Identify the ZFS Snapshot Disk Hog

[Jason Breitman]

What is my best tool or set of command line scripts to find the snapshot or snapshots that are the disk hogs?
 
I am familiar with the scripts
  zfs list -r -o space,refer -t snapshot tank/username

and with the command below to identify the estimated space savings
  zfs destroy -nv tank/username at zfs-auto-snap_monthly-2013-11-01-03h00

When I go through and destroy the snapshots from oldest to the most recent, I do not seem to reclaim any space and am forced to believe there must be a better way.
The users in questions are developers so there is churn causing the snapshots to be larger than an average user which means I will need to create a process I can use on a regular basis.
I am using refquota for each user.


OS: Freebsd 9.1

# zpool upgrade -v
This system is currently running ZFS pool version 28.

The following versions are supported:

VER  DESCRIPTION
---  --------------------------------------------------------
 1   Initial ZFS version
 2   Ditto blocks (replicated metadata)
 3   Hot spares and double parity RAID-Z
 4   zpool history
 5   Compression using the gzip algorithm
 6   bootfs pool property
 7   Separate intent log devices
 8   Delegated administration
 9   refquota and refreservation properties
 10  Cache devices
 11  Improved scrub performance
 12  Snapshot properties
 13  snapused property
 14  passthrough-x aclinherit
 15  user/group space accounting
 16  stmf property support
 17  Triple-parity RAID-Z
 18  Snapshot user holds
 19  Log device removal
 20  Compression using zle (zero-length encoding)
 21  Deduplication
 22  Received properties
 23  Slim ZIL
 24  System attributes
 25  Improved scrub stats
 26  Improved snapshot deletion performance
 27  Improved snapshot creation performance
 28  Multiple vdev replacements

For more information on a particular version, including supported releases,
see the ZFS Administration Guide.

# zfs upgrade -v
The following filesystem versions are supported:

VER  DESCRIPTION
---  --------------------------------------------------------
 1   Initial ZFS filesystem version
 2   Enhanced directory entries
 3   Case insensitive and filesystem user identifier (FUID)
 4   userquota, groupquota properties
 5   System attributes

For more information on a particular version, including supported releases,
see the ZFS Administration Guide.



Jason Breitman
jbreitman at zxcvm.com