在 2015年10月23日 23:05, Johannes Henninger 写道:
On 23.10.2015 01:47, Qu Wenruo wrote:
在 2015年10月23日 04:38, Johannes Henninger 写道:
I'm having a weird problem with snapshots and exclusive quotas. After
creating a snapshot of a subvolume and setting an exclusive quota of
50MB for the snapshot, everything seems to work fine. I can write
approximately 50MB before the quota kicks in.
However, if I create a snapshot, set an exclusive quota and just wait
for some time, I suddenly cannot even create an empty file because I'm
getting a "quota exceeded" error. The time until the bug appears seems
to vary. During the waiting time, I'm changing neither the snapshot nor
the original subvolume. "qgroup show -e" reports an exclusive use of
only a few kilobytes for the snapshot, which is nowhere near the limit.
Steps to reproduce (/media/extern is a fresh and empty btrfs partition):
Enable quota and create an empty subvolume:
root@t420:/media/extern# btrfs quota enable .
root@t420:/media/extern# btrfs subvolume create sub
Create subvolume './sub'
Snapshot the subvolume and set a limit:
root@t420:/media/extern# btrfs subvolume snapshot sub snap
Create a snapshot of 'sub' in './snap'
root@t420:/media/extern# cd snap/
root@t420:/media/extern/snap# btrfs qgroup limit -e 50M .
Sometimes it takes "longer" for the quota to kick in, so I'm touching a
file every 5 minutes here:
root@t420:/media/extern/snap# for file in {1..100}; do touch $file;
sleep 5m; done
touch: cannot touch ‘7’: Disk quota exceeded
^C
root@t420:/media/extern/snap# btrfs qgroup show -e .
qgroupid rfer excl max_excl
-------- ---- ---- --------
0/5 16.00KiB 16.00KiB none
0/257 16.00KiB 16.00KiB none
0/258 16.00KiB 16.00KiB 50.00MiB
Any idea why this happens?
BTW, to make btrfs qgroup show work, it's better to call sync before
qgroup show.
It's a known bug that even after qgroup accounting rework, qgroup
reserve still has bug and can cause reserved space to underflow,
making such problem happen.
For such case, btrfs qgroup show won't help as reserved space is not
shown in the output.
One workaround would be, umount the filesystem and mount again.
Which will reset the underflow reserved space and work for sometime.
If it's OK for you to recompile the kernel, you can try the following
patchset:
[PATCH v3 00/21] Rework btrfs qgroup reserved space framework
Which should solve the problem.
Thanks,
Qu
Thanks a lot for your reply!
While remounting the filesystem fixes the issue temporary, it doesn't
take very long for the bug to happen again so it's not really a
workaround I can work with.
I did recompile the kernel using your patches, but unfortunately the
problem still appears.
Thanks,
Johannes
Interesting, just touching file will cause EQUOTA is quite a big problem.
I'll try to reproduce it with my patchset and see what really caused the
problem.
The problem seems to do with snapshot qgroup hacking.
But I'm not completely sure yet.
BTW, does "sync; btrfs qgroup show -prce" still show excl as 16K?
16K is the correct number with only 6 empty files, just in case.
Thanks,
Qu
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