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?
Thanks,
Johannes
System info:
Linux t420 4.3.0-rc5 #1 SMP Tue Oct 13 13:21:02 CEST 2015 x86_64
GNU/Linux
Label: none uuid: 9551e3ca-1608-469c-9d8c-77b99ce0e8ec
Total devices 1 FS bytes used 816.00KiB
devid 1 size 931.51GiB used 2.04GiB path /dev/sdb1
btrfs-progs v4.1.2
Data, single: total=8.00MiB, used=256.00KiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=1.00GiB, used=544.00KiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B
[249174.151820] sdb: sdb1
[249184.387377] sdb: sdb1
[249184.573096] sdb: sdb1
[249184.656274] BTRFS: device fsid
9551e3ca-1608-469c-9d8c-77b99ce0e8ec devid 1 transid 3 /dev/sdb1
[249186.323915] sdb: sdb1
[249186.534505] sdb: sdb1
[249186.538420] sdb: sdb1
[249196.781978] BTRFS info (device sdb1): disk space caching is enabled
[249196.781986] BTRFS: has skinny extents
[249196.781990] BTRFS: flagging fs with big metadata feature
[249196.818164] BTRFS: creating UUID tree
[249202.311983] BTRFS info (device sdb1): qgroup scan completed
(inconsistency flag cleared)
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