Listing Virtual Device I/O Statistics

In addition to pool-wide I/O statistics, the zpool iostat command can display I/O statistics for virtual devices. This command can be used to identify abnormally slow devices or 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
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
rpool       6.05G  61.9G      0      0    785    107
  mirror    6.05G  61.9G      0      0    785    107
    c1t0d0s0    -      -      0      0    578    109
    c1t1d0s0    -      -      0      0    595    109
----------  -----  -----  -----  -----  -----  -----
tank        36.5G  31.5G      4      1   295K   146K
  mirror    36.5G  31.5G    126     45  8.13M  4.01M
    c1t2d0      -      -      0      3   100K   386K
    c1t3d0      -      -      0      3   104K   386K
----------  -----  -----  -----  -----  -----  -----

Note two important points when viewing I/O statistics for virtual devices:

• First, disk space usage statistics are only available for top-level virtual devices. The way in which disk 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, which is not accounted for at the mirror level. Over time, these numbers gradually equalize. However, broken, unresponsive, or offline devices can affect this symmetry as well.

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