Commands Reference, Volume 3

iