[linux-lvm] Scrub errors after extending LVM RAID1 mirror [full email]

[Steve Dodd]

Sorry, user error sent the last email before I'd finished typing, trying
again..

Hi everyone,

I am experiencing a mystery scrub failure after extending a particular LV
which is a raid1 type mirror. I am using Ubuntu 18.04, LVM
2.02.176-4.1ubuntu3, Ubuntu kernel 4.15.0-29-generic. I mentioned this on
IRC, thought an email might reach more people and allow me to provide more
detail.

As far as I can tell, the LV was *not* created with --nosync:

# lvs rvg/backups
>   LV      VG  Attr       LSize  Pool Origin Data%  Meta%  Move Log
> Cpy%Sync Convert
>   backups rvg rwi-aor--- 96.64G                                    100.00
>


The only odd thing I tend to do is specify extents for the extension
manually, being a bit OCD about on disk segment layouts. Having mined
.bash_history, it seems that last time I ran:

lvextend -l+2561 rvg/backups /dev/sdc3:20480-23041 /dev/sdb3:80097-82658


After that, a *lvchange --syncaction check rvg/backups* showed a huge
number for raid_mismatch_count (seemed roughly consistent with the newly
extended portion not being synced), but dumping the actual filesystem with
partclone from both legs of the mirror through md5sum showed no
inconsistencies; the contents are mostly borg repositories and for good
measure I verified the data in those using borg as well - no problems.

After a full resync all is well again. This is the second time this
happened to me on the same LV (I think - certainly the same VG.)

Any clues? Any known bugs fixed recently that might not have made it into
Ubuntu 1804? I am trying to reproduce with a test LV but can't. Only other
thing I can think might be relevant was that the volume was mounted (but
quiescent) at the time.

Thanks,
Steve
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