Subject: RFC: Read repair for md RAID1 after mirror read failures

Hello list,

Recently I encountered an interesting failure mode while migrating data 
from an aging backup system.

The source Btrfs filesystem spanned several LVM logical volumes, each 
backed by an md RAID1 array. One of these logical volumes resided on a 
degraded RAID1 array, so I added a new logical volume on a new RAID1 
array and started migrating the data using:

btrfs device remove

During the migration, many read requests encountered unrecoverable read 
errors (UNC) on the remaining member of the degraded RAID1 array. The 
migration continued, which is exactly what I had hoped for, since only a 
small fraction of the media appeared to be affected.

This made me wonder whether md RAID1 has, or has ever considered, a read 
repair mechanism.

Consider the following situation:

mirror A:
read -> UNC

mirror B:
read -> OK

During a RAID1 read request, if md can satisfy the read from the 
alternate mirror, no data is lost. However, this also represents an 
opportunity to repair the degraded copy. Once a read has failed on one 
mirror, the array has effectively lost its ability to tolerate a second 
read failure for that logical block. This window persists until the 
affected block is rewritten by normal filesystem activity, which may 
never happen on cold archival data. Repairing the degraded copy 
immediately may restore full redundancy while a valid copy is still 
available. Writing the recovered block back to the the corresponding 
block on the failed mirror would give the drive an opportunity to 
recover that location, for example by successfully rewriting the sector 
or remapping it if the write cannot be verified.

In other words, the first successfully recovered read request could 
automatically become a repair opportunity. The repair could even be 
scheduled asynchronously, so the successful read is returned immediately 
while the rewrite is performed in the background. Unlike a periodic 
resync, this repair would be driven by an actual read failure, making it 
targeted rather than rewriting the entire mirror.

With today's 18–24 TB HDDs and backup/archive workloads, where data may 
remain unchanged for years, latent media degradation seems increasingly 
relevant. A successful read from the alternate mirror may be one of the 
last opportunities to refresh such a sector before it becomes 
permanently unreadable.

One advantage of such an approach is that it does not require md to 
decide between two conflicting copies. One mirror has already reported 
an unrecoverable read error, while the other has successfully 
reconstructed the requested block. The proposal only applies to this 
specific case.

Has this idea been discussed before, or is there a reason why md 
deliberately avoids this type of read repair?

Regards,

Dion Kant