Resolving ZFS Space Issues

Review the following sections if you are unsure how ZFS reports file system and pool space accounting. Also review ZFS Disk Space Accounting.

ZFS File System Space Reporting

The zpool list and zfs list commands are better than the previous df and du commands for determining your available pool and file system space. With the legacy commands, you cannot easily discern between pool and file system space, nor do the legacy commands account for space that is consumed by descendent file systems or snapshots.

For example, the following root pool (rpool) has 5.46 GB allocated and 68.5 GB free.

# zpool list rpool
NAME   SIZE  ALLOC   FREE  CAP  DEDUP  HEALTH  ALTROOT
rpool   74G  5.46G  68.5G   7%  1.00x  ONLINE  -

If you compare the pool space accounting with the file system space accounting by reviewing the USED column of your individual file systems, you can see that the pool space that is reported in ALLOC is accounted for in the file systems' USED total. For example:

# zfs list -r rpool
NAME                      USED  AVAIL  REFER  MOUNTPOINT
rpool                    5.41G  67.4G  74.5K  /rpool
rpool/ROOT               3.37G  67.4G    31K  legacy
rpool/ROOT/solaris       3.37G  67.4G  3.07G  /
rpool/ROOT/solaris/var    302M  67.4G   214M  /var
rpool/dump               1.01G  67.5G  1000M  -
rpool/export             97.5K  67.4G    32K  /rpool/export
rpool/export/home        65.5K  67.4G    32K  /rpool/export/home
rpool/export/home/admin  33.5K  67.4G  33.5K  /rpool/export/home/admin
rpool/swap               1.03G  67.5G  1.00G  -

ZFS Storage Pool Space Reporting

The SIZE value that is reported by the zpool list command is generally the amount of physical disk space in the pool, but varies depending on the pool's redundancy level. See the examples below. The zfs list command lists the usable space that is available to file systems, which is disk space minus ZFS pool redundancy metadata overhead, if any.

• Non-redundant storage pool – When a pool is created with one 136-GB disk, the zpool list command reports SIZE and initial FREE values as 136 GB. The initial AVAIL space reported by the zfs list command is 134 GB, due to a small amount of pool metadata overhead. For example:

  # zpool create tank c0t6d0
  # zpool list tank
  NAME   SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
  tank   136G  95.5K   136G     0%  1.00x  ONLINE  -
  # zfs list tank
  NAME   USED  AVAIL  REFER  MOUNTPOINT
  tank    72K   134G    21K  /tank

• Mirrored storage pool– When a pool is created with two 136-GB disks, zpool list command reports SIZE as 136 GB and initial FREE value as 136 GB. This reporting is referred to as the deflated space value. The initial AVAIL space reported by the zfs list command is 134 GB, due to a small amount of pool metadata overhead. For example:

  # zpool create tank mirror c0t6d0 c0t7d0   
  # zpool list tank
  NAME   SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
  tank   136G  95.5K   136G     0%  1.00x  ONLINE  -
  # zfs list tank
  NAME   USED  AVAIL  REFER  MOUNTPOINT
  tank    72K   134G    21K  /tank

• RAID-Z storage pool – When a raidz2 pool is created with three 136-GB disks, the zpool list commands reports SIZE as 408 GB and initial FREE value as 408 GB. This reporting is referred to as the inflated disk space value, which includes redundancy overhead, such as parity information. The initial AVAIL space reported by the zfs list command is 133 GB, due to the pool redundancy overhead. The space discrepancy between the zpool list and the zfs list output for a RAID-Z pool is because zpool list reports the inflated pool space.

  # zpool create tank raidz2 c0t6d0 c0t7d0 c0t8d0
  # zpool list tank
  NAME   SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
  tank   408G   286K   408G     0%  1.00x  ONLINE  -
  # zfs list tank
  NAME   USED  AVAIL  REFER  MOUNTPOINT
  tank  73.2K   133G  20.9K  /tank