Scheduling of Housekeeping Operations in Flash Memory Systems

• US 20090265508A1
• Filed: 06/29/2009
• Published: 10/22/2009
• Est. Priority Date: 01/20/2005
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Sandisk Technologies Incorporated (Western Digital Corporation)
• Inventors: Tomlin, Andrew, Bennett, Alan David, Gorobets, Sergey Anatolievich, Schroter, Charles
• Granted Patent: US 8,364,883 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 2212/7205 Cleaning, compaction, garbage collection, erase control
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles

Abstract

A re-programmable non-volatile memory system, such as a flash EEPROM system, having its memory cells grouped into blocks of cells that are simultaneously erasable is operated to perform memory system housekeeping operations in the foreground during execution of a host command, wherein the housekeeping operations are unrelated to execution of the host command. Both one or more such housekeeping operations and execution of the host command are performed within a time budget established for executing that particular command. One such command is to write data being received to the memory. One such housekeeping operation is to level out the wear of the individual blocks that accumulates through repetitive erasing and re-programming.

Claims

1. A method of operating an erasable and re-programmable non-volatile memory system, which comprises, in response to receiving a command from outside the memory system:

perform any function necessary to execute the command,

assert a busy signal outside of the memory system for a time extending beyond that utilized to perform said any necessary function, and

performing during the extended busy signal time at least a portion of one housekeeping operation within the memory system that is unnecessary to execute the received command,

wherein said command has a time budget for its executions, and said busy signal is asserted for a time remaining within the time budget.

2. The method according to claim 1, wherein the received command includes a write command followed by one or more units of data to be written into the memory system.

3. The method according to claim 2, wherein a plurality of units of data to be written into the memory system are received, and wherein the busy signal is asserted in at least two intervals between receipt of the units of data, during which time said at least a portion of one housekeeping operation is performed by completing said at least one housekeeping operation.

4. The method according to claim 3, wherein said at least one housekeeping operation is performed during more than one of said at least two intervals.

5. The method according to claim 4, wherein said any function necessary to execute the command includes a garbage collection operation and writing the received data to the memory system.

6. The method according to any one of claims 1-5, wherein said one housekeeping operation includes transferring a quantity of data from one location to another location within the memory system, and wherein performing at least a portion of such a housekeeping operation includes transferring only a portion of the quantity of data.

7. The method according to claim 6, wherein remaining portions of the quantity of data are subsequently transferred during execution of a garbage collection operation not directed specifically to perform said one housekeeping operation.

8. The method to any one of claims 1-5, wherein said at least one housekeeping operation includes a wear leveling operation performed in a portion of the memory not involved in the execution of the command.

9. The method according to any one of claims 1-5, wherein said at least one housekeeping operation includes refreshing data stored in a portion of the memory not involved in the execution of the command.

10. The method according to any one of claims 1-5, wherein said at least one housekeeping operation includes a garbage collection operation performed in a portion of the memory not involved in the execution of the command.

11. The method according to claim 1, wherein the received command is a command to transfer one or more units of data within said given time budget, wherein:

a) performing any function necessary to execute the command includes;

determining whether any housekeeping operation is necessary to be able to transfer the one or more units of data,

if any housekeeping operation is necessary to be able to transfer the one or more units of data, performing within the time budget at least the necessary housekeeping operation, and

b) wherein performing during the extend busy signal time at least a portion of one housekeeping operation within the memory system that is unnecessary to execute the received command comprises;

determining whether a housekeeping operation unnecessary to the transfer of one or more units of data is pending,

if a housekeeping operation unnecessary to the data transfer is pending,

determine whether any remaining time within the time budget is sufficient for performing the pending housekeeping operation unnecessary to the data transfer, and

if it is determined that sufficient time remains within the time budget, performing within the time budget at least a portion of the pending housekeeping operation unnecessary to the data transfer during execution of the received command,

c) transferring the one or more units of data within the given time budget.

12. The method according to claim 11, wherein determining whether any housekeeping operation is necessary to be able to write the one or more units of data includes determining whether data within two or more locations of the memory need to be consolidated into a single location, and, if so, consolidating such data within the given time budget.

13. The method according to claim 11, wherein determining whether a housekeeping operation unnecessary to the data transfer is pending includes identifying a pending wear leveling exchange, and wherein performing the pending housekeeping operation unnecessary to the data transfer includes performing at least a portion of the pending wear leveling exchange within the given time budget.

14. The method according to claim 13, wherein performing the pending housekeeping operation unnecessary to the data transfer includes performing all of the pending wear leveling exchange within the given time budget.

15. The method according to claim 11, wherein determining whether a housekeeping operation unnecessary to the data transfer is pending includes identifying an operation to refresh specified data stored in a portion of the memory system different than a portion involved in executing the received data transfer command, and wherein performing the pending housekeeping operation unnecessary to the data transfer includes refreshing at least a portion of the specified data within the given time budget.

16. The method according to claim 15, wherein performing the pending housekeeping operation unnecessary to the data transfer includes refreshing all of the specified data within the given time budget.

17. The method according to claim 11, wherein determining whether a housekeeping operation unnecessary to the data transfer is pending includes identifying an operation for performing garbage collection in a portion of the memory system different than a portion involved in executing the received write command, and thereafter performing at least a portion of such garbage collection within the given time budget.

18. The method according to claim 17, wherein performing the pending housekeeping operation unnecessary to the data transfer includes performing all of such data transfer within the given time budget.

19. The method according to claim 11, additionally comprising receiving two or more units of data in succession, and wherein determining any remaining amounts of time within the time budget for performing the housekeeping operation unnecessary to the data transfer results in determining that sufficient time remains, and additionally comprises asserting at least one busy period that is unnecessary to complete execution of the data transfer command.

20. The method according to claim 11, wherein if there is not time within the time budget sufficient for performing at least a portion of the pending housekeeping operation unnecessary to the data transfer, the unperformed housekeeping operation is added to a count of such unperformed operations.

21. The method according to claim 20, in response to multiple unperformed housekeeping operations being included in the count, causing each in order to be rendered as pending for execution during execution of all subsequent data write commands until the count is zero.

22. The method according to claim 20, wherein, in response to multiple unperformed housekeeping operations being included in the count, each unperformed housekeeping operation is caused to be rendered pending one at a time in order for execution during execution of all subsequent data transfer commands until the count is zero.

23. The method according to claim 20, wherein housekeeping operations unrelated to execution of the data transfer command are normally rendered at a pre-defined interval, and wherein, in response to multiple unperformed housekeeping operations being included in the count, the unperformed housekeeping operations are caused to be rendered pending one at a time in order to be performed during execution of subsequent data transfer commands at intervals less than said pre-defined interval until the count is zero.

24. The method according to claim 20, wherein housekeeping operations unrelated to execution of the data transfer command are normally rendered at a pre-defined interval, and wherein, in response to multiple unperformed housekeeping operations being included in the count, at least one of the unperformed housekeeping operations is caused to be rendered pending to be performed during execution of a subsequent data transfer command at an interval less than said pre-defined interval while at least one other of the unperformed housekeeping operations is not performed.