Re: Exclusive quota of snapshot exceeded despite no space used

[Johannes Henninger <johannes+btrfs@henninger.io>]

On 25.10.2015 02:44, Qu Wenruo wrote:
>
>
> 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

I ran my example from the first mail again and managed to write 7 files
this time, "qgroup show" still shows 16kB after sync:

root@t420:/media/extern/snap# btrfs qg limit -e 50M .
root@t420:/media/extern/snap# for file in {1..100}; do touch $file;
sleep 5m; done
touch: cannot touch ‘8’: Disk quota exceeded
^C
root@t420:/media/extern/snap# sync
root@t420:/media/extern/snap# btrfs qgroup show -pcre .
qgroupid         rfer         excl     max_rfer     max_excl parent  child
--------         ----         ----     --------     -------- ------  -----
0/5          16.00KiB     16.00KiB         none         none ---     --- 
0/257        16.00KiB     16.00KiB         none         none ---     --- 
0/258        16.00KiB     16.00KiB         none     50.00MiB ---     --- 
root@t420:/media/extern/snap# btrfs fi sync .
FSSync '.'
root@t420:/media/extern/snap# btrfs qgroup show -pcre .
qgroupid         rfer         excl     max_rfer     max_excl parent  child
--------         ----         ----     --------     -------- ------  -----
0/5          16.00KiB     16.00KiB         none         none ---     --- 
0/257        16.00KiB     16.00KiB         none         none ---     --- 
0/258        16.00KiB     16.00KiB         none     50.00MiB ---     --- 

By the way, I don't if its relevant but the problem is not limited to
exclusive quotas, but also happens when setting a "referenced" limit
(qgroup limit without "-e").

Thanks,
Johannes