Linux User's Manual IOSTAT

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

NAME
iostat - Report Central Processing Unit (CPU) statistics and input/out-
put statistics for devices, partitions and network filesystems (NFS).

SYNOPSIS
iostat [ -c ] [ -d ] [ -N ] [ -n ] [ -h ] [ -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
statistics for each device that is configured. When option -n is used,
an NFS header row is displayed followed by a line of statistics for
each network filesystem that is mounted.

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 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 device
expressed in a number of blocks per second. Blocks are
equivalent to sectors with kernels 2.4 and later 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 device
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 device
expressed in kilobytes per second.

kB_wrtn/s
Indicate the amount of data written to the device
expressed in kilobytes per second.

kB_read
The total number of kilobytes read.

kB_wrtn
The total number of kilobytes written.

MB_read/s
Indicate the amount of data read from the device
expressed in megabytes per second.

MB_wrtn/s
Indicate the amount of data written to the device
expressed in megabytes per second.

MB_read
The total number of megabytes read.

MB_wrtn
The total number of megabytes written.

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 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.

rMB/s
The number of megabytes read from the device per second.

wMB/s
The number of 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.

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%.

Network Filesystem report
The Network Filesystem (NFS) report provides statistics for each
mounted network filesystem. The report shows the following
fields:

Filesystem:
This columns shows the hostname of the NFS server fol-
lowed by a colon and by the directory name where the net-
work filesystem is mounted.

rBlk_nor/s
Indicate the number of blocks read by applications via
the read(2) system call interface. A block has a size of
512 bytes.

wBlk_nor/s
Indicate the number of blocks written by applications via
the write(2) system call interface.

rBlk_dir/s
Indicate the number of blocks read from files opened with
the O_DIRECT flag.

wBlk_dir/s
Indicate the number of blocks written to files opened
with the O_DIRECT flag.

rBlk_svr/s
Indicate the number of blocks read from the server by the
NFS client via an NFS READ request.

wBlk_svr/s
Indicate the number of blocks written to the server by
the NFS client via an NFS WRITE request.

rkB_nor/s
Indicate the number of kilobytes read by applications via
the read(2) system call interface.

wkB_nor/s
Indicate the number of kilobytes written by applications
via the write(2) system call interface.

rkB_dir/s
Indicate the number of kilobytes read from files opened
with the O_DIRECT flag.

wkB_dir/s
Indicate the number of kilobytes written to files opened
with the O_DIRECT flag.

rkB_svr/s
Indicate the number of kilobytes read from the server by
the NFS client via an NFS READ request.

wkB_svr/s
Indicate the number of kilobytes written to the server by
the NFS client via an NFS WRITE request.

rMB_nor/s
Indicate the number of megabytes read by applications via
the read(2) system call interface.

wMB_nor/s
Indicate the number of megabytes written by applications
via the write(2) system call interface.

rMB_dir/s
Indicate the number of megabytes read from files opened
with the O_DIRECT flag.

wMB_dir/s
Indicate the number of megabytes written to files opened
with the O_DIRECT flag.

rMB_svr/s
Indicate the number of megabytes read from the server by
the NFS client via an NFS READ request.

wMB_svr/s
Indicate the number of megabytes written to the server by
the NFS client via an NFS WRITE request.

ops/s
Indicate the number of operations that were issued to the
filesystem per second.

rops/s
Indicate the number of 'read' operations that were issued
to the filesystem per second.

wops/s
Indicate the number of 'write' operations that were
issued to the filesystem per second.

OPTIONS
-c Display the CPU utilization report.

-d Display the device utilization report.

-h Make the NFS report displayed by option -n easier to read by a
human.

-k Display statistics in kilobytes per second instead of blocks per
second. Data displayed are valid only with kernels 2.4 and
later.

-m Display statistics in megabytes per second instead of blocks or
kilobytes per second. Data displayed are valid only with ker-
nels 2.4 and later.

-N Display the registered device mapper names for any device mapper
devices. Useful for viewing LVM2 statistics.

-n Display the network filesystem (NFS) report. This option works
only with kernel 2.6.17 and later.

-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. Note that this option works only with post 2.5
kernels.

-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. This option 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 ker-
nels (e.g. 2.4) only if extended statistics are available in
/proc/partitions (the kernel needs to be patched for that).

-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-
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. 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 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.

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/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).

/proc/self/mountstats contains statistics for network filesystems.

AUTHOR
Sebastien Godard (sysstat <at> orange.fr)

SEE ALSO
sar(1), pidstat(1), mpstat(1), vmstat(8)

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

Linux APRIL 2009 IOSTAT(1)