Efficiency of ZFS ZVOLs.

[Adam Nowacki]

On 2013-07-14 09:55, Zaphod Beeblebrox wrote:
> [1:3:303]root at virtual:~> zfs list -rt all vr2/Steam
> NAME                      USED  AVAIL  REFER  MOUNTPOINT
> vr2/Steam                3.25T  9.27T  1.18T  -
> vr2/Steam at 20130528-0029   255M      -  1.18T  -
> vr2/Steam at 20130529-0221   172M      -  1.18T  -
>
> vr2/Steam is a ZVOL exported by iSCSI to my desktop and it contains an NTFS
> filesystem which is mounted into C:\Program Files (x86)\Steam.  Windows
> sees this drive as a 1.99T drive of which 1.02T is used.
>
> Now... the value of "REFER" seems quite right: 1.18T vs. 1.02T is pretty
> good... but the value of "USED" seems _way_ out.  3.25T ... even regarding
> that more of the disk might have been "touched" (ie: used from the ZVOL's
> impression) than is used, it seems too large.  Neither is it 1.18T + 255M +
> 172M.

This is how much space would be required to store the snapshots plus 2TB 
volume with no shared blocks between any of the snapshots. 1.18T from 
snapshots + 2T reservation = 3.18T, just about the 3.25T displayed. You 
can remove the reservation with 'zfs set refreservation=none vr2/Steam'.