Variable bit encoding per NAND flash cell to extend life of flash-based storage devices and preserve over-provisioning

• US 9,864,525 B2
• Filed: 02/24/2017
• Issued: 01/09/2018
• 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:
  US15/441,808
• Publication Number:
  US 20170168713A1
• Time in Patent Office: 319 Days
• Field of Search: None
• CPC Class Codes: G06F 12/02 Addressing or allocation; Relocation program address sequencing G06F9/00; arrangements for selecting an address in a digital store G11C8/00
• CPC Class Codes: G06F 3/061 Improving I/O performance
• 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 3/0644 Management of space entities, e.g. partitions, extents, pools
• CPC Class Codes: G06F 3/0655 Vertical data movement, i.e. input-output transfer; data movement between one or more hosts and one or more storage devices
• CPC Class Codes: G06F 3/0688 Non-volatile semiconductor memory arrays

Abstract

Systems, methods, and/or devices are used to implement variable bit encoding per NAND flash cell to extend life of flash-based storage devices and preserve over-provisioning. In some embodiments, the method includes detecting a trigger condition with respect to one or more non-volatile memory portions (e.g., portions configured to store data encoded in a first encoding format and having a first storage density) of a plurality of non-volatile memory portions of a storage device. In response to detecting the trigger condition and in accordance with a first determination that a projected amount of over-provisioning (e.g., corresponding to over-provisioning for the storage device after reconfiguring the one or more non-volatile memory portions to store data encoded in a second encoding format and having a second storage density) meets predefined over-provisioning criteria, the method includes reconfiguring the one or more non-volatile memory 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;

detecting a trigger condition, wherein, when the trigger condition is detected, one or more non-volatile memory portions 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;

in response to detecting the trigger condition, and in accordance with a first determination that a projected amount of over-provisioning meets predefined over-provisioning criteria, where the projected amount of over-provisioning corresponds to over-provisioning for the storage device after reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density corresponding to the second encoding format, reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density.

2. The method of claim 1, wherein the second storage density is a lower storage density than the first storage density, and the storage device has reduced over-provisioning after the reconfiguring.

3. The method of claim 1, further comprising:

determining a current endurance metric for the plurality of non-volatile memory portions of the storage device; and

determining an estimated endurance metric for the plurality of non-volatile memory portions of the storage device, corresponding to an estimated endurance for the plurality of non-volatile memory portions of the storage device after the reconfiguring; and

wherein reconfiguring the one or more non-volatile memory portions of the storage device further comprises;

reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density in accordance with the first determination that the projected amount of over-provisioning meets the predefined over-provisioning criteria and in accordance with a second determination that the estimated endurance metric comprises an improvement over the current endurance metric in accordance with predefined endurance improvement criteria.

4. The method of claim 3, 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.

5. The method of claim 4, 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.

6. 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;

wherein detecting the trigger condition comprises;

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

7. 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 mode.

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

9. The method of claim 1, wherein the projected amount of over-provisioning comprises a first quantity of storage units in the storage device, comprising mapped storage units remaining after reconfiguring the one or more non-volatile memory portions and unmapped storage units that are usable for storing data and that are remaining after reconfiguring the one or more non-volatile memory portions, and subtracting from the first quantity a quantity corresponding to a declared storage capacity of the storage device.

10. 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;

detect a trigger condition, wherein, when the trigger condition is detected, one or more non-volatile memory portions 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

in response to detecting the trigger condition, and in accordance with a first determination that a projected amount of over-provisioning meets predefined over-provisioning criteria, where the projected amount of over-provisioning corresponds to over-provisioning for the storage device after reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density corresponding to the second encoding format, reconfigure the one or more non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density.

11. The storage device of claim 10, wherein the one or more controller modules include:

a trigger condition detection module to detect the trigger condition; and

a reconfiguration module to reconfigure the one or more non-volatile memory portions of the storage device in response to the trigger condition and in accordance with the first determination.

12. The storage device of claim 11, wherein the one or more controller modules further include an over-provisioning module to determine the projected amount of over-provisioning remaining for the storage device after reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density.

13. The storage device of claim 10, wherein the projected amount of over-provisioning meets the predefined over-provisioning criteria only when the projected amount of over-provisioning is greater than a predetermined minimum amount of over-provisioning for the storage device.

14. The storage device of claim 10, wherein the second storage density is a lower storage density than the first storage density, and the storage device has reduced over-provisioning after the reconfiguring.

15. The storage device of claim 10, wherein the one or more controller modules are further configured to:

determine a current endurance metric for the plurality of non-volatile memory portions of the storage device; and

determine an estimated endurance metric for the plurality of non-volatile memory portions of the storage device, corresponding to an estimated endurance for the plurality of non-volatile memory portions of the storage device after the reconfiguring; and

wherein reconfiguring the one or more non-volatile memory portions of the storage device further comprises;

reconfigure the one or more non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density in accordance with the first determination that the projected amount of over-provisioning meets the predefined over-provisioning criteria and in accordance with a second determination that the estimated endurance metric comprises an improvement over the current endurance metric in accordance with predefined endurance improvement criteria.

16. The storage device of claim 15, 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.

17. The storage device of claim 16, 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.

18. The storage device of claim 10, 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;

wherein detecting the trigger condition comprises;

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

19. The storage device of claim 10, wherein the one or more non-volatile memory devices comprise one or more flash memory devices.

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:

detect a trigger condition, wherein, when the trigger condition is detected, one or more non-volatile memory portions of the plurality of non-volatile memory portions are configured to store data encoded in a first encoding format and having a first storage density corresponding to the first encoding format; and

in response to detecting the trigger condition, and in accordance with a first determination that a projected amount of over-provisioning meets predefined over-provisioning criteria, where the projected amount of over-provisioning corresponds to over-provisioning for the storage device after reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in a second encoding format and having a second storage density corresponding to the second encoding format, reconfiguring the one or more non-volatile memory portions of the storage device to store data encoded in the second encoding format and having the second storage density.