iostat(1) — Linux manual page
iostat(1) — Linux manual page
https://man7.org/linux/man-pages/man1/iostat.1.html
iostat(1) — Linux manual page NAME | SYNOPSIS | DESCRIPTION | REPORTS | OPTIONS | ENVIRONMENT | EXAMPLES | BUGS | FILES | AUTHOR | SEE ALSO | COLOPHON
IOSTAT(1) Linux User’s Manual IOSTAT(1) NAME top iostat - Report Central Processing Unit (CPU) statistics and input/output statistics for devices and partitions. SYNOPSIS top iostat [ -c ] [ -d ] [ -h ] [ -k | -m ] [ -N ] [ -s ] [ -t ] [ -U ] [ -V ] [ -x ] [ -y ] [ -z ] [ –compact ] [ –dec={ 0 | 1 | 2 } ] [ { -f | +f } directory ] [ -j { ID | LABEL | PATH | UUID | … } ] [ -o JSON ] [ [ -H ] -g group_name ] [ –human ] [ –pretty ] [ -p [ device[,…] | ALL ] ] [ device […] | ALL ] [ interval [ count ] ] DESCRIPTION top 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 parameter, the iostat command
generates reports continuously. REPORTS top
The iostat command generates two types of reports, the CPU
Utilisation report and the Device Utilisation report.
CPU Utilisation Report
The first report generated by the iostat command is the CPU
Utilisation 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 utilisation that occurred
while executing at the user level (application).
%nice Show the percentage of CPU utilisation that occurred
while executing at the user level with nice
priority.
%system
Show the percentage of CPU utilisation 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 Utilisation Report
The second report generated by the iostat command is the
Device Utilisation 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
1024-byte 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 (kibibytes,
mebibytes) 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 (kibibytes,
mebibytes) 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 (kibibytes,
mebibytes) 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
(kibibytes, mebibytes) per second.
Blk_read (kB_read, MB_read)
The total number of blocks (kibibytes, mebibytes)
read.
Blk_wrtn (kB_wrtn, MB_wrtn)
The total number of blocks (kibibytes, mebibytes)
written.
Blk_dscd (kB_dscd, MB_dscd)
The total number of blocks (kibibytes, mebibytes)
discarded.
Blk_w+d (kB_w+d, MB_w+d)
The total number of blocks (kibibytes, mebibytes)
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 (kibibytes, mebibytes) read
from, written to or discarded for the device per
second.
rsec/s (rkB/s, rMB/s)
The number of sectors (kibibytes, mebibytes) read
from the device per second.
wsec/s (wkB/s, wMB/s)
The number of sectors (kibibytes, mebibytes) written
to the device per second.
dsec/s (dkB/s, dMB/s)
The number of sectors (kibibytes, mebibytes)
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 kibibytes) 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 kibibytes) of the read requests
that were issued to the device.
wareq-sz
The average size (in kibibytes) of the write
requests that were issued to the device.
dareq-sz
The average size (in kibibytes) 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 utilisation 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 top
-c Display the CPU utilisation report.
--compact
Don't break the Device Utilisation Report into sub-reports
so that all the metrics get displayed on a single line.
-d Display the device utilisation report.
--dec={ 0 | 1 | 2 }
Specify the number of decimal places to use (0 to 2,
default value is 2).
-f directory
+f directory
Specify an alternative directory for iostat to read devices
statistics. Option -f tells iostat to use only the files
located in the alternative directory, whereas option +f
tells it to use both the standard kernel files and the
files located in the alternative directory to read device
statistics.
directory is a directory containing files with statistics
for devices managed in userspace. It may contain:
- a "diskstats" file whose format is compliant with that
located in "/proc",
- statistics for individual devices contained in files
whose format is compliant with that of files located in
"/sys".
In particular, the following files located in directory may
be used by iostat:
directory/block/device/stat
directory/block/device/partition/stat
partition files must have an entry in directory/dev/block/
directory, e.g.:
directory/dev/block/major:minor -->
../../block/device/partition
-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 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 This option is equivalent to specifying --human --pretty.
--human
Print sizes in human readable format (e.g. 1.0k, 1.2M,
etc.) The units displayed with this option supersede any
other default units (e.g. kibibytes, sectors...)
associated with the metrics.
-j { ID | LABEL | PATH | UUID | ... } [ device [...] | ALL ]
Display persistent device names. Keywords ID, LABEL, etc.
specify the type of the persistent name. These keywords 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 persistent device names are
usually long, option --pretty is implicitly set with this
option.
-k Display statistics in kibibytes per second.
-m Display statistics in mebibytes per second.
-N Display the registered device mapper names for any device
mapper devices. Useful for viewing LVM2 statistics.
-o JSON
Display the statistics in JSON (JavaScript Object Notation)
format. JSON output field order is undefined, and new
fields may be added in the future.
-p [ { device[,...] | ALL } ]
Display 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.
--pretty
Make the Device Utilisation Report easier to read by a
human. The device name will be printed on the right side.
The report may also be broken into sub-reports if there are
many metrics to display (use --compact option to prevent
this).
-s Display a short (narrow) version of the report that should
fit in 80 characters wide screens.
-t Print the time for each report displayed. The timestamp
format may depend on the value of the S_TIME_FORMAT
environment variable (see below) and on whether option -U
has been used.
-U Display timestamp (UTC - Coordinated Universal Time) in
seconds from the epoch.
-V Print version number then exit.
-x Display extended statistics.
-y Omit first report with statistics since system boot, if
displaying 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 top
The iostat command takes into account the following environment
variables:
POSIXLY_CORRECT
When this variable is set, transfer rates are shown in
512-byte blocks instead of the default 1024-byte blocks.
S_COLORS
By default statistics are displayed in colour when the
output is connected to a terminal. Use this variable to
change the settings. Possible values for this variable are
never, always or auto (the latter is equivalent to the
default settings).
Please note that the colour (being red, yellow, or some
other colour) used to display a value is not indicative of
any kind of issue simply because of the colour. It only
indicates different ranges of values.
S_COLORS_SGR
Specify the colours and other attributes used to display
statistics on the terminal. Its value is a colon-separated
list of capabilities that defaults to
I=32;22:N=34;1:W=35;1:X=31;1:Z=34;22. Supported
capabilities are:
I= SGR (Select Graphic Rendition) substring for device
names.
N= SGR substring for non-zero statistics values.
W= (or M=)
SGR substring for percentage values in the range
from 75% to 90% (or in the range 10% to 25%
depending on the metric's meaning).
X= (or H=)
SGR substring for percentage values greater than or
equal to 90% (or lower than or equal to 10%
depending on the metric's meaning).
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 top
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
intervals 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 top
/proc filesystem must be mounted for iostat to work.
Kernels older than 2.6.x are no longer supported.
Although iostat displays units corresponding to kilobytes (kB),
megabytes (MB)..., it actually uses kibibytes (kiB), mebibytes
(MiB)... A kibibyte is equal to 1024 bytes, and a mebibyte is
equal to 1024 kibibytes. FILES top
/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 top
Sebastien Godard (sysstat <at> orange.fr) SEE ALSO top
sar(1), pidstat(1), mpstat(1), vmstat(8), tapestat(1),
nfsiostat(1), cifsiostat(1)
https://github.com/sysstat/sysstat
https://sysstat.github.io/ COLOPHON top
This page is part of the sysstat (sysstat performance monitoring
tools) project. Information about the project can be found at
⟨http://sebastien.godard.pagesperso-orange.fr/⟩. If you have a bug
report for this manual page, send it to sysstat-AT-orange.fr.
This page was obtained from the project's upstream Git repository
⟨https://github.com/sysstat/sysstat.git⟩ on 2026-01-16. (At that
time, the date of the most recent commit that was found in the
repository was 2025-12-07.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
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Linux JULY 2025 IOSTAT(1)