More on ZFS filesystem sizes.

[Brooks Davis]

On Wed, Dec 17, 2008 at 04:51:00PM -0500, Zaphod Beeblebrox wrote:
> 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.

Have you tried asking the zfs developers?  I'd tend to assume zfs is
reporting the amount of space it thinks it's using and that as long as
the numbers are close to expected it's not likely to be a FreeBSD issue.

It might well be the case that a given bit of data takes different
amounts of space when stored on different pool types due to needing
different meta data.

-- Brooks
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