Variable bit encoding per NAND flash cell to improve device endurance and extend life of flash-based storage devices

• US 9,891,844 B2
• Filed: 04/11/2017
• Issued: 02/13/2018
• Est. Priority Date: 05/20/2015
• Status: Active Grant

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Inventors: Kankani, Navneeth, Truong, Linh Tien
• Primary Examiner(s): YU, JAE UN
• Application Number:
  US15/485,158
• Publication Number:
  US 20170220269A1
• Time in Patent Office: 308 Days
• Field of Search: 711103
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• CPC Class Codes: G06F 2212/1036 Life time enhancement
• CPC Class Codes: G06F 2212/1044 Space efficiency improvement
• CPC Class Codes: G06F 2212/7204 Capacity control, e.g. partitioning, end-of-life degradation
• CPC Class Codes: G06F 3/0616 in relation to life time, e.g. increasing Mean Time Between Failures [MTBF]
• CPC Class Codes: G06F 3/0629 Configuration or reconfiguration of storage systems
• CPC Class Codes: G06F 3/0634 by changing the state or mode of one or more devices
• CPC Class Codes: G06F 3/0638 Organizing or formatting or addressing of data
• CPC Class Codes: G06F 3/0653 Monitoring storage devices or systems
• CPC Class Codes: G06F 3/0688 Non-volatile semiconductor memory arrays

Abstract

Systems, methods, and/or devices are used to implement variable bit encoding to improve device endurance and extend life of storage devices. In some embodiments, the method includes determining a current endurance metric for a plurality of non-volatile memory portions configured to store data encoded in a first encoding format and determining an estimated endurance metric for the plurality of non-volatile memory portions (e.g., corresponding to estimated endurance after reconfiguration of the one or more portions to store data encoded in a second encoding format), and in accordance with a determination that reconfiguration criteria are satisfied (e.g., the estimated endurance metric comprises an improvement over the current endurance metric), reconfiguring the one or more portions to store data encoded in the second encoding format.

Claims

1. A method of operation in a storage device that comprises a storage controller and a plurality of non-volatile memory portions in one or more memory devices, the method comprising:

at the storage controller, the storage controller having one or more physical processors and memory;

determining a current endurance metric for the plurality of non-volatile memory portions of the storage device, wherein at least a first subset of the plurality of non-volatile memory portions are configured to store data encoded in a first encoding format and have a first storage density corresponding to the first encoding format

determining an estimated endurance metric for the plurality of non-volatile memory portions of the storage device, the estimated endurance metric corresponding to an estimated endurance for the plurality of non-volatile memory portions of the storage device after a reconfiguration of the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density; and

in accordance with a determination that reconfiguration criteria are satisfied, reconfiguring the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density, the reconfiguration criteria including a determination that the estimated endurance metric comprises an improvement over the current endurance metric in accordance with predefined endurance improvement criteria.

2. The method of claim 1, wherein the second storage density is a lower storage density than the first storage density.

3. The method of claim 1, wherein the current endurance metric for the plurality of non-volatile memory portions corresponds to an average value of an endurance metric with respect to the plurality of non-volatile memory portions, and the estimated endurance metric for the plurality of non-volatile memory portions of the storage device corresponds to a projected average value of the endurance metric with respect to the plurality of non-volatile memory portions after the reconfiguring.

4. The method of claim 3, wherein the endurance metric with respect to a single memory portion of the plurality of non-volatile memory portions is a value corresponding to a projected number of write operations that can be performed, prior to end of life, by the single memory portion, or a projected number of program/erase cycles that can be performed, prior to end of life, by the single memory portion.

5. The method of claim 1, further comprising:

maintaining one or more status metrics for each memory portion of the plurality of non-volatile memory portions of the storage device, wherein a respective status metric for a respective memory portion corresponds to the respective memory portion'"'"'s ability to retain data; and

in accordance with a determination that at least one of the status metrics for one or more non-volatile memory portions of the storage device satisfies a threshold, detecting a trigger condition as to the one or more non-volatile memory portions of the storage device.

6. The method of claim 1, further comprising:

maintaining one or more status metrics for each of the one or more memory devices, wherein a first status metric of the one or more status metrics corresponds to a respective memory device'"'"'s ability to retain data; and

in accordance with a determination that the one or more status metrics for a respective memory device of the one or more memory devices satisfy one or more end-of-life criteria, operating the respective memory device in read-only mope.

7. The method of claim 1, wherein the one or more memory devices comprise one or more flash memory devices.

8. A storage device, comprising:

a set of one or more non-volatile memory devices, the set of one or more non-volatile memory devices including a plurality of non-volatile memory portions; and

a storage controller, the storage controller including one or more controller modules configured to;

determine a current endurance metric for the plurality of non-volatile memory portions of the storage device, wherein at least a first subset of the plurality of non-volatile memory portions are configured to store data encoded in a first encoding format and have a first storage density corresponding to the first encoding format

determine an estimated endurance metric for the plurality of non-volatile memory portions of the storage device, the estimated endurance metric corresponding to an estimated endurance for the plurality of non-volatile memory portions of the storage device after a reconfiguration of the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density; and

in accordance with a determination that reconfiguration criteria are satisfied, reconfigure the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density, the reconfiguration criteria including a determination that the estimated endurance metric comprises an improvement over the current endurance metric in accordance with predefined endurance improvement criteria.

