Retention-drift-history-based non-volatile memory read threshold optimization

1. A method comprising:

determining whether an optimal read threshold voltage associated with a group of pages of a non-volatile memory (“NVM”) is out of tolerance based at least in part on a retention drift history associated with the group of pages;

upon determining that the optimal read threshold voltage associated with the group of pages is out of tolerance, writing reference cells associated with the group of pages with a pattern having a known statistical distribution of ones and zeroes;

determining a new optimal read threshold voltage associated with the group of pages by reading the reference cells; and

updating the retention drift history associated with the group of pages with the new optimal read threshold voltage and an indication of a new reference cell generation.

2. The method of claim 1 , wherein determining that the optimal read threshold voltage associated with the group of pages is out of tolerance is based on a time elapsed since the reference cells were last written according to the retention drift history.

3. The method of claim 1 , wherein determining that the optimal read threshold voltage associated with the group of pages is out of tolerance is based on an exposure of the reference cells to temperature according to the retention drift history.

4. The method of claim 1 , wherein determining a new optimal read threshold voltage associated with the group of pages comprises:

reading data from the reference cells;

determining a distribution of ones and zeroes in the data read;

determining a disparity between the distribution of ones and zeroes in the data read and the known statistical distribution of ones and zeroes; and

determining a new read threshold voltage based at least in part upon a magnitude of the determined disparity.

5. The method of claim 4 , wherein determining the new read threshold voltage based upon the magnitude of the disparity comprises looking up the read threshold voltage value from a lookup table indexed at least in part by magnitude of determined disparity.

6. The method of claim 1 , wherein the reference cells associated with the group of pages occur in at least two pages from the group of pages.

7. The method of claim 1 , wherein the reference cells associated with the group of pages comprise a last page of a block of the NVM containing the group of pages.

8. The method of claim 1 , wherein the NVM comprises MLC memory and the optimal read threshold voltage is one of a set of optimal read threshold voltages associated with the group of pages.

9. A storage system comprising:

a non-volatile memory (“NVM”); and

a storage controller communicatively coupled to the NVM, the storage controller configured to

determine whether an optimal read threshold voltage associated with a group of pages of the NVM is out of tolerance based at least in part on a retention drift history associated with the group of pages,

upon determining that the optimal read threshold voltage associated with the group of pages is out of tolerance, determine a new optimal read threshold voltage associated with the group of pages by reading reference cells associated with the group of pages with a pattern having a known statistical distribution of ones and zeroes, and

update the retention drift history associated with the group of pages with the new optimal read threshold voltage and an indication of a new reference cell generation.

10. The storage system of claim 9 , wherein the storage controller is further configured to, before determining the new optimal read threshold voltage associated with the group of pages, writing the reference cells with the pattern.

11. The storage system of claim 10 , wherein the storage controller is further configured to, upon updating the retention drift history with the new optimal read threshold, updating the retention drift history associated with the group of pages with an indication of a new reference cell generation.

12. The storage system of claim 9 , wherein the determining whether an optimal read threshold voltage associated with a group of pages of the NVM is repeated periodically based on a time elapsed since the reference cells were last written according to the retention drift history.

13. The storage system of claim 9 , wherein determining a new optimal read threshold voltage associated with the group of pages comprises:

reading data from the reference cells;

determining a distribution of ones and zeroes in the data read;

determining a disparity between the distribution of ones and zeroes in the data read and the known statistical distribution of ones and zeroes; and

determining a new read threshold voltage based at least in part upon a magnitude of the determined disparity.

14. The storage system of claim 13 , wherein determining the new read threshold voltage based upon the magnitude of the disparity comprises looking up the read threshold voltage value from a lookup table indexed at least in part by magnitude of determined disparity.

15. The storage system of claim 9 , wherein the reference cells associated with the group of pages occur in at least two pages from the group of pages.

16. The storage system of claim 9 , wherein the reference cells associated with the group of pages comprise a last page of a block of the NVM containing the group of pages.

17. A non-transitory computer readable storage medium having processor-executable instructions stored thereon that, when executed by a processor, cause the processor to:

determine whether an optimal read threshold voltage associated with a group of pages of a non-volatile memory (“NVM”) is out of tolerance based at least in part on a retention drift history associated with the group of pages;

upon determining that the optimal read threshold voltage associated with the group of pages is out of tolerance, write reference cells associated with the group of pages with a pattern having a known statistical distribution of ones and zeroes;

determine a new optimal read threshold voltage associated with the group of pages by reading the reference cells; and

update the retention drift history associated with the group of pages with the new optimal read threshold voltage and an indication of a new reference cell generation.

18. The non-transitory computer readable storage medium of claim 17 , wherein determining that the optimal read threshold voltage associated with the group of pages is out of tolerance is based on a time elapsed since the reference cells were last written according to the retention drift history.

19. The non-transitory computer readable storage medium of claim 17 , wherein determining a new optimal read threshold voltage associated with the group of pages comprises:

reading data from the reference cells;

determining a distribution of ones and zeroes in the data read;

determining a disparity between the distribution of ones and zeroes in the data read and the known statistical distribution of ones and zeroes; and

determining a new read threshold voltage based at least in part upon a magnitude of the determined disparity.

20. The non-transitory computer readable storage medium of claim 19 , wherein determining the new read threshold voltage based upon the magnitude of the disparity comprises looking up the read threshold voltage value from a lookup table indexed at least in part by magnitude of determined disparity.