Iostat

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.)