WO2008029389A1

WO2008029389A1 - Device and method for prioritized erasure of flash memory

Info

Publication number: WO2008029389A1
Application number: PCT/IL2007/001081
Authority: WO
Inventors: Eran Erez
Original assignee: Sandisk Il Ltd.
Priority date: 2006-09-04
Filing date: 2007-09-02
Publication date: 2008-03-13
Also published as: KR101429898B1; TWI375227B; JP2010503103A; JP5065395B2; JP5486047B2; JP2012216234A; CN101529370A; TW200822124A; CN101529370B; KR20090047513A

Abstract

A storage device and method 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

DEVICE AND METHOD FOR PRIORITIZED ERASURE OF FLASH MEMORY

FIELD AND BACKGROUND OF THE INVENTION

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 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.

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.

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

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. 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.

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.

Table 1 shows three alternate embodiments of the present invention.

Table 1. Three alternate embodiments of the present invention.

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.

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's erasure-priority.

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.

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 ah, 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.

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. 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.

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. 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.

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. 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.

Preferably, the erasing by the controller includes aborting erasure, before completing the erasure, for at least some of the plurality of blocks. 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. These and further embodiments will be apparent from the detailed description and examples that follow.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

Figure 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; Figure 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;

Figure 3 is a simplified flowchart of a prioritized-erasure procedure, according to a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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.

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. 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.

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.

Referring now to the drawings, Figure 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.

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.

Some of blocks 28 in flash units 26 are selected to accommodate high erasure- priority data (shown as blocks H in Figure 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 Figure 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. 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 Figure 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.

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.

A physical erase-slice 30 is shown in Figure 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 Figure 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.

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 Figure 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. 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.

Figure 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. Patent No. 6,898,662.

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.

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.

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 Figure 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.

Flash-memory device 22 can erase either a physical erase-slice or a logical erase- slice in a single erasure cycle. In Figure 1 , the data is organized in an architecture that is optimal for erasure of blocks in physical erase-slices. In Figure 2, the data is organized arbitrarily; thus, a mechanism that erases the blocks by logical erase-slices has to be implemented.

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 Figure 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.

Figure 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).

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. 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.

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/K5L5

628JBM/K5L5628JBM.htm;

(2) a "suspend erase" command in NOR-type flash memory, such as that described in: http : //www. electron! cstalk.com/news/sor/sor 100.htm 1 ; and

(3) a "reset" command in NAND-type flash memory, such as that described in the data sheet of the Samsung K9F 1 G08U0A in: http://www.samsung.com/Products/Semiconductor/NANDFlash/SLC Large Block/1 Gbit/K9F1 G08U0A/ds k9fl g08x0a reyl O.pdf.

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."

These commands are much shorter than the foil-erase cycle (in NAND-type flash memory, a reset command takes a maximum of 0.5 ms while a foil-erase cycle takes 2.5 ms). When a foil-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 foil-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 foil-erase cycles. Both alternatives consume the foil 5-ms interval. 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 Figure 2). 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. 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.

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.

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.

Claims

WHAT IS CLAIMED IS:

1. A non-volatile storage device having prioritized-erasure capabilities, the device comprising:

(a) a storage memory for storing data in the storage device, said 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 said data into said plurality of blocks;

(ii) to assign an erasure-priority to each said block, wherein said erasure- priority correlates with an erasure-priority of said data; and

(iii) to erase said data in said each block according to said erasure- priority of said each block upon receiving an emergency-erase command.

2. The device of claim 1, wherein said controller is configured to perform said writing on arbitrarily-selected blocks, and said assigning is performed according to said erasure-priority of said data.

3. The device of claim 1, wherein said controller is configured to perform said assigning of said erasure-priority prior to said writing of said data, and said writing is performed according to said erasure-priority of said each block.

4. The device of claim 1, wherein said controller is configured to perform said writing of said data into said plurality of blocks in an arbitrary order in a first said flash unit, and said writing into subsequent said flash units is performed in correlation with said order in said first flash unit.

5. The device of claim 4, wherein said controller is further configured:

(iv) to assign a common erasure-priority to blocks having a common relative position in each respective said flash unit.

6. The device of claim 1, wherein said controller is further configured: (iv) to store a log of said erasure-priority for said each block; and

(v) to erase said data in said each block according to said erasure- priority stored in said log upon receiving said emergency-erase command.

7. The device of claim 1, wherein said erasing by said controller includes aborting erasure, before completing said erasure, for at least some of said plurality of blocks.

8. A hard-disk drive having prioritized-erasure capabilities, the drive comprising:

(a) a storage memory for storing data in the hard-disk drive, said storage memory having at least two sectors;

(b) a mechanism for assigning an erasure-priority to each said sector, wherein said erasure-priority correlates with an erasure-priority of said data; and

(c) a mechanism for erasing said at least two sectors according to said erasure- priority of said each sector.

9. A method for prioritized erasure of a non- volatile storage device, the method comprising the steps of:

(a) providing at least one flash unit of the storage device, wherein each flash unit has a plurality of blocks;

(b) writing data into said plurality of blocks;

(c) assigning an erasure-priority to each said block, wherein said erasure- priority correlates with an erasure-priority of said data; and

(d) erasing said data in said each block according to said erasure-priority of said each block upon receiving an emergency-erase command.

10. The method of claim 9, wherein said step of writing is performed on arbitrarily-selected blocks, and said step of assigning is performed according to said erasure-priority of said data.

11. The method of claim 9, wherein said step of assigning is performed prior to said step of writing, and said step of writing is performed according to said erasure-priority of said each block.

12. The method of claim 9, wherein said step of writing data into said plurality of blocks is performed in an arbitrary order in a first said flash unit, and said step of writing into subsequent said flash units is performed in correlation with said order in said first flash unit.

13. The method of claim 12, wherein said step of assigning includes assigning a common erasure-priority to blocks having a common relative position in each respective said flash unit.

14. The method of claim 9, the method further comprising the steps of:

(e) storing a log of said erasure-priority for said each block; and

(f) erasing said data in said each block according to said erasure-priority stored in said log upon receiving said emergency-erase command.

15. The method of claim 9, wherein said step of erasing includes aborting erasure, before completing said erasure, for at least some of said plurality of blocks.

16. A method for prioritized erasure of a hard-disk drive, the method comprising the steps of:

(a) providing at least two sectors of the hard-disk drive;

(b) writing data into said at least two sectors;

(c) assigning an erasure-priority to each said sector, wherein said erasure- priority correlates with an erasure-priority of said data; and

(d) erasing said data in said each sector according to said erasure-priority of said each sector upon receiving an emergency-erase command.