Re: Activating space_cache after read-only snapshots without space_cache have been taken

[Sander <sander@humilis.net>]

Liu Bo wrote (ao):
> On Tue, Apr 16, 2013 at 02:28:51AM +0200, Ochi wrote:
> > 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?

No, he first created two ro snapshots, then (some time later) mounted
with nospace_cache, and then continued to take 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'.

AFAIUI the problem is not there anymore, but this is a good tip for the
future.

	Sander