zfs destroy snapshot doesn't free space

[Andreas Mayer]

Hi,

I have a problem with my ZFS storage: some application filled a
certain directory in /var completely up with data and the server runs
a script which takes a snapshot every night. So, ~650 GB of the
available 700 GB were filled up.

Then I destroyed the last two snapshots (each referencing about 300
GB) and the files on the live system so that now there are no
snapshots for
/var and "du -hs /var" reports a size of 2 GB.

However, zfs still reports that 623G are referenced:

# zfs list rpool/var
NAME        USED  AVAIL  REFER  MOUNTPOINT
rpool/var   623G  48.4G   623G  /var

# zfs get all rpool/var
...
rpool/var  usedbysnapshots       0                      -
rpool/var  usedbydataset         623G                   -
rpool/var  usedbychildren        0                      -
rpool/var  usedbyrefreservation  0                      -
...

If I take a snapshot again, this snapshot also references 623G.

What can I do to reclaim this space? I have to do this before I can
set a quota (I have set quotas for all other file systems now :) ).

-- 
Best regards,
Andreas