Listing Virtual Device Statistics

In addition to pool-wide I/O statistics, the zpool iostat command can display statistics for virtual devices. This command can be used to identify abnormally slow devices, or simply to observe the distribution of I/O generated by ZFS. To request the complete virtual device layout as well as all I/O statistics, use the zpool iostat -v command. For example:

# zpool iostat -v
               capacity     operations    bandwidth
tank         used  avail   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
mirror      20.4G  59.6G      0     22      0  6.00K
  c1t0d0        -      -      1    295  11.2K   148K
  c1t1d0        -      -      1    299  11.2K   148K
----------  -----  -----  -----  -----  -----  -----
total       24.5K   149M      0     22      0  6.00K

Note two important things when viewing I/O statistics on a virtual device basis:

• First, space usage is only available for top-level virtual devices. The way in which space is allocated among mirror and RAID-Z virtual devices is particular to the implementation and not easily expressed as a single number.

• Second, the numbers might not add up exactly as you would expect them to. In particular, operations across RAID-Z and mirrored devices will not be exactly equal. This difference is particularly noticeable immediately after a pool is created, as a significant amount of I/O is done directly to the disks as part of pool creation that is not accounted for at the mirror level. Over time, these numbers should gradually equalize, although broken, unresponsive, or offlined devices can affect this symmetry as well.

You can use the same set of options (interval and count) when examining virtual device statistics.