iostat



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)

       http://perso.wanadoo.fr/sebastien.godard/



Linux                              JUNE 2004                         IOSTAT(1)

Man(1) output converted with man2html