The iostat utility iteratively reports terminal, disk, and tape I/O activity, as well as CPU utilization. The first line of output is for all time since boot; each subsequent line is for the prior interval only.

To compute this information, the kernel maintains a number of counters. For each disk, the kernel counts reads, writes, bytes read, and bytes written. The kernel also takes hi-res time stamps at queue entry and exit points, which allows it to keep track of the residence time and cumulative residence-length product for each queue. Using these values, iostat produces highly accurate measures of throughput, utilization, queue lengths, transaction rates and service time. For terminals collectively, the kernel simply counts the number of input and output characters.

During execution of this kernel status command, the "state" of the kernel can change. An example would be CPUs going online or offline. iostat reports this as one or more of the following messages:

device_name added 
device_name removed
NFS_filesystem mounted
NFS_filesystem unmounted
cpu[s] taken offline: cpuid
cpu[s] brought online: cpuid

where device_name, NFS_filesystem and cpuid are replaced with the actual name or names of the entities formatted according to other options.

For more general system statistics, use sar(1), sar(1M), or vmstat(1M).

The iostat utility's activity class options default to tdc (terminal, disk, and CPU). If any activity class options are specified, the default is completely overridden. Therefore, if only -d is specified, neither terminal nor CPU statistics will be reported. The last disk option specified (-d, -D, or -x) is the only one that is used.

The following options are supported:

-c

Report the percentage of time the system has spent in user mode, in system mode, waiting for I/O, and idling.

-C

When the -n and -x options are also selected, report extended disk statistics aggregated by controller id.

-d

For each disk, report the number of kilobytes transferred per second, the number of transfers per second, and the average service time in milliseconds.

-D

For each disk, report the reads per second, writes per second, and percentage disk utilization.

-e

Display device error summary statistics. The total errors, hard errors, soft errors, and transport errors are displayed.

-E

Display all device error statistics.

-i

In -E output, display the "Device Id" instead of the "Serial No". The "Device Id" is a unique identifier registered by a driver through ddi_devid_register(9F).

-I

Report the counts in each interval, rather than rates (where applicable).

-l n

Limit the number of disks included in the report to n; the disk limit defaults to 4 for -d and -D, and unlimited for -x. Note: disks explicitly requested (see disk below) are not subject to this disk limit.

-m

Report file system mount points. This option is most useful if the -P or -p option is also specified.

-M

Display data throughput in MB/sec instead of KB/sec.

-n

Display names in descriptive format (for example, cXtYdZ, rmt/N, server:/export/path).

-p

For each disk, report per-partition statistics in addition to per-device statistics.

-P

For each disk, report per-partition statistics only, no per-device statistics.

-r

Display data in a comma-separated format.

-s

Suppress messages related to "state changes."

-t

Report the number of characters read and written to terminals per second.

-T u | d

Display a time stamp.

Specify u for a printed representation of the internal representation of time. See time(2). Specify d for standard date format. See ctime(3C).

-x

For each disk, report extended disk statistics. The output is in tabular form.

-X

For disks under scsi_vhci control, also report statistics in the form of target.controller.

-z

Do not print lines whose underlying data values are all zeros.

The following operands are supported:

disk

Explicitly specify the disks to be reported; in addition to any explicit disks, any active disks up to the disk limit (see -l above) will also be reported.

count

Display only count reports.

interval

Report once each interval seconds.

See attributes(5) for descriptions of the following attributes:

sar(1), sar(1M), vmstat(1M), time(2), ctime(3C), attributes(5), scsi_vhci(7D)

System Administration Guide: Basic Administration

The sum of CPU utilization might vary slightly from 100 because of rounding errors in the production of a percentage figure.