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iostat
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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 ] [ -k ] [ -t ] [ -V ] [ -x ] [ { 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
statistics 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 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.
%sys
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.
%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 for which
statistics are to be displayed may be entered on the command
line. Partitions may also be entered on the command line provid-
ing that option -x is not used. 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. The report may show
the following fields, depending on the flags used:
Device:
This column gives the device (or partition) name, which
is displayed as hdiskn with 2.2 kernels, for the nth
device. It is displayed as devm-n with 2.4 kernels, where
m is the major number of the device, and n a distinctive
number. With newer kernels, the device name as listed in
the /dev directory is displayed.
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
Indicate the amount of data read from the drive expressed
in a number of blocks per second. Blocks are equivalent
to sectors with 2.4 kernels and newer and therefore have
a size of 512 bytes. With older kernels, a block is of
indeterminate size.
Blk_wrtn/s
Indicate the amount of data written to the drive
expressed in a number of blocks per second.
Blk_read
The total number of blocks read.
Blk_wrtn
The total number of blocks written.
kB_read/s
Indicate the amount of data read from the drive expressed
in kilobytes per second. Data displayed are valid only
with kernels 2.4 and newer.
kB_wrtn/s
Indicate the amount of data written to the drive
expressed in kilobytes per second. Data displayed are
valid only with kernels 2.4 and newer.
kB_read
The total number of kilobytes read. Data displayed are
valid only with kernels 2.4 and newer.
kB_wrtn
The total number of kilobytes written. Data displayed are
valid only with kernels 2.4 and newer.
rrqm/s
The number of read requests merged per second that were
issued to the device.
wrqm/s
The number of write requests merged per second that were
issued to the device.
r/s
The number of read requests that were issued to the
device per second.
w/s
The number of write requests that were issued to the
device per second.
rsec/s
The number of sectors read from the device per second.
wsec/s
The number of sectors written to the device per second.
rkB/s
The number of kilobytes read from the device per second.
wkB/s
The number of kilobytes 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.
svctm
The average service time (in milliseconds) for I/O
requests that were issued to the device.
%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 The -c option is exclusive of the -d option and displays only
the CPU usage report.
-d The -d option is exclusive of the -c option and displays only
the device utilization report.
-k Display statistics in kilobytes per second instead of blocks per
second. Data displayed are valid only with kernels 2.4 and
newer.
-p device | ALL
The -p option is exclusive of the -x option and displays statis-
tics 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 dis-
played for all the block devices and partitions defined by the
system, including those that have never been used. Note that
this option works only with post 2.5 kernels.
-t Print the time for each report displayed.
-V Print version number and usage then exit.
-x Display extended statistics. This option is exclusive of the -p
one, and works with post 2.5 kernels since it needs
/proc/diskstats file or a mounted sysfs to get the statistics.
This option may also work with older kernels (e.g. 2.4) only if
extended statistics are available in /proc/partitions (the ker-
nel needs to be patched for that).
ENVIRONMENT
The iostat command takes into account the following environment vari-
able:
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.
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 hda hdb 2 6
Display six reports of extended statistics at two second inter-
vals for devices hda and hdb.
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.
Extended statistics are available only with post 2.5 kernels.
Because of what seems to be a Linux kernel bug, iostat -x may display
huge I/O response times (svctm) and a bandwidth utilization (%util) of
100% for some devices. Indeed these devices have a value for the field
#9 in /proc/{partitions,diskstats} which is always different from 0,
and even negative sometimes. Yet this field should go to zero, since it
gives the number of I/Os currently in progress (it is incremented as
requests are submitted, and decremented as they finish).
FILES
/proc/stat contains system statistics.
/proc/partitions contains disk statistics (for pre 2.5 kernels that
have been patched).
/proc/diskstats contains disks statistics (for post 2.5 kernels).
/sys contains statistics for block devices (post 2.5 kernels).
AUTHOR
Sebastien Godard (sysstat <at> wanadoo.fr)
SEE ALSO
sar(1), mpstat(1), vmstat(8)
Linux JUNE 2004 IOSTAT(1)
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