On Tue, Apr 16, 2013 at 02:28:51AM +0200, Ochi wrote:
> 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.
So are you using '-o nospace_cache' when creating two RO snapshots?
>
> 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 don't know why this happens, but maybe you can observe the umount process's
very slow behaviour by using 'cat /proc/{umount-pid}/stack' or 'perf top'.
thanks,
liubo
>
> 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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