ZFS pool/filesystem space oddity

	Hi,

	My NAS runs a ZFS pool called archive striped over two mirrored
pairs of 4TB drives. It all works fine but I had a space eater recently and
so have been paying closer attention to space reports than normal - which
is why I noticed the oddity that the filesystems in the pool are reported
as having more free space than the pool.

	Why is this ? Which is right ?

From zpool iostat -v
                     capacity     operations     bandwidth 
pool               alloc   free   read  write   read  write
-----------------  -----  -----  -----  -----  -----  -----
archive            4.38T  2.87T     59     52   880K  1.14M

One of the filesystems in the archive pool:

✓ steve@holdall ~ $ df -H /data
Filesystem      Size    Used   Avail Capacity  Mounted on
archive/data    7.0T    4.0T    3.0T    57%    /data

-- 
Steve O'Hara-Smith <steve@sohara.org>