On 20/01/20 10:02, Andrey ``Bass'' Shcheglov wrote: > Greetings, > > I have a question on how to repair a RAID1 array (consisting of 2 > physical hard drives, metadata version 1.2) which went split-brain. > > One of my md-devices repeatedly shows a non-zero mismatch_cnt: > > # cat /sys/block/md4/md/mismatch_cnt > 1024 > > Zeroing out free space (with `zerofree`, as recommended here: > <http://decafbad.net/2017/01/03/mismatch_cnt,-raid1,-and-a-clever-fix/>) > and disabling the swap both retain the mismatch count at the very same > level. > Also, none of the drives is failing (18x and 19x SMART attributes are ok). > Checking file systems (ext4) doesn't show any problem, either, so the > file system metadata is most probably correct, too. > > The usual suspects ruled out, I'm starting to think it my data got > corrupted, and at least one out of two replicas is affected. > Of course I can > > # echo repair > /sys/block/md0/md/sync_action > > but I have a 50% chance of losing information stored on the "right" replica. > > > So, assuming my /dev/md0 is now assembled from /dev/sda1 and /dev/sdb1, > I feel like assemble and run two separate degraded mirrors from > /dev/sda1 and /dev/sdb1, respectively (`mdadm -A`), > mount the corresponding file systems R/O, > create two backups (one backup per replica) > and then compare them with each other (`diff -urN`). > > > The question is: is it possible to assemble an array in a read-only mode, > so that the underlying block device is never written to, > the metadata in the superblock remains intact and the event count is > not incremented? > > My intention is to avoid the resync when my original /dev/md0 is > reassembled from /dev/sda1 and /dev/sdb1. > Then how (assuming one drive is corrupt) are you going to re-assemble the array without forcing a resync on that drive? > > If you have any other recommendations on how to interactively repair > the array (I want to be able to peek at the data being synced), > I'd appreciate you sharing them. > My inclination (no warranty included!) would be to shut down the array, then assemble it with "/dev/sda1 missing" and --force if necessary. fsck that, then rinse and repeat with the second drive. Assuming neither drive has problems, you should then be able to assemble --assume-clean, which will prevent the sync, otherwise you'll have to just re-add the duff drive and let it resync. (In other words, why worry about the resync, because if you find the problem then you're going to have to resync to fix it, anyway.) Hint - look at dm-integrity. I believe you can put the integrity information elsewhere (if you've got a spare bit of disk space) so this issue won't arise again. It's new with raid, but apparently works fine with raid-1. Don't try it with the higher raids - 5 or 6 - yet. > Regards, > Andrey. > Cheers, Wol
