Until recently, I've been confused and frustrated by the zfs list output as I try to clear up space on my hard drive.

Take this example using a 1 GB zpool:

bleonard@os200906:~# mkfile 1G /dev/dsk/disk1

bleonard@os200906:~# zpool create tank disk1

bleonard@os200906:~# zpool list tank

NAME   SIZE   USED  AVAIL    CAP  HEALTH  ALTROOT

tank  1016M    73K  1016M     0%  ONLINE  -

Now let's create some files and snapshots:

bleonard@os200906:~# mkfile 100M /tank/file1

bleonard@os200906:~# zfs snapshot tank@snap1

bleonard@os200906:~# mkfile 100M /tank/file2

bleonard@os200906:~# zfs snapshot tank@snap2

bleonard@os200906:~# zfs list -t all -r tank

NAME         USED  AVAIL  REFER  MOUNTPOINT

tank         200M   784M   200M  /tank

tank@snap1    17K      -   100M  -

tank@snap2      0      -   200M  -

The output here looks as I'd expect. I have used 200 MB of disk space, neither of which is used by the snapshots. snap1 refers to 100 MBs of data (file1) and snap2 refers to 200 MBs of data (file1 and file2).

Now let's delete file1 and look at our zfs list output again:

bleonard@os200906:~# rm /tank/file1

bleonard@os200906:~# zpool scrub tank

bleonard@os200906:~# zfs list -t all -r tank

NAME         USED  AVAIL  REFER  MOUNTPOINT

tank         200M   784M   100M  /tank

tank@snap1    17K      -   100M  -

tank@snap2    17K      -   200M  -

Only 1 thing has changed - tank now only refers to 100 MB of data. file1 has been deleted and is only referenced by the snapshots. So why don't the snapshots reflect this in their USED column? You may think we should show 100 MB used by snap1, however, this would be misleading as deleting snap1 has no effect on the data used by the tank file system. Deleting snap1 would only free up 17K of disk space. We'll come back to this test case in a moment.

There is an option to get more detail on the space consumed by the snapshots. Although you can pretty easily deduct from the example above that the snapshots are using 100 MB, by using the zfs space option you can save yourself from doing the math:

bleonard@os200906:~# zfs list -t all -o space -r tank

NAME        AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD

tank         784M   200M      100M    100M              0       154K

tank@snap1      -    17K         -       -              -          -

tank@snap2      -    17K         -       -              -          -

Here we can clearly see that of the 200 MB used by our file system, 100 MB is used by snapshots (file1) and 100 MB is used by the dataset itself (file2). Of course, there are other factors that can affect the total amount used - see the zfs man page for details.

Now, if we were to delete snap1 (we know this is safe, because it's not using any space):

bleonard@os200906:~# zfs destroy tank@snap1

bleonard@os200906:~# zfs list -t all -r tank

NAME         USED  AVAIL  REFER  MOUNTPOINT

tank         200M   784M   100M  /tank

tank@snap2   100M      -   200M  -

We can see that snap2 now shows 100 MBs used. If I were to delete snap2, I would be deleting 100 MB of data (or reclaiming 100 MB of space):

Now let's look at a more realistic example - my home directory where I have Time Slider running:

bleonard@opensolaris:~$ zfs list -t all -r -o space rpool/export/home

NAME                                                       AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD

rpool/export/home                                          25.4G  35.2G     17.9G   17.3G              0          0

rpool/export/home@zfs-auto-snap:monthly-2010-08-03-09:30       -   166M         -       -              -          -

rpool/export/home@zfs-backup-2010-08-12-15:30                  -  5.06M         -       -              -          -

rpool/export/home@zfs-backup-2010-08-12-15:56                  -  5.15M         -       -              -          -

rpool/export/home@zfs-backup-2010-08-31-14:12                  -  54.6M         -       -              -          -

rpool/export/home@zfs-auto-snap:monthly-2010-09-01-00:00       -  53.8M         -       -              -          -

rpool/export/home@zfs-auto-snap:weekly-2010-09-08-00:00        -  95.8M         -       -              -          -

rpool/export/home@zfs-backup-2010-09-09-09:04                  -  53.9M         -       -              -          -

rpool/export/home@zfs-auto-snap:weekly-2010-09-15-00:00        -  2.06G         -       -              -          -

rpool/export/home@zfs-auto-snap:weekly-2010-09-22-00:00        -  89.7M         -       -              -          -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:00      -  18.3M         -       -              -          -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:15      -   293K         -       -              -          -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:30      -   293K         -       -              -          -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:45      -  1.18M         -       -              -          -

My snapshots are consuming almost 18 GBs of space. However, it would appear that I could only reclaim about 2.5 GBs of space by deleting all of my snapshots. In reality, 15.5 GBs of space is referenced by 2 or more snapshots.

