Hello everyone,
I've ran into problems with _very_ slow unmounting of my btrfs-formatted
"backup" volume. I have a suspicion what might be the cause but maybe
someone with more experience with the btrfs code could enlighten me
whether it is actually correct.
The situation is the following: I have created a backup-volume to which
I regularly rsync a backup of my system into a subvolume. After
rsync'ing, I take a _read-only_ snapshot of that subvolume with a
timestamp added to its name.
Now at the time I started using this backup volume, I was _not_ using
the space_cache mount option and two read-only snapshots were taken
during this time. Then I started using the space_cache option and
continued doing snapshots.
A bit later, I started having very long lags when unmounting the backup
volume (both during shutdown and when unmounting manually). I scrubbed
and fsck'd the volume but this didn't show any errors. Defragmenting the
root and subvolumes took a long time but didn't improve the situation much.
Now I started having the suspicion that maybe the space cache possibly
couldn't be written to disk for the readonly subvolumes/snapshots that
were created during the time when I wasn't using the space_cache option,
forcing the cache to be rebuilt every time.
Clearing the cache didn't help. But when I deleted the two snapshots
that I think were taken during the time without the mount option, the
unmounting time seems to have improved considerably.
I will have to observe whether unmounting stays quick now but my
question is whether it is possible that the read-only snapshots taken
during the time when I wasn't using space_cache might actually have been
the culprits.
Best,
Sebastian
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