The -c option of btrfs qgroup limit

[Koen De Wit <koen.de.wit@oracle.com>]

All,

The "btrfs qgroup limit" command has an option -c which means "limit amount
of data after compression". Whether this option is specified or not, I seem
to be able to write more well-compressible data than the specified limit. I
was expecting that omitting the -c option would never allow me to write
more data than specified by the limit. Am I missing something? Which
difference in behavior should I expect from the -c option?

Koen.

---

This is what I did:

Mount a btrfs filesystem with the compress option and turn on quota:
   # mkfs.btrfs -f /dev/sdg2
   # mount -o compress /dev/sdg2 /mnt
   # cd /mnt
   # btrfs quota enable ./

Create a subvol and limit the amount of data after compression. Write some
well-compressible files:
   # btrfs subvol create subvol1
   # btrfs qgroup limit -c 3m ./subvol1
   # for i in `seq 1 5`; do
         dd if=/dev/zero of=subvol1/file$i bs=1024 count=1000;
         sync;
     done
   (no errors)
   # du -s subvol1
   5000 subvol1
We can write 5 MB of data to a file that is limited to 3 MB, which is 
normal
in this case because data from /dev/zero is very good compressible.

Create a subvol and limit the amount of data after compression. Write some
poorly compressible files:
   # btrfs subvol create subvol2
   # btrfs qgroup limit -c 3m ./subvol2
   # for i in `seq 1 5`; do
         dd if=/dev/urandom of=subvol2/file$i bs=1024 count=1000;
         sync;
     done
   dd: writing `subvol2/file4': Disk quota exceeded
   dd: opening `subvol2/file5': Disk quota exceeded
   # du -s subvol2
   3056 subvol2
Here we get quota violations, because data from /dev/urandom is poorly
compressible.

Now write some well-compressible data to a subvol that is limited without
the -c option:
   # btrfs subvol create subvol3
   # btrfs qgroup limit 3m ./subvol3
   # for i in `seq 1 5`; do
         dd if=/dev/zero of=subvol3/file$i bs=1024 count=1000;
         sync;
   done
   (no errors)
   # du -s subvol3
   5000 subvol3
We're still able to write 5 MB of data to a subvol that is limited to 3 MB.