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   iostat    ( 1 )

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REPORTS

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 and partitions for which statistics are to be displayed may be entered on the command line. 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. Transfer rates are shown in 1K blocks by default, unless the environment variable POSIXLY_CORRECT is set, in which case 512-byte blocks are used. The report may show the following fields, depending on the flags used (e.g. -x, -s and -k or -m):

Device: This column gives the device (or partition) name as listed in the /dev directory.

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 (kB_read/s, MB_read/s) Indicate the amount of data read from the device expressed in a number of blocks (kilobytes, megabytes) per second. Blocks are equivalent to sectors and therefore have a size of 512 bytes.

Blk_wrtn/s (kB_wrtn/s, MB_wrtn/s) Indicate the amount of data written to the device expressed in a number of blocks (kilobytes, megabytes) per second.

Blk_dscd/s (kB_dscd/s, MB_dscd/s) Indicate the amount of data discarded for the device expressed in a number of blocks (kilobytes, megabytes) per second.

Blk_w+d/s (kB_w+d/s, MB_w+d/s) Indicate the amount of data written to or discarded for the device expressed in a number of blocks (kilobytes, megabytes) per second.

Blk_read (kB_read, MB_read) The total number of blocks (kilobytes, megabytes) read.

Blk_wrtn (kB_wrtn, MB_wrtn) The total number of blocks (kilobytes, megabytes) written.

Blk_dscd (kB_dscd, MB_dscd) The total number of blocks (kilobytes, megabytes) discarded.

Blk_w+d (kB_w+d, MB_w+d) The total number of blocks (kilobytes, megabytes) written or discarded.

r/s The number (after merges) of read requests completed per second for the device.

w/s The number (after merges) of write requests completed per second for the device.

d/s The number (after merges) of discard requests completed per second for the device.

f/s The number (after merges) of flush requests completed per second for the device. This counts flush requests executed by disks. Flush requests are not tracked for partitions. Before being merged, flush operations are counted as writes.

sec/s (kB/s, MB/s) The number of sectors (kilobytes, megabytes) read from, written to or discarded for the device per second.

rsec/s (rkB/s, rMB/s) The number of sectors (kilobytes, megabytes) read from the device per second.

wsec/s (wkB/s, wMB/s) The number of sectors (kilobytes, megabytes) written to the device per second.

dsec/s (dkB/s, dMB/s) The number of sectors (kilobytes, megabytes) discarded for the device per second.

rqm/s The number of I/O requests merged per second that were queued to the device.

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.

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

%rrqm The percentage of read requests merged together before being sent to the device.

%wrqm The percentage of write requests merged together before being sent to the device.

%drqm The percentage of discard requests merged together before being sent to the device.

areq-sz The average size (in kilobytes) of the I/O requests that were issued to the device. Note: In previous versions, this field was known as avgrq-sz and was expressed in sectors.

rareq-sz The average size (in kilobytes) of the read requests that were issued to the device.

wareq-sz The average size (in kilobytes) of the write requests that were issued to the device.

dareq-sz The average size (in kilobytes) of the discard 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.

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

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

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

f_await The average time (in milliseconds) for flush requests issued to the device to be served. The block layer combines flush requests and executes at most one at a time. Thus flush operations could be twice as long: Wait for current flush request, then execute it, then wait for the next one.

aqu-sz The average queue length of the requests that were issued to the device. Note: In previous versions, this field was known as avgqu-sz.

%util Percentage of elapsed 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% for devices serving requests serially. But for devices serving requests in parallel, such as RAID arrays and modern SSDs, this number does not reflect their performance limits.