I have set up a btrfs volume with subvolumes and qgroups on those subvolumes.
Currently I'm getting the "Disk quota exceeded" error but according to qgroup show I have only used about 60% of the total quota. There is also still enough free space on the volume:

# mount
/dev/sdb on /var/log/squid type btrfs (rw,relatime,space_cache,subvolid=257,subvol=/logging)
/dev/sdb on /srv/www/squid type btrfs (rw,relatime,space_cache,subvolid=258,subvol=/statistics)
#df -h
/dev/sdb        150G   85G   65G  57% /var/log/squid
/dev/sdb        150G   85G   65G  57% /srv/www/squid
# btrfs subvolume show /srv/www/squid
        Name:                   statistics
        uuid:                   8ddfb9aa-0818-ea45-a37e-e5b659987569
        Parent uuid:            -
        Creation time:          2015-11-13 12:46:45
        Object ID:              258
        Generation (Gen):       9842
        Gen at creation:        7
        Parent:                 5
        Top Level:              5
        Flags:                  -
Data is set to 'single' instead of 'DUP':

# btrfs fi df /srv/www/squid
Data, single: total=80.00GiB, used=79.39GiB
System, DUP: total=32.00MiB, used=16.00KiB
Metadata, DUP: total=3.00GiB, used=2.30GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
# btrfs qgroup show -prce  /srv/www/squid
qgroupid         rfer         excl     max_rfer     max_excl parent  child 
--------         ----         ----     --------     -------- ------  ----- 
0/5          16.00KiB     16.00KiB        0.00B        0.00B ---     ---  
0/257         8.14GiB      8.14GiB     25.00GiB        0.00B ---     ---  
0/258        73.22GiB     73.22GiB    150.00GiB        0.00B ---     ---
# touch /srv/www/squid/tt
touch: cannot touch /srv/www/squid/tt: Disk quota exceeded
Balancing the btrfs subvolume also doesn't help..

I know the quota set for /srv/www/squid is currently equal to the total disk space. At first the quota was set to 100Gb but then I increased it to 150Gb for testing purposes because I was already bumping the 100Gb quota limit when using only about 60Gb actual data. And as you can see above, the other subvolume on that volume only uses 8Gb of data, so I should bump into a "no space left on device error" before i would be able to bump onto the quota limit currently.

I do have to note that this filesystem currently contains 4,209,707 small files (in several subdirectories) but I don't see how that could be a reason to "lose" 70Gb of free space ?

What is happening here ? Is this a bug in the quota system ?