash unit 
                   
                   
                 1 and 2 
               
               
                   
               
             
          
         
       
     
         [0013]    In a preferred embodiment of the present invention, data is stored in a flash memory in an arbitrary manner that does not dictate any constraints on the writing allocation, as shown in Table 1, Embodiment 1, and described below. Rather, the location of the written blocks that contain critical data is recorded, and the erasure is performed according to these records. 
         [0014]    In another preferred embodiment of the present invention, the data is stored in a flash unit in a manner that ensures the fastest erasure of the high erasure-priority data, as shown in Table 1, Embodiment 2, and described below. Certain areas of the flash units are reserved for high erasure-priority blocks. In emergency situations, the reserved areas are erased before any other parts of the flash units are erased. The locations of blocks available for writing data are predetermined according to each block&#39;s erasure-priority. 
         [0015]    In another preferred embodiment of the present invention (as shown in Table 1, Embodiment 3, and described below), the data is stored in the flash unit in a way that combines the advantages of Embodiments 1 and 2. In Embodiment 3, writing is performed as fast as in Embodiment 1, and erasure is performed as fast as in Embodiment 2. The writing of the first flash unit is performed in a random order in Embodiment 3 (similar to Embodiment 1), and then the locations of the high erasure-priority areas in the rest of the flash units are aligned according to the first flash unit. Embodiment 3 is as fast as Embodiment 1 in writing the first flash unit, and as fast as Embodiment 2 upon emergency erasure. 
         [0016]    The prioritized erasure of flash memory can be implemented using at least three different procedures:
       (1) an ordinary erase command, such as that described in the technical data sheet of the K9F1G08U0A flash memory available from Samsung Electronics, Suwon, South Korea;   (2) a sanitized erasure, such as that taught in Koren et al., US Patent Application No. 20040188710; and   (3) an interrupted erasure, such as that described in detail below.       
 
