On 2017年09月17日 22:45, Antoine Belvire wrote:
Hello,
The behavior of 'cp --reflink' puzzles me from the quota point of view.
It looks like it's possible to easily exceeds a qgroup's max_rfer limit
like this:
~# # Create '/test' and '/test/a' subvolumes
~# btrfs subvolume create /test
~# btrfs subvolume create /test/a
~#
~# # Set limits
~# btrfs qgroup limit 10m (qgroupid) /
~# btrfs qgroup limit -e 5m (qgroupid) /
~#
~# # Limits are set
~# btrfs qgroup show -erF /test/a/
qgroupid rfer excl max_rfer max_excl
-------- ---- ---- -------- --------
(the id) 16.00KiB 16.00KiB 10.00MiB 5.00MiB
~#
~# # Write a big file in '/test'
~# dd if=/dev/urandom of=/test/bigfile bs=1M count=50
~#
~# # Copy it to '/test/a' using cp --reflink
~# cp --reflink /test/bigfile /test/a
~#
~# # No EDQUOT error, quota limit is exceeded
~# btrfs qgroup show --sync -erF /test/a/
qgroupid rfer excl max_rfer max_excl
-------- ---- ---- -------- --------
(the id) 50.02MiB 16.00KiB 10.00MiB 5.00MiB
~#
However, now the limit is exceeded, EDQUOT is raised upon a new 'cp
--reflink':
~# touch /test/nothing
~# cp --reflink /test/nothing /test/a
cp: cannot create regular file 'a/nothing': Disk quota exceeded
~#
As far as I understand the code, btrfs_clone doesn't try to reserve data
it clones. It only reserves metadata through btrfs_start_transaction.
That's a bug, and should be fixed.
But it's a little complicated to fix.
The biggest problem is, until reflink is done and we commit a
transaction, we don't know if the the reflink operation will increase
"rfer" or not.
(Considering a case where both reflink source and destination is inside
the same subvolume)
Yes, we can always try to reserve the size of the data, and cause false
alert, but the handling of reserved data range will be another problem.
I'll check if we can solve it.
Thanks,
Qu
Is it an expected behavior?
Regards,
--
Antoine
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