I can get a better idea of which snapshots might reclaim the most space by removing the space option so I get the REFER field in the output:

bleonard@opensolaris:~$ zfs list -t all -r rpool/export/home

NAME                                                        USED  AVAIL  REFER  MOUNTPOINT

rpool/export/home                                          35.2G  25.4G  17.3G  /export/home

rpool/export/home@zfs-auto-snap:monthly-2010-08-03-09:30    166M      -  15.5G  -

rpool/export/home@zfs-backup-2010-08-12-15:30              5.06M      -  28.5G  -

rpool/export/home@zfs-backup-2010-08-12-15:56              5.15M      -  28.5G  -

rpool/export/home@zfs-backup-2010-08-31-14:12              54.6M      -  15.5G  -

rpool/export/home@zfs-auto-snap:monthly-2010-09-01-00:00   53.8M      -  15.5G  -

rpool/export/home@zfs-auto-snap:weekly-2010-09-08-00:00    95.8M      -  15.5G  -

rpool/export/home@zfs-backup-2010-09-09-09:04              53.9M      -  17.4G  -

rpool/export/home@zfs-auto-snap:weekly-2010-09-15-00:00    2.06G      -  19.4G  -

rpool/export/home@zfs-auto-snap:weekly-2010-09-22-00:00    89.7M      -  15.5G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:15   293K      -  17.3G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:30   293K      -  17.3G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:45  1.18M      -  17.3G  -

rpool/export/home@zfs-auto-snap:hourly-2010-09-28-12:00        0      -  17.3G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-12:00      0      -  17.3G  -

In the above output, I can see that 2 snapshots, taken 26 seconds apart, are referring to 28.5 GBs of disk space. If I were to delete one of those snapshots and check the zfs list output again:

bleonard@opensolaris:~$ pfexec zfs destroy rpool/export/home@zfs-backup-2010-08-12-15:30

bleonard@opensolaris:~$ zfs list -t all -r rpool/export/home

NAME                                                        USED  AVAIL  REFER  MOUNTPOINT

rpool/export/home                                          35.2G  25.4G  17.3G  /export/home

rpool/export/home@zfs-auto-snap:monthly-2010-08-03-09:30    166M      -  15.5G  -

rpool/export/home@zfs-backup-2010-08-12-15:56              12.5G      -  28.5G  -

rpool/export/home@zfs-backup-2010-08-31-14:12              54.6M      -  15.5G  -

rpool/export/home@zfs-auto-snap:monthly-2010-09-01-00:00   53.8M      -  15.5G  -

rpool/export/home@zfs-auto-snap:weekly-2010-09-08-00:00    95.8M      -  15.5G  -

rpool/export/home@zfs-backup-2010-09-09-09:04              53.9M      -  17.4G  -

rpool/export/home@zfs-auto-snap:weekly-2010-09-15-00:00    2.06G      -  19.4G  -

rpool/export/home@zfs-auto-snap:weekly-2010-09-22-00:00    89.7M      -  15.5G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:15   293K      -  17.3G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:30   293K      -  17.3G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-11:45  1.18M      -  17.3G  -

rpool/export/home@zfs-auto-snap:frequent-2010-09-28-12:00   537K      -  17.3G  -

I can now clearly see that the remaining snapshot is using 12.5 GBs of space and deleting this snapshot would reclaim much needed space on my laptop:

bleonard@opensolaris:~$ zpool list rpool

NAME    SIZE   USED  AVAIL    CAP  HEALTH  ALTROOT

rpool   149G   120G  28.5G    80%  ONLINE  -

bleonard@opensolaris:~$ pfexec zfs destroy rpool/export/home@zfs-backup-2010-08-12-15:56

bleonard@opensolaris:~$ zpool list rpool

NAME    SIZE   USED  AVAIL    CAP  HEALTH  ALTROOT

rpool   149G   108G  41.0G    72%  ONLINE  -

And that should be enough to keep Time Slider humming along smoothly and prevent the warning dialog from appearing (lucky you if you haven't seen that yet).