       The prioritized-erasure procedures that are described in the present invention include both the selection of the erasure procedure and the erasure order. 
       [0020]    Therefore, according to the present invention, there is provided for the first time a non-volatile storage device having prioritized-erasure capabilities, the device including: (a) a storage memory for storing data in the storage device, the storage memory having at least one flash unit, wherein each flash unit has a plurality of blocks; and (b) a storage-device controller configured: (i) to write the data into the plurality of blocks; (ii) to assign an erasure-priority to each block, wherein the erasure-priority correlates with an erasure-priority of the data; and (iii) to erase the data in each block according to the erasure-priority of each block upon receiving an emergency-erase command. 
         [0021]    Preferably, the controller is configured to perform the writing on arbitrarily-selected blocks, and the assigning is performed according to the erasure-priority of the data. 
         [0022]    Preferably, the controller is configured to perform the assigning of the erasure-priority prior to the writing of the data, and the writing is performed according to the erasure-priority of each block. 
         [0023]    Preferably, the controller is configured to perform the writing of the data into the plurality of blocks in an arbitrary order in a first flash unit, and the writing into subsequent flash units is performed in correlation with the order in the first flash unit. 
         [0024]    Most preferably, the controller is further configured: (iv) to assign a common erasure-priority to blocks having a common relative position in each respective flash unit. 
         [0025]    Preferably, the controller is further configured: (iv) to store a log of the erasure-priority for each block; and (v) to erase the data in each block according to the erasure-priority stored in the log upon receiving the emergency-erase command. 
         [0026]    Preferably, the erasing by the controller includes aborting erasure, before completing the erasure, for at least some of the plurality of blocks. 
         [0027]    According to the present invention, there is provided for the first time a hard-disk drive having prioritized-erasure capabilities, the drive including: (a) a storage memory for storing data in the hard-disk drive, the storage memory having at least two sectors; (b) a mechanism for assigning an erasure-priority to each sector, wherein the erasure-priority correlates with an erasure-priority of the data; and (c) a mechanism for erasing the sectors according to the erasure-priority of each sector. 
         [0028]    These and further embodiments will be apparent from the detailed description and examples that follow. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The present invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0030]      FIG. 1  is a simplified schematic block diagram of a flash-memory system using a prioritized-erasure procedure that erases using physical erase-slices, according to a preferred embodiment of the present invention; 
           [0031]      FIG. 2  is a simplified schematic block diagram of a flash-memory system using a prioritized-erasure procedure that erases using logical erase-slices, according to a preferred embodiment of the present invention; 
           [0032]      FIG. 3  is a simplified flowchart of a prioritized-erasure procedure, according to a preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    The present invention relates to devices for managing the storage and erasure of data in a storage device in such a way that more critical data is erased before less critical data is erased. The principles and operation for managing the storage and erasure of data in a storage device, according to the present invention, may be better understood with reference to the accompanying description and the drawings. 
         [0034]    In Embodiment 1 of Table 1 above, the writing process is arbitrary, a log documents the allocation of blocks to various erasure-priority levels, and the erasure is performed according to the log. 
         [0035]    In Embodiment 2 of Table 1 above, the writing process is performed according to a reserved allocation of blocks to high erasure-priority levels, and the erasure is performed according to the allocation. 
         [0036]    In Embodiment 3 of Table 1 above, the writing of the first flash unit is performed arbitrarily, as in Embodiment 1. Such an arbitrary allocation then prescribes the allocation in the rest of the flash units, and the erasure is performed as in Embodiment 2. 
         [0037]    Referring now to the drawings,  FIG. 1  is a simplified schematic block diagram of a flash-memory system using a prioritized-erasure procedure that erases using physical erase-slices, according to a preferred embodiment of the present invention. A host system  20  is shown connected to a flash-memory device  22 , having a flash controller  24  and a plurality of flash units  26 . 
         [0038]    Each flash unit  26  has a number of blocks  28  that can be individually addressed for erasure. Such a structure for a flash-memory disk drive is well-known in the art, and is found in components such as FFD-25-UATA-8192-A, available from SanDisk IL Ltd., Kefar Saba, Israel. 
         [0039]    Some of blocks  28  in flash units  26  are selected to accommodate high erasure-priority data (shown as blocks H in  FIG. 1 ). The positions of blocks H are made known to the writing mechanism of host system  20 . The writing mechanism is typically flash controller  24 , but can also be host system  20 . The writing mechanism then allocates data of high erasure-priority to blocks H. Other blocks  28  in flash units  26  are selected to accommodate medium and low erasure-priority data (shown as blocks M and L, respectively, in  FIG. 1 ). The positions of blocks M and L are also made known to the writing mechanism of host system  20 . The writing mechanism then allocates data of lower erasure-priority to blocks M and L. There can be any number of erasure-priority levels for the selective allocation of data to blocks  28 . 
         [0040]    In a preferred embodiment of the present invention, some blocks  28  are designated with an excluded erasure-priority excluding the designated blocks from the prioritized-erasure procedure (shown as blocks E in  FIG. 1 ). Data can be allocated to blocks E that do not need to be erased in the event of an emergency-erase situation. Upon the need to urgently erase the data in flash-memory device  22 , the erasure mechanism erases blocks  28  according to their designated erasure-priority, ensuring that the data is erased in the correct order. 
         [0041]    A “physical erase-slice” is a collection of blocks from several flash units, where each of the selected blocks has the same address in its respective flash unit. If the blocks are allocated, upon writing, so that selected erasure-priority blocks in each flash unit share the same address, then the optimal prioritized erasure can be performed by physical erase-slices. 
         [0042]    A physical erase-slice  30  is shown in  FIG. 1  representing a collection of blocks  28  across several flash units  26 . While blocks H are shown as part of physical erase-slice  30  in  FIG. 1 , physical erase-slice  30  can include any “slice” of blocks  28 . In a preferred embodiment of the present invention, erasure of blocks  28  in physical erase-slice  30  is performed simultaneously. 
         [0043]    In some flash-memory architectures, the erasure of some blocks is faster than for other blocks due to the heterogeneous structure of the flash memory. In the present embodiment described in  FIG. 1 , in which blocks are pre-allocated for high erasure-priority, it is preferable to use the inherently fast-erasing blocks for allocation to the high erasure-priority data. A protocol can be applied where high erasure-priority data resides in fast-erasing blocks. In such systems, the fast-erasing blocks are selected to accommodate the higher erasure-priority data. By doing so, the high erasure-priority data will be erased faster. 
