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The iostat command provides data about the I/O device usage of the system.
To compute this information, seeks, data transfer completions, and the
number of words transferred are counted for each disk.
Iostat varies
significantly between vendors. The command is normally issued with
both an interval and a count specified. The interval specifies the
time interval, in seconds, between records included in the output.
The initial record contains cumulative statistics since system boot.
It is of little value for performance tuning. Succeeding records contain
data for the preceding interval. The count specifies the number of
records to include in the report. The following output is from an
iostat -x for Solaris
extended device statistics
device r/s w/s kr/s kw/s wait actv svc_t %w %b
fd0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0
sd3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0
sd6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0
st4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0
nfs1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0
nfs2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0 0
The main columns of interest are
| wait |
average number of transactions waiting for service (queue length) |
| actv |
average number of transactions actively being serviced (removed from
the queue but not yet completed) |
| svc_t |
average service time, in milliseconds. This is the time that the disk
reports that a request has been completed. If there is RAID in the way then,
for example, the write confirmation will be coming from the NVRAM and not
the real disk. |
| %w |
percent of time there are transactions waiting for service (queue non-empty) |
| %b |
percent of time the disk is busy (transactions in progress) |
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