iostat - Report Central Processing Unit (CPU) statistics and
input/output statistics for devices and partitions.
iostat [ -c ] [ -d ] [ -h ] [ -k | -m ] [ -N ] [ -s ] [ -t ] [
-V ] [ -x ] [ -y ] [ -z ] [ --dec={ 0 | 1 | 2 } ] [ -j { ID | LABEL |
PATH | UUID | ... } ] [ -o JSON ] [ [ -H ] -g group_name
] [ --human ] [ -p [ device [,...] | ALL ] ] [
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, 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.
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:
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_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.
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.
sec/s (kB/s, MB/s)
The number of sectors (kilobytes, megabytes) read from or
written to 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.
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.
%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.
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.
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.
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.
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.
- -c
- Display the CPU utilization report.
- -d
- Display the device utilization report.
- --dec={ 0 | 1 | 2 }
- Specify the number of decimal places to use (0 to 2, default value is
2).
- -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
- Make the Device Utilization Report easier to read by a human.
--human is enabled implicitly with this option.
- --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.
kilobytes, sectors...) associated with the metrics.
- -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 persistent device names are
usually long, 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.
- -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 }
]
- 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.
- -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).
- -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.
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 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).
Note: On Debian sysstems the colors are displayed by default
when output is connected to the terminal, even if this variable is not
set (i.e. unset variable is treated as if it were set to
auto).
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 statistics on the
terminal. Its value is a colon-separated list of capabilities that
defaults to H=31;1:I=32;22:M=35;1:N=34;1:Z=34;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.
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.)
/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.
/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.
Sebastien Godard (sysstat <at> orange.fr)