IOSTAT(1) Linux User's Manual IOSTAT(1)NAMEiostat - Report Central Processing Unit (CPU) statistics and input/out‐
put statistics for devices and partitions.
SYNOPSISiostat [ -c ] [ -d ] [ -h ] [ -N ] [ -k | -m ] [ -t ] [ -V ] [ -x ] [
-z ] [ device [...] | ALL ] [ -p [ device [,...] | ALL ] ] [ interval [
count ] ]
DESCRIPTION
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 sys‐
tems, CPU statistics are calculated system-wide as averages among all
processors. A device header row is displayed followed by a line of sta‐
tistics 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 col‐
lected during the interval since the previous report. The count parame‐
ter 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 parame‐
ter is specified without the count parameter, the iostat command gener‐
ates reports continuously.
REPORTS
The iostat command generates three types of reports, the CPU Utiliza‐
tion report, the Device Utilization report and the Network Filesystem
report.
CPU Utilization Report
The first report generated by the iostat command is the CPU Uti‐
lization Report. For multiprocessor systems, the CPU values are
global averages among all processors. The report has the fol‐
lowing 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 servic‐
ing 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 parti‐
tions 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_COR‐
RECT is set, in which case 512-byte blocks are used. The report
may show the following fields, depending on the flags used:
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 inde‐
terminate 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 there‐
fore 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_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) writ‐
ten.
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 (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.
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.
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 servic‐
ing 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 servic‐
ing 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 servic‐
ing them.
svctm
The average service time (in milliseconds) for I/O
requests that were issued to the device. Warning! Do not
trust this field any more. This field will be removed in
a future sysstat version.
%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%.
OPTIONS-c Display the CPU utilization report.
-d Display the device utilization report.
-h Make the Device Utilization Report easier to read by a human.
-k Display statistics in kilobytes per second.
-m Display statistics in megabytes per second.
-N Display the registered device mapper names for any device mapper
devices. Useful for viewing LVM2 statistics.
-p [ { device [,...] | ALL } ]
The -p option 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.
-t Print the time for each report displayed. The timestamp format
may depend on the value of the S_TIME_FORMAT environment vari‐
able (see below).
-V Print version number then exit.
-x Display extended statistics.
-z Tell iostat to omit output for any devices for which there was
no activity during the sample period.
ENVIRONMENT
The iostat command takes into account the following environment vari‐
ables:
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. The timestamp displayed with option -t will
also be compliant with ISO 8601 format.
POSIXLY_CORRECT
When this variable is set, transfer rates are shown in 512-byte
blocks instead of the default 1K blocks.
EXAMPLESiostat
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 sda sdb 2 6
Display six reports of extended statistics at two second inter‐
vals for devices sda and sdb.
iostat-p sda 2 6
Display six reports at two second intervals for device sda and
all its partitions (sda1, etc.)
BUGS
/proc filesystem must be mounted for iostat to work.
Kernels older than 2.6.x are no longer supported.
The average service time (svctm field) value is meaningless, as I/O
statistics are calculated at block level, and we don't know when the
disk driver starts to process a request. For this reason, this field
will be removed in a future sysstat version.
FILES
/proc/stat contains system statistics.
/proc/uptime contains system uptime.
/proc/diskstats contains disks statistics.
/sys contains statistics for block devices.
/proc/self/mountstats contains statistics for network filesystems.
AUTHOR
Sebastien Godard (sysstat <at> orange.fr)
SEE ALSOsar(1), pidstat(1), mpstat(1), vmstat(8), nfsiostat(1), cifsiostat(1)
http://pagesperso-orange.fr/sebastien.godard/
Linux MAY 2011 IOSTAT(1)
