Strange ZFS problem, filesystem claims to be full when clearly not full

[Torbjorn Kristoffersen]

I have a ZFS "tank" called tpool, the server runs a couple of jails (each
with a zfs filesystem).  There is a problem with one of these filesystems.
First, its disk usage as shown in ``df -h'':
...
tpool/rb.org      100G     95G    4.6G    95%    /jails/rb.org
...

The command ``zfs list'' shows the same:
..
tpool/rb.org    95.4G  4.56G  95.4G  /jails/rb.org
..

However, there is a very mysterious problem somewhere.
Something inside this jail is eating diskspace, but we can't find any
directories that is actually taking the diskspace. We first suspected either
fetchmail or spamassassin of causing a lot of space to be used, since some
of their directories were huge. (These were later deleted, and which is why
you see that 4.6GB is now available, before that 0GB was available).

However, we can't find *any trace* of an actual directory or file that is
taking all the spac.e

Take this for instance:

outsidejail# du -sh rb.org
 43G    rb.org

How can this be?  df and zfs are showing that the entire drive is nearly
full, yet I can't find any directory that is actually taking all this space.
 I've carefully looked through every single directory within the jail trying
to find something that's taking all that space, but to no avail.

----
My system stats:
# uname -a
FreeBSD grim 8.1-RELEASE FreeBSD 8.1-RELEASE #0: Mon Jul 19 02:36:49 UTC
2010     root at mason.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  amd64
# zpool get version tpool
NAME   PROPERTY  VALUE    SOURCE
tpool  version   14       default
# zpool status
  pool: tpool
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        tpool       ONLINE       0     0     0
          mirror    ONLINE       0     0     0
            ad4s1d  ONLINE       0     0     0
            ad6s1d  ONLINE       0     0     0

errors: No known data errors

[ Note that I've also done a scrub recently ]
----