NAND FLASH-BASED STORAGE DEVICE AND METHODS OF USING

• US 20130124787A1
• Filed: 11/15/2012
• Published: 05/16/2013
• Est. Priority Date: 09/09/2011
• Status: Active Grant

• Current Assignee: OCZ Storage Solutions Incorporated (Toshiba Corporation)
• Original Assignee: OCZ Technology Group, Inc. (Toshiba Corporation)
• Inventors: Schuette, Franz Michael
• Granted Patent: US 9,298,603 B2
• Time in Patent Office: Days
• US Class Current: 711/103
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• CPC Class Codes: G06F 2212/7202 Allocation control and policies
• CPC Class Codes: G06F 2212/7204 Capacity control, e.g. partitioning, end-of-life degradation
• CPC Class Codes: G06F 2212/7205 Cleaning, compaction, garbage collection, erase control
• CPC Class Codes: G06F 2212/7206 Reconfiguration of flash memory system
• CPC Class Codes: G06F 2212/7211 Wear leveling
• CPC Class Codes: G11C 11/5628 Programming or writing circuits; Data input circuits
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/10 Programming or data input circuits
• CPC Class Codes: G11C 16/3495 Circuits or methods to detect or delay wearout of nonvolatile EPROM or EEPROM memory devices, e.g. by counting numbers of erase or reprogram cycles, by using multiple memory areas serially or cyclically
• CPC Class Codes: G11C 2211/5641 Multilevel memory having cells with different number of storage levels

Abstract

A solid state drive having at least one NAND flash memory component organized in blocks, pages and cells. Each cell is adapted to store at least two bits. Each block of the memory component is adapted to be dynamically configured to store at least one bit per cell using a first mode of operation and dynamically configured to store at least two bits per cell using a second mode of operation while the mass storage device is operating, wherein the first mode of operation entails programming fewer bits of a cell in fewer passes as compared to the second mode of operation.

Claims

1. A NAND flash memory-based solid state drive comprising:

at least one NAND flash memory component that is organized in blocks, pages and cells and defines a memory space partitioned into at least an overprovisioning pool and a user memory pool for host writes and permanent storage, respectively, a first number of the blocks being dynamically assigned to the overprovisioning pool and at least one bit per cell is stored in the first number of the blocks using a first mode of operation, and data being consolidated in a second number of the blocks that are dynamically assigned to the user memory pool and two or more bits per cell are stored in the second number of the blocks using a second mode of operation; and

means for dynamically cycling the first and second numbers of the blocks between the overprovisioning pool and the user memory pool on demand and based on wear history thereof;

wherein the first mode of operation entails programming fewer bits of a cell in fewer passes as compared to the second mode of operation.

2. A NAND flash memory-based solid state mass storage device comprising:

a substrate;

at least one NAND flash memory component on the substrate, the NAND flash memory component being organized in blocks, pages and cells, each of the cells being adapted to store at least two bits;

a flash memory controller on the substrate;

an interface on the substrate adapted to functionally connect the mass storage device to a host computer system; and

a non-volatile memory on the substrate and storing firmware of the mass storage device; and

means for dynamically configuring each block to store at least one bit per cell using a first mode of operation and dynamically configuring each block to store two or more bits per cell using a second mode of operation while the mass storage device is operating, wherein the first mode of operation entails programming fewer bits of a cell in fewer passes as compared to the second mode of operation.

3. The storage device of claim 2, wherein the at least one NAND flash memory component defines a memory space partitioned into at least an overprovisioning pool and a user memory pool, each of the overprovisioning and user memory pools containing a plurality of the blocks that can be cycled from the overprovisioning pool to the user memory pool, wherein data written by the host computer system to the storage device are written to the overprovisioning pool, and wherein blocks written to by the host computer system are written in the first mode.

4. The storage device of claim 3, wherein writing in the second mode includes writing lower and upper bits to the same cell, wherein the lower bits are logically associated with a lower page and the upper bits are logically associated with an upper page, and wherein writing in the first mode includes writing only the lower bit associated with a lower page.

5. The storage device of claim 4, wherein writing in the first mode further includes skipping the upper pages when programming a block.

6. The storage device of claim 4, wherein writing in the first mode further includes writing the least programmable charge to the upper pages but flagging the upper pages as invalid for the file system.

7. The storage device of claim 5, wherein blocks programmed in the first mode are prioritized for data coalescence and space reclamation after all lower pages have been written.

8. The storage device of claim 7, wherein the data coalescence and space reclamation includes copying data from first and second blocks programmed in the first mode into a third block using the second mode followed by erasing the first and second blocks.

9. The storage device of claim 7, wherein the data coalescence and space reclamation includes copying the data from a first block programmed in the first mode to a second block programmed in the first mode by using the upper pages of the second block in the second mode.

10. The storage device of claim 8, wherein a number of the blocks exhibit increased error rates and are permanently allocated to the overprovisioning pool to work in the first mode.

11. A method for increasing write performance and endurance of a NAND flash-based solid state drive having at least one NAND flash memory component organized in blocks, pages and cells with each cell adapted to store at least two bits, the method comprising:

directing all host writes of data to an overprovisioning pool containing a first number of the blocks that are temporarily and dynamically reconfigured to be programmed to store at least one bit per cell using a first mode of operation; and

subsequently coalescing the data in a second number of the blocks that are programmed to store two or more bits per cell using a second mode of operation;

wherein the first mode of operation entails programming fewer bits of a cell in fewer passes as compared to the second mode of operation.

12. A method for increasing write performance of a NAND flash based solid state storage device operating with a host computer and having an array of NAND flash memory components that are organized into blocks and pages and having cells capable of storing at least two bits per cell, the method comprising:

partitioning the array into an over-provisioning pool and a user memory pool, wherein the blocks of the memory components are dynamically allocated to either the over-provisioning pool or the user memory pool; and

configuring the blocks allocated to the overprovisioning pool to write fewer bits per cell if the write originates from the host computer as compared to the blocks allocated to the user memory pool.

13. The method of claim 12, wherein the blocks allocated to the user memory pool are written in the second mode.

14. The method of claim 13, wherein error rates of the blocks are monitored and, if the error rate of any one of the blocks written to in the second mode exceeds a threshold, the one block is permanently marked to be written to in the first mode only.

15. The method of claim 13, wherein data from the blocks written in the first mode are combined in other of the blocks using the second mode during garbage collection.

16. The method of claim 14, wherein pool management and write mode configuration of the blocks is controlled by a firmware stored in a non-volatile memory IC component of the storage device.

17. A NAND flash-based solid state drive comprising:

a substrate;

at least one NAND flash memory component mounted on the substrate, organized in blocks, pages and cells, and defining a memory space of the solid state drive, the blocks containing at least first and second pages associated with first and second bits stored in each of the cells;

a flash memory controller operatively coupled to the memory component, the flash memory controller receiving control and data signals from a host computer system, the flash memory controller further being configured through firmware to partition the memory space into an overprovisioning pool and a user memory pool, the controller further allocating the blocks to the user memory pool and to the overprovisioning pool and configuring the blocks allocated to the overprovisioning pool to disallow writing data to the second pages.

18. The solid state drive of claim 17, the blocks further containing third pages associated with third bits stored in each of the cells and configured to disallow writing data to the second and third pages of any one of the blocks if the one block is dynamically allocated to the overprovisioning pool.

19. The solid state drive of claim 18, further disallowing writing data to the third pages if a monitored bit error rate of one of the blocks exceeds a threshold.

20. The solid state drive of claim 18, wherein flash memory controller causes all host writes to be written to the blocks dynamically allocated to the overprovisioning pool.

21. A method of increasing endurance of a solid state drive having NAND flash memory components organized in blocks, pages and cells, each of the cells being adapted to store at least two bits, the method comprising:

monitoring a bit error rate of each of the blocks; and

dynamically reconfiguring the block to allow storing of a single bit per cell only if the bit error rate increases beyond a threshold.

22. The method of claim 21, wherein the solid state drive is partitioned into an overprovisioning pool and a user memory pool, and all of the blocks configured to allow storing of a single bit per cell only are allocated to the overprovisioning pool.

23. The method of claim 22, wherein all data written to the solid state drive by a host computer are written to the blocks in the overprovisioning pool using a single bit per cell, and wherein data from the blocks in the overprovisioning pool are consolidated in other of the blocks allocated to the user memory pool using at least two bits per cell.

24. The method of claim 21, wherein wear leveling information of the reconfigured blocks is adjusted to match a new configuration thereof.