Iostat

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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 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 first report contains statistics for the time since system startup (boot). Each subsequent report contains statistics collected during the interval since the previous 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.

Installing Iostat

yum -y install sysstat

Using Iostat

To see the status of your hard drives try

iostat -dmx 10 2

When you do, you should recevie an screen like this.

Device:         rrqm/s   wrqm/s   r/s   w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util
sda               9.00    14.50 27.00 13.90     0.27     0.11    19.19     0.93   22.67   3.80  15.56
sdb               0.00     2.20  0.10 22.00     0.01     1.32   123.66     0.03    1.55   1.19   2.64
sdc               0.00   167.40  0.10  8.40     0.00     0.69   165.55     0.02    2.71   1.38   1.17
dm-0              0.00     0.00 36.30 28.50     0.27     0.11    12.11     1.38   21.36   2.40  15.57
dm-1              0.00     0.00  0.00  0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00

The Man File

The Man File Reads:

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.
             %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  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  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.

             %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.

      -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 newer.

      -n     Displays  the NFS-directory statistic.  Data displayed are valid
             only with kernels 2.6.17 and newer.  This option is exclusive ot
             the -x option.

      -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 then exit.

      -x     Display extended statistics.  This option is exclusive of the -p
             and  -n,  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.)