On 04/16/2013 10:10 AM, Sander wrote:
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.
I need to clarify this: The NOspace_cache option was never used, I just
didn't explicitly activate space_cache in the beginning. However, I was
not aware that space_cache is the default anyways (at least in Arch
which is the distro I'm using). I reviewed old system logs and it
actually looks like space caching was always being used right from the
beginning, even when I didn't explicitly use the space_cache mount
option. So I guess this wasn't the problem after all :\
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
That's correct, the problem has vanished after the deletion of the
oldest two snapshots. Mounting and unmounting is reasonably fast now. I
will just continue to use the volume normally (i.e. making regular
backups and snapshotting) and report back if the problem appears again.
Just for the records: The btrfs volume and the first snapshots were
originally created under kernel 3.7.10. I then updated to 3.8.3. I don't
know if this information is useful - just in case... :)
Thanks,
Sebastian
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