iostat - Report Central Processing Unit (CPU) statistics and input/output statistics for devices and partitions.

iostat [ -c | -d ] [ -k | -m ] [ -t ] [ -V ] [ -x ] [ device [ ... ] | ALL ] [ -p [ device | ALL ] ] [ interval [ count ] ]

The iostat command is used for monitoring system input/output device loading by observing the time the devices are active in relation to their average transfer rates. The iostat command generates reports that can be used to change system configuration to better balance the input/output load between physical disks.

The first report generated by the iostat command provides statistics concerning the time since the system was booted. Each subsequent report covers the time since the previous report. All statistics are reported each time the iostat command is run. The report consists of a CPU header row followed by a row of CPU statistics. On multiprocessor systems, CPU statistics are calculated system-wide as averages among all processors. A device header row is displayed followed by a line of statistics for each device that is configured.

The interval parameter specifies the amount of time in seconds between each report. The first report contains statistics for the time since system startup (boot). Each subsequent report contains statistics collected during the interval since the previous report. The count parameter can be specified in conjunction with the interval parameter. If the count parameter is specified, the value of count determines the number of reports generated at interval seconds apart. If the interval parameter is specified without the count parameter, the iostat command generates reports continuously.

The iostat command generates two types of reports, the CPU Utilization report and the Device Utilization report.

CPU Utilization Report

The first report generated by the iostat command is the CPU Utilization Report. For multiprocessor systems, the CPU values are global averages among all processors. The report has the following format:

%user Show the percentage of CPU utilization that occurred while executing at the user level (application). %nice Show the percentage of CPU utilization that occurred while executing at the user level with nice priority. %system Show the percentage of CPU utilization that occurred while executing at the system level (kernel). %iowait Show the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request. %steal Show the percentage of time spent in involuntary wait by the virtual CPU or CPUs while the hypervisor was servicing another virtual processor. %idle Show the percentage of time that the CPU or CPUs were idle and the system did not have an outstanding disk I/O request.

Device Utilization Report

The second report generated by the iostat command is the Device Utilization Report. The device report provides statistics on a per physical device or partition basis. Block devices for which statistics are to be displayed may be entered on the command line. Partitions may also be entered on the command line providing that option -x is not used. If no device nor partition is entered, then statistics are displayed for every device used by the system, and providing that the kernel maintains statistics for it. If the ALL keyword is given on the command line, then statistics are displayed for every device defined by the system, including those that have never been used. The report may show the following fields, depending on the flags used:

Device: This column gives the device (or partition) name, which is displayed as hdiskn with 2.2 kernels, for the nth device. It is displayed as devm-n with 2.4 kernels, where m is the major number of the device, and n a distinctive number. With newer kernels, the device name as listed in the /dev directory is displayed.

tps Indicate the number of transfers per second that were issued to the device. A transfer is an I/O request to the device. Multiple logical requests can be combined into a single I/O request to the device. A transfer is of indeterminate size.

Blk_read/s Indicate the amount of data read from the device expressed in a number of blocks per second. Blocks are equivalent to sectors with 2.4 kernels and newer and therefore have a size of 512 bytes. With older kernels, a block is of indeterminate size.

Blk_wrtn/s Indicate the amount of data written to the device expressed in a number of blocks per second.

Blk_read The total number of blocks read.

Blk_wrtn The total number of blocks written.

kB_read/s Indicate the amount of data read from the device expressed in kilobytes per second.

kB_wrtn/s Indicate the amount of data written to the device expressed in kilobytes per second.

kB_read The total number of kilobytes read.

kB_wrtn The total number of kilobytes written.

MB_read/s Indicate the amount of data read from the device expressed in megabytes per second.

MB_wrtn/s Indicate the amount of data written to the device expressed in megabytes per second.

MB_read The total number of megabytes read.

MB_wrtn The total number of megabytes written.

rrqm/s The number of read requests merged per second that were queued to the device.

wrqm/s The number of write requests merged per second that were queued to the device.

r/s The number of read requests that were issued to the device per second.

w/s The number of write requests that were issued to the device per second.

rsec/s The number of sectors read from the device per second.

wsec/s The number of sectors written to the device per second.

rkB/s The number of kilobytes read from the device per second.

wkB/s The number of kilobytes written to the device per second.

rMB/s The number of megabytes read from the device per second.

wMB/s The number of megabytes written to the device per second.

avgrq-sz The average size (in sectors) of the requests that were issued to the device.

avgqu-sz The average queue length of the requests that were issued to the device.

await The average time (in milliseconds) for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.

svctm The average service time (in milliseconds) for I/O requests that were issued to the device.

%util Percentage of CPU time during which I/O requests were issued to the device (bandwidth utilization for the device). Device saturation occurs when this value is close to 100%.

-c

The -c option is exclusive of the -d option and displays only the CPU usage report.

-d

The -d option is exclusive of the -c option and displays only the device utilization report.

-k

Display statistics in kilobytes per second instead of blocks per second. Data displayed are valid only with kernels 2.4 and newer.

-m

Display statistics in megabytes per second instead of blocks or kilobytes per second. Data displayed are valid only with kernels 2.4 and newer.

-p [ { device | ALL } ]

The -p option is exclusive of the -x option and displays statistics for block devices and all their partitions that are used by the system. If a device name is entered on the command line, then statistics for it and all its partitions are displayed. Last, the ALL keyword indicates that statistics have to be displayed for all the block devices and partitions defined by the system, including those that have never been used. Note that this option works only with post 2.5 kernels.

-t

Print the time for each report displayed.

-V

Print version number then exit.

-x

Display extended statistics. This option is exclusive of the -p one, and works with post 2.5 kernels since it needs /proc/diskstats file or a mounted sysfs to get the statistics. This option may also work with older kernels (e.g. 2.4) only if extended statistics are available in /proc/partitions (the kernel needs to be patched for that).

The iostat command takes into account the following environment variable:

S_TIME_FORMAT

If this variable exists and its value is ISO then the current locale will be ignored when printing the date in the report header. The iostat command will use the ISO 8601 format (YYYY-MM-DD) instead.

iostat Display a single history since boot report for all CPU and Devices.

iostat -d 2 Display a continuous device report at two second intervals.

iostat -d 2 6 Display six reports at two second intervals for all devices.

iostat -x hda hdb 2 6 Display six reports of extended statistics at two second intervals for devices hda and hdb.

iostat -p sda 2 6 Display six reports at two second intervals for device sda and all its partitions (sda1, etc.)

/proc filesystem must be mounted for iostat to work.

Extended statistics are available only with post 2.5 kernels.

/proc/stat contains system statistics.

/proc/partitions contains disk statistics (for pre 2.5 kernels that have been patched).

/proc/diskstats contains disks statistics (for post 2.5 kernels).

/sys contains statistics for block devices (post 2.5 kernels).

Sebastien Godard (sysstat <at> wanadoo.fr)

sar(1), mpstat(1), vmstat(8)

http://perso.wanadoo.fr/sebastien.godard/