         [0044]    It should be noted that in the present embodiment, high erasure-priority blocks are allocated to flash units randomly. Since the erasure procedure erases one block in each flash unit during an erase cycle, it may occur that, in some flash units, there will be remaining blocks to be erased, while in other flash units, all the high erasure-priority blocks have already been erased. Such a situation results in a loss in efficiency. The erasure procedure continues to erase a smaller number of blocks in each cycle until the last high erasure-priority block in the last flash unit is erased. 
         [0045]      FIG. 2  is a simplified schematic block diagram of a flash-memory system using a prioritized-erasure procedure that erases using logical erase-slices, according to a preferred embodiment of the present invention. In this embodiment, the writing process is optimized by engineering considerations that are not related to the present invention, and are taught in the prior art of flash-memory device configurations such as in Gorobets, U.S. Pat. No. 6,898,662. 
         [0046]    Typically, writing will not be performed by a contiguous set of blocks  28 , one flash unit  26  after the other, but rather “across the board” where data is written on several flash units  26  in parallel. Upon writing, a record is made in a log table (not shown) of the erasure-priority of each written block  28 , or at least blocks H (i.e. the blocks in which high erasure-priority data is written). For the purpose of the disclosure of present invention, it is assumed that there are three levels of erasure-priority, and that the majority of the blocks do not have any erasure-priority (i.e. blocks E). Therefore, there is no risk if the data in blocks E are left un-erased. 
         [0047]    As is known in the art of flash-memory engineering, the erasure of a multi-unit flash-memory device is most efficient when a block is simultaneously erased in each of the flash units during an erasure cycle. However, if the blocks are not pre-arranged according to their erasure-priority, a physical erase-slice cannot be performed. In such a case, a “logical erase-slice” can be performed. 
         [0048]    A “logical erase-slice” is a collection of one, arbitrary, representative block  28  out of each flash unit  26  (e.g. blocks H, M, or L in  FIG. 2 ). The present embodiment makes use of the fact that a single erasure cycle can erase blocks of different position in each flash unit  26  in one operation. A logical erase-slice can be erased simultaneously by providing the address of the selected block  28  in each flash unit  26 , and then applying the erase command to all flash units  26 , where in each flash unit  26 , the selected block  28  is erased. Such a procedure provides a way to erase blocks  28 , in each flash unit  26 , according to their relative erasure-priority. 
         [0049]    Flash-memory device  22  can erase either a physical erase-slice or a logical erase-slice in a single erasure cycle. In  FIG. 1 , the data is organized in an architecture that is optimal for erasure of blocks in physical erase-slices. In  FIG. 2 , the data is organized arbitrarily; thus, a mechanism that erases the blocks by logical erase-slices has to be implemented. 
         [0050]    When there is a need to urgently erase flash-memory device  22 , flash controller  24  checks the log table to find the highest erasure-priority blocks in each of flash units  26 . A set of blocks H from each flash unit  26  becomes a logical erase-slice  32 , as shown in  FIG. 2 . Logical erase-slice  32  is erased, and the log table is updated to reflect that these blocks have been erased. Note that not all blocks H are erased in logical erase-slice  32 , only one block H from each flash unit  26 . Flash controller  24  then picks the next highest erasure-priority block in each flash unit  26  (e.g. blocks M). A set of blocks M from each flash unit  26  becomes a logical erase-slice  34  that is erased, and the log table is updated again. This process continues (e.g. logical erase-slices  36  and  38 ) until there are no more high erasure-priority blocks in any of flash units  26 . In practice, after logical erase-slice  32  is erased, the next logical erase-slice chosen may also include only blocks H. Such a procedure can continue until blocks H are no longer in the log table, then logical erase-slice  34  (i.e. blocks M) can be erased, or until the process of erasure is externally stopped. 
         [0051]      FIG. 3  is a simplified flowchart of a prioritized-erasure procedure, according to a preferred embodiment of the present invention. Upon receiving an emergency-erase command from the host system, the controller of the flash-memory device begins the prioritized-erasure procedure (Step  40 ). The controller checks whether or not there are erasure-priority blocks to be erased (Step  42 ). If there are no erasure-priority blocks to be erased, the prioritized-erasure procedure ends (Step  44 ). If there are erasure-priority blocks to be erased, the controller checks whether or not there are any flash units left to examine (Step  46 ). If there are still flash units to be examined, the controller seeks the highest erasure-priority block in the next flash unit (Step  48 ), and proceeds to add the block to the current logical erase-slice (Step  50 ). Then, the controller again checks whether or not there are any flash units left to examine (Step  46 ). Once all flash units have been examined, the current logical erase-slice is erased (Step  52 ), with all blocks contained in the logical erase-slice erased in parallel. The log table is then updated accordingly (Step  54 ). 
         [0052]    In a preferred embodiment of the present invention, an “interrupted-erase” cycle is used instead of a “full-erase” cycle. A full-erase cycle is an erasure procedure that takes a relatively long time, typically 2.5 milliseconds, and ensures that the erasure is “clean” in the sense that all of the bits of the memory block have been set to one-logic. If the erasure procedure is made shorter, there is a risk that some of the bits will not be set to one-logic. When a flash memory, having thousands of blocks, has to be erased, and each block is erased using a full-erase cycle, the total erase time may take tens of seconds. In emergency situations, there is a risk that the erasure procedure will be cut short before all of the blocks are erased. 
         [0053]    The emergency-erase time can be used more effectively by dedicating a fraction of the 2.5-ms cycle for erasing a block, enabling a lot more blocks to be erased in 2.5 ms. Typically, most of the bits lose their original logical state after less than 50% of the full-erase cycle has been performed. The remaining amount of bits that are not fully erased are so few as to render the information virtually useless. It is thus preferable to erase double the amount of blocks using 50% of the full-erase cycle time, rather than erase 50% of the blocks using the full-erase cycle time. Clearly, the determination of the duration of the interrupted-erase cycle can be set anywhere between 0% and 100% of the nominal full-erase cycle using engineering considerations and assumptions on the total time available for the prioritized-erasure procedure. 
         [0054]    One possible way to implement an interrupted-erase cycle is to make use of the fact that flash memory (for both NOR- and NAND-type flash memory), while “blind” to many commands during an erase cycle, is responsive to special “abort” commands such as:
       (1) a “reset” command in NOR-type flash memory, such as that described in:   http://www.samsung.com/Products/Semiconductor/MCP/NOR_based/K5L5628JBM/K5L5628JBM.htm;   (2) a “suspend erase” command in NOR-type flash memory, such as that described in: http://www.electronicstalk.com/news/sor/sor100.html; and   (3) a “reset” command in NAND-type flash memory, such as that described in the data sheet of the Samsung K9F1G08U0A in: http://www.samsung.com/Products/Semiconductor/NANDFlash/SLC_LargeBlock/1Gbit/K9F1G08U0A/ds_k9f1g08x0a ‘ rev10.pdf.       
 
         [0059]    The data sheet of the Samsung K9F1G08U0A states that, “The device offers a reset feature, executed by writing FFh to the command register. When the device is in Busy state during random read, program or erase mode, the reset operation will abort these operations.” 
         [0060]    These commands are much shorter than the full-erase cycle (in NAND-type flash memory, a reset command takes a maximum of 0.5 ms while a full-erase cycle takes 2.5 ms). When a full-erase cycle is aborted, the memory is left in a random state that is not useful for any purpose. It is preferable to utilize a 5-ms interval to start five full-erase cycles and abort the cycles after 0.5 ms each (using five reset commands that each consume 0.5 ms), than to perform two complete full-erase cycles. Both alternatives consume the full 5-ms interval. 
         [0061]    In a preferred embodiment of the present invention (as shown in Table 1, Embodiment 3), the writing of the first flash unit can be carried out in an arbitrary manner, logging the blocks that receive higher erasure-priority data. Upon writing to the first flash unit, the subsequent flash units are written in an order that is correlated to the order of the first flash unit. Typically, the subsequent flash units are aligned with the high erasure-priority blocks of the first flash unit, so that blocks having the same (or correlated) addresses in all of the flash units receive data having the same erasure-priority. This enables the system to erase high erasure-priority data by erasing blocks having the same address in parallel, without having to go through the construction of logical erase-slices (as described above and shown in  FIG. 2 ). 
         [0062]    It should be noted that using fast-erasing blocks for high erasure-priority data and aligning the high erasure-priority data in common physical erase-slices are not conflicting protocols, and can preferably be implemented together. While the high erasure-priority blocks are stored in the “faster parts” of the flash units, the blocks are stored in the first flash unit in a random order. This random order prescribes the order for all the other flash units, resulting in the high erasure-priority data residing in common physical erase-slices. 
         [0063]    It should be noted that while erasure in logical erase-slices and erasure in physical erase-slices may lead to the same order of erasure (and therefore, to the same level of optimization), physical erase-slices: (1) are simpler to implement, and (2) require less management and storage of administrative data than in implementations using logical erase-slices. Therefore, implementations using physical erase-slices are preferable. 
         [0064]    It should be noted that hard-disk drives are a typical example of storage devices that are covered by the present invention. The present invention is not limited in any way only to flash-memory storage devices, and applies to, and is intended to cover, any storage system that is characterized by at least some of the following features:
       (1) the storage device is divided into many sub-units, each of which can be erased individually;   (2) erasure time of a sub-unit is a relatively long process;   (3) complete erasure of a sub-unit takes a lot longer amount of time than an aborted erasure; and   (4) the storage-device controller has the flexibility to store data in any sub-unit.       
 
         [0069]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications, and other applications of the invention may be made.