iostat Command

Purpose

Reports Central Processing Unit (CPU) statistics, asynchronous input/output (AIO) and input/output statistics for the entire system, adapters, TTY devices, disks CD-ROMs, tapes and file systems.

Syntax

iostat [ -a ] [-b] [ -l ] [ -s ] [ -t ] [ -T ] [-V] [ -z ] [ { -A [ -P ] [ -q | -Q ] } | { [ -d | -p ] [ -D ] [ -R ] }[ -m ] ] [ { -f | -F } [ filesystems,… ] ][ -S power] [ -@ wparname | ALL | Global ] [ drives ... ] [ interval] [ count ]

iostat [-X [-o filename]] [interval[count]]

Restriction: The -a, -b, -t, -z, -A, -P, -q, -Q, -p, -R, -m flags and the wparname parameter are restricted inside workload partitions.

Description

The iostat command is used to monitor system input/output (I/O) devices (physical and logical) that are loaded, by observing the time for which these devices are active. The iostat command also generates reports that can be used to change system configuration to better balance the I/O load between file systems, physical volumes, and adapters.

The iostat command generates an XML file when the –X option is specified.

The iostat command generates various utilization and throughput reports based on the options that you specify. On multiprocessor systems, CPU statistics are calculated system-wide as averages among all processors.

A report generated by the iostat command consists of system configuration information and various utilization and throughput reports. The system configuration row displays at the start of the iostat command and whenever there is a change in monitored configuration. In addition to system configuration, WPAR configuration is also displayed for the WPAR that has enforced resource limits when the -@ flag is used.

The Interval parameter specifies the amount of time in seconds between each report. If the Interval parameter is not specified, the iostat command generates a single report containing statistics for the time since system startup (boot). 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.

The iostat command is useful in determining whether a physical volume is becoming a performance bottleneck and if there is potential to improve the situation. The % utilization field for the physical volumes indicates how evenly the file activity is spread across the drives. A high % utilization on a physical volume is a good indication that there may be contention for this resource. Since the CPU utilization statistics are also available with the iostat report, the percentage of time the CPU is in I/O wait can be determined at the same time. Consider distributing data across drives if the I/O wait time is significant and the disk utilization is not evenly distributed across volumes.

Beginning with AIX® 5.3, the iostat command reports number of physical processors consumed (physc) and the percentage of entitlement consumed (% entc) in Micro-Partitioning® environments. These metrics will only be displayed on Micro-Partitioning environments.

If the Drive parameter is specified, only those names specified are displayed. One or more alphabetic or alphanumeric values can be specified for Drives. If you specify the Drive parameter, the TTY and CPU reports are displayed and the disk or tape report contains statistics for the specified drives. If a drive name that you specified is not found, the report lists that name and displays the message Drive Not Found and gives the report of all the available drives on the system. If you did not configure drives on the system, no disk or tape report is generated.

Restriction: The first character in the Drive parameter cannot be numeric.

Tape utilization report is generated only if you specified the –p or -a flag.

Reports

The iostat command generates four types of reports, the TTY and CPU utilization report, the disk/tape utilization report, the file system utilization report, the system throughput report and the adapter throughput report.

TTY and CPU Utilization Report

The first report generated by the iostat command is the TTY and CPU utilization report. For multiprocessor systems, the CPU values are global averages among all processors. Also, the I/O wait state is defined system-wide and not per processor. The report has the following format:

This information is updated at regular intervals by the kernel (typically sixty times per second). The TTY report provides a collective account of characters per second received from all terminals on the system as well as the collective count of characters output per second to all terminals on the system.

Methods Used to Compute CPU Disk I/O Wait Time

Operating system version 4.3.3 and later contain enhancements to the method used to compute the percentage of CPU time spent waiting on disk I/O (wio time). The method used in AIX 4.3.2 and earlier versions of the operating system can, under certain circumstances, give an inflated view of wio time on SMPs. The wio time is reported by the commands sar (%wio), vmstat (wa) and iostat (% iowait).

The method used in AIX 4.3.2 and earlier versions is as follows: At each clock interrupt on each processor (100 times a second per processor), a determination is made as to which of the four categories (usr/sys/wio/idle) to place the last 10 ms of time. If the CPU was busy in usr mode at the time of the clock interrupt, then usr gets the clock tick added into its category. If the CPU was busy in kernel mode at the time of the clock interrupt, then the sys category gets the tick. If the CPU was not busy, a check is made to see if any I/O to disk is in progress. If any disk I/O is in progress, the wio category is incremented. If no disk I/O is in progress and the CPU is not busy, the idle category gets the tick. The inflated view of wio time results from all idle CPUs being categorized as wio regardless of the number of threads waiting on I/O. For example, systems with just one thread doing I/O could report over 90 percent wio time regardless of the number of CPUs it has.

The method used in AIX 4.3.3 and later is as follows: The change in operating system version 4.3.3 is to only mark an idle CPU as wio if an outstanding I/O was started on that CPU. This method can report much lower wio times when just a few threads are doing I/O and the system is otherwise idle. For example, a system with four CPUs and one thread doing I/O will report a maximum of 25 percent wio time. A system with 12 CPUs and one thread doing I/O will report a maximum of 8 percent wio time. NFS client reads/writes go through the VMM, and the time that biods spend in the VMM waiting for an I/O to complete is now reported as I/O wait time.

Disk/Tape Utilization Report

The second report generated by the iostat command is the disk/tape utilization report. By default, the disk utilization report is displayed, and you must specify the -p flag to display the tape utilization report.

When you specify the -m flag, the path utilization report is displayed.

The disk report provides statistics on a per-physical-disk basis, and tape utilization report provides statistics on a per-tape-basis. The default report has the following format:

If you specify the -D flag, the report has the following metrics for disk/tape. Extended metrics for disk are displayed by default and users need to specify the –p option for tape utilization report:

Statistics for CD-ROM devices are also reported.

Block IO Device Utilization Report

The Block IO Device Utilization report provides statistics per IO device. The report helps you in analyzing the IO statistics at VMM or filesystem, and disk layers of IO stack. The report also helps you in analyzing the performance of the IO stack. The default report has the following format:

System Throughput Report

This report is generated if the -s flag is specified. This report provides statistics for the entire system. This report has the following format:

Tip: The -s flag, when used with the -@ or -f flag, displays logical and physical volume throughput, which corresponds to File Systems and Disks respectively.

Adapter Throughput Report

This report is generated if the -a flag is specified. This report provides statistics on an adapter-by-adapter basis (for both physical and virtual adapters). This report has the following format for a physical adapter report:

The virtual adapter's default throughput report has the following format:

The virtual adapter's extended throughput report (-D option) has the following format:

Asynchronous I/O Report

File System Utilization Report

The file system utilization report provides statistics on a per-file-system basis. The default report has the following format:

Important: You must specify the disk names before you invoke the -f or -F flag. If you specify the -f or -F flag, separate file system names to be monitored by commas.

Flags

Security

Examples

1. To display a single history since boot report for all TTY, CPU, and Disks, enter the following command:

   iostat

2. To display a continuous disk report at two second intervals for the disk with the logical name disk1, enter the following command:

   iostat -d disk1 2

3. To display six reports at two second intervals for the disk with the logical name disk1, enter the following command:

   iostat disk1 2 6

4. To display six reports at two second intervals for all disks, enter the following command:

   iostat -d 2 6

5. To display six reports at two second intervals for three disks named disk1, disk2, disk3, enter the following command:

   iostat disk1 disk2 disk3 2 6

6. To print the System throughput report since boot, enter the following command:

    
   iostat -s

7. To print the adapter throughput reports at 5-second intervals, enter the following command:

   iostat -a 5

8. To print 10 system and adapter throughput reports at 20-second intervals, with only the TTY and CPU report (no disk reports), enter the following command:

     
   iostat -sat 20 10

9. To print the system and adapter throughput reports with the disk utilization reports of hdisk0 and hdisk7 every 30 seconds, enter the following command:

   iostat -sad hdisk0 hdisk7 30

10. To display time stamp next to each line of output of iostat, enter the following command:

    iostat -T 60

11. To display 6 reports at 2-second intervals on AIO, enter the following command:

    iostat -A 2 6 

12. To display AIO statistics since boot for queues associated with all mounted file systems, enter the following command:

    iostat -A -Q

13. To display extended drive report for all disks, enter the following command:

    iostat -D 

14. To display extended drive report for all tapes, enter the following command:

    iostat -Dp

15. To display extended drive report for a specific disk, enter the following command:

    iostat –D hdisk0 

16. To reset the disk input/output statistics, enter the following command:

    iostat –z 

17. To display only file system statistics for all workload partitions, enter the following command:

    iostat –F -@ ALL 

18. To display system throughput of all workload partitions along with the system, enter the following command:

    iostat -f -s -@ ALL

19. To display file system statistics that are appended with default O/P, enter the following command:

    iostat –f

20. To display logical and physical system throughput, enter the following command:

    iostat –s -f

21. To display throughput for user-specified drives and file systems, enter the following command:

    iostat hdisk0 hdisk1 –f /dev/fslv00 /dev/fslv01 /dev/fslv02

22. To display the processor statistics that are multiplied by a factor of 10, enter the following command:

    iostat -S 1

File