More on ZFS filesystem sizes.

[Zaphod Beeblebrox]

So... I posted before about the widly different sizes reported by zfs list
and du -h for my ports repository.  Nobody explained this to any satisfying
degree.

I now have another quandry.  I have ZFS on my laptop (two drives, mirrored)
and I "zfs send" backups to my big array (6 drives, raid-Z1).  The problem
is that they don't match up:

On the 6 drive array:

vr2/backup/canoe/64/usr at 20080307-1541       746M      -  4.82G  -
vr2/backup/canoe/64/usr at 20080309-1443       221M      -  4.79G  -
vr2/backup/canoe/64/usr at 20080319-1722       334M      -  4.97G  -
vr2/backup/canoe/64/usr at 20080329-0041      27.8M      -  5.24G  -
vr2/backup/canoe/64/usr at 20080402-2300      21.9M      -  5.27G  -
vr2/backup/canoe/64/usr at 20080416-0223      18.5M      -  5.29G  -
vr2/backup/canoe/64/usr at 20080417-0117      18.6M      -  5.29G  -

On the 2 drive laptop:

canoe/64/usr at 20080307-1541             738M      -  4.76G  -
canoe/64/usr at 20080309-1443             217M      -  4.73G  -
canoe/64/usr at 20080319-1722             330M      -  4.90G  -
canoe/64/usr at 20080329-0041            26.7M      -  5.17G  -
canoe/64/usr at 20080402-2300            20.6M      -  5.20G  -
canoe/64/usr at 20080416-0223            17.5M      -  5.22G  -
canoe/64/usr at 20080417-0117            17.5M      -  5.22G  -

... note that the snapshot sizes differ by many megabytes ... and not
seemingly any fixed amount, either.