Why data is moved upon fixable bit-flip

[Danesh Daroui]

Hi all,

I am running some torture tests on a NAND flash memory having UBIFS on it. Running the tests for few days shows some errors which are apparently fixable by UBI. Some errors look like:

UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 557
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 1627
UBI: run torture test for PEB 1627
UBI: PEB 1627 passed torture test, do not mark it a bad
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 557
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 1627
UBI: run torture test for PEB 1627
UBI: PEB 1627 passed torture test, do not mark it a bad
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 557
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 1627
UBI: run torture test for PEB 1627
UBI: PEB 1627 passed torture test, do not mark it a bad

which are promising since it seems that the file system has been able to fix the error as it says "fixable bit-flip'. However, some other errors are like:

UBI DBG (pid 1691): ubi_io_read: fixable bit-flip detected at PEB 1546
UBI DBG (pid 1691): ubi_wl_scrub_peb: schedule PEB 1546 for scrubbing
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 1546
UBI: scrubbed PEB 1546 (LEB 3:177), data moved to PEB 1875
UBI DBG (pid 1691): ubi_io_read: fixable bit-flip detected at PEB 217
UBI DBG (pid 1691): ubi_wl_scrub_peb: schedule PEB 217 for scrubbing
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 217
UBI: scrubbed PEB 217 (LEB 3:937), data moved to PEB 1875
UBI DBG (pid 1691): ubi_io_read: fixable bit-flip detected at PEB 1546
UBI DBG (pid 1691): ubi_wl_scrub_peb: schedule PEB 1546 for scrubbing
UBI DBG (pid 25): ubi_io_read: fixable bit-flip detected at PEB 1546
UBI: scrubbed PEB 1546 (LEB 3:1054), data moved to PEB 1875

This is a bit strange though, because the data is moved even when the error was fixable because of bit-flip as the log says.

Now I would like to know why the data is moved even when it is fixable? Is it something that we need to be worried about? Why the bit-flip is not simply corrected as it is in the former case but it is rather scrubbed and the data is moved? Also, will the PEB be marked as unusable bad cell when it is scrubbed and the data is moved to another PEB?

Thanks a lot for your help,

Danesh Daroui