Disk Space Accounting for ZFS Snapshots

When you create a snapshot, its disk space is initially shared between the snapshot and the file system, and possibly with previous snapshots. As the file system changes, disk space that was previously shared becomes unique to the snapshot, and thus is counted in the snapshot's used property. Additionally, deleting snapshots can increase the amount of disk space unique to (and thus used by) other snapshots.

A snapshot's space referenced property value is the same as the file system's value was when the snapshot was created.

You can identify additional information about how the values of the used property are consumed. New read-only file system properties describe disk space usage for clones, file systems, and volumes. For example:

$ zfs list -o space -r rpool
NAME                      AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
rpool                      124G  9.57G         0    302K              0      9.57G
rpool/ROOT                 124G  3.38G         0     31K              0      3.38G
rpool/ROOT/solaris         124G  20.5K         0       0              0      20.5K
rpool/ROOT/solaris/var     124G  20.5K         0   20.5K              0          0
rpool/ROOT/solaris-1       124G  3.38G     66.3M   3.14G              0       184M
rpool/ROOT/solaris-1/var   124G   184M     49.9M    134M              0          0
rpool/VARSHARE             124G  39.5K         0   39.5K              0          0
rpool/dump                 124G  4.12G         0   4.00G           129M          0
rpool/export               124G    63K         0     32K              0        31K
rpool/export/home          124G    31K         0     31K              0          0
rpool/swap                 124G  2.06G         0   2.00G          64.7M          0

For a description of these properties, see the used properties in the zfs(8) man page.