9. The storage device of claim 8, wherein the one or more controller modules include:

a metric maintaining module to determine the estimated endurance metric; and

a reconfiguration module to reconfigure one or more of the plurality of non-volatile memory portions of the storage device in accordance with the determination that reconfiguration criteria are satisfied.

10. The storage device of claim 8, wherein the second storage density is a lower storage density than the first storage density.

11. The storage device of claim 8, wherein the current endurance metric for the plurality of non-volatile memory portions corresponds to an average value of an endurance metric with respect to the plurality of non-volatile memory portions, and the estimated endurance metric for the plurality of non-volatile memory portions of the storage device corresponds to a projected average value of the endurance metric with respect to the plurality of non-volatile memory portions after the reconfiguring.

12. The storage device of claim 11, wherein the endurance metric with respect to a single memory portion of the plurality of non-volatile memory portions is a value corresponding to a projected number of write operations that can be performed, prior to end of life, by the single memory portion, or a projected number of program/erase cycles that can be performed, prior to end of life, by the single memory portion.

13. The storage device of claim 8, wherein the one or more controller modules are further configured to:

maintain one or more status metrics for each memory portion of the plurality of non-volatile memory portions of the storage device, wherein a respective status metric for a respective memory portion corresponds to the respective memory portion'"'"'s ability to retain data; and

in accordance with a determination that at least one of the status metrics for one or more non-volatile memory portions of the storage device satisfies a threshold, detecting a trigger condition as to the one or more non-volatile memory portions of the storage device.

14. The storage device of claim 8, wherein the one or more controller modules are further configured to:

maintain one or more status metrics for each of the one or more memory devices, wherein a first status metric of the one or more status metrics corresponds to a respective memory device'"'"'s ability to retain data; and

in accordance with a determination that the one or more status metrics for a respective memory device of the one or more memory devices satisfy one or more end-of-life criteria, operate the respective memory device in read-only mode.

15. The storage device of claim 8, wherein the one or more memory devices comprise one or more flash memory devices.

16. A storage device, comprising:

a set of one or more non-volatile memory devices, the set of one or more non-volatile memory devices including a plurality of non-volatile memory portions;

metric maintaining means for determining a current endurance metric for the plurality of non-volatile memory portions of the storage device, wherein at least a first subset of the plurality of non-volatile memory portions are configured to store data encoded in a first encoding format and have a first storage density corresponding to the first encoding format, and for determining an estimated endurance metric for the plurality of non-volatile memory portions of the storage device, the estimated endurance metric corresponding to an estimated endurance for the plurality of non-volatile memory portions of the storage device after a reconfiguration of the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density; and

reconfiguration means, enabled in accordance with a determination that reconfiguration criteria are satisfied, for reconfiguring the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density, the reconfiguration criteria including a determination that the estimated endurance metric comprises an improvement over the current endurance metric in accordance with predefined endurance improvement criteria.

17. The storage device of claim 16, wherein the second storage density is a lower storage density than the first storage density.

18. The storage device of claim 16, wherein the current endurance metric for the plurality of non-volatile memory portions corresponds to an average value of an endurance metric with respect to the plurality of non-volatile memory portions, and the estimated endurance metric for the plurality of non-volatile memory portions of the storage device corresponds to a projected average value of the endurance metric with respect to the plurality of non-volatile memory portions after the reconfiguring.

19. The storage device of claim 18, wherein the endurance metric with respect to a single memory portion of the plurality of non-volatile memory portions is a value corresponding to a projected number of write operations that can be performed, prior to end of life, by the single memory portion, or a projected number of program/erase cycles that can be performed, prior to end of life, by the single memory portion.

20. A non-transitory computer readable storage medium, storing one or more programs configured for execution by one or more processors of a storage device having a set of one or more non-volatile memory devices, the set of one or more non-volatile memory devices including a plurality of non-volatile memory portions, and a storage controller, the one or more programs including instructions that when executed by the one or more processors cause the storage device to:

determine a current endurance metric for the plurality of non-volatile memory portions of the storage device, wherein at least a first subset of the plurality of non-volatile memory portions are configured to store data encoded in a first encoding format and have a first storage density corresponding to the first encoding format

determine an estimated endurance metric for the plurality of non-volatile memory portions of the storage device, the estimated endurance metric corresponding to an estimated endurance for the plurality of non-volatile memory portions of the storage device after a reconfiguration of the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density; and

in accordance with a determination that reconfiguration criteria are satisfied, reconfigure the first subset of the plurality of non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density, the reconfiguration criteria including a determination that the estimated endurance metric comprises an improvement over the current endurance metric in accordance with predefined endurance improvement criteria.