iostat Man page

IOSTAT(1) Linux User’s Manual IOSTAT(1)

NAME

iostat – Report Central Processing Unit (CPU) statistics and input/out‐
put statistics for devices and partitions.

SYNOPSIS

iostat [ -c ] [ -d ] [ -h ] [ -k | -m ] [ -N ] [ -t ] [ -V ] [ -x ] [
-y ] [ -z ] [ -j { ID | LABEL | PATH | UUID | … } ] [ [ -H ] -g
group_name ] [ -p [ device [,…] | ALL ] ] [ 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, unless the -y option
is used (in this case, this first report is omitted). 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 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 parame‐
ter, the iostat command generates reports continuously.

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 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 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.

OPTIONS

-c Display the CPU utilization report.

-d Display the device utilization report.

-g group_name { device […] | ALL }
Display statistics for a group of devices. The iostat command
reports statistics for each individual device in the list then a
line of global statistics for the group displayed as group_name
and made up of all the devices in the list. The ALL keyword
means that all the block devices defined by the system shall be
included in the group.

-h Make the Device Utilization Report easier to read by a human.

-j { ID | LABEL | PATH | UUID | … } [ device […] | ALL ] Display persistent device names. Options ID, LABEL, etc. specify
the type of the persistent name. These options are not limited,
only prerequisite is that directory with required persistent
names is present in /dev/disk. Optionally, multiple devices can
be specified in the chosen persistent name type. Because per‐
sistent device names are usually long, option

-H This option must be used with option -g and indicates that only
global statistics for the group are to be displayed, and not
statistics for individual devices in the group. -h is enabled
implicitly with this option.

-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. If option -j is defined before this option,
devices entered on the command line can be specified with the
chosen persistent name type.

-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.

-y Omit first report with statistics since system boot, if display‐
ing multiple records at given interval.

-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:

POSIXLY_CORRECT
When this variable is set, transfer rates are shown in 512-byte
blocks instead of the default 1K blocks.

S_COLORS
When this variable is set, display statistics in color on the
terminal. Possible values for this variable are never, always
or auto (the latter is the default).

Please note that the color (being red, yellow, or some other
color) used to display a value is not indicative of any kind of
issue simply because of the color. It only indicates different
ranges of values.

S_COLORS_SGR
Specify the colors and other attributes used to display statis‐
tics on the terminal. Its value is a colon-separated list of
capabilities that defaults to
H=31;1:I=32;22:M=34;1:N=33;1:Z=33;22. Supported capabilities
are:

H= SGR (Select Graphic Rendition) substring for percentage
values greater than or equal to 75%.

I= SGR substring for device names.

M= SGR substring for percentage values in the range from 50%
to 75%.

N= SGR substring for non-zero statistics values.

Z= SGR substring for zero values.

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.

EXAMPLES
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 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 now 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.

/dev/disk contains persistent device names.

AUTHOR

Sebastien Godard (sysstat orange.fr)

SEE ALSO

sar, pidstat, mpstat, vmstat(8), tapestat, nfsiostat,
cifsiostat

http://pagesperso-orange.fr/sebastien.godard/

Linux OCTOBER 2015 IOSTAT(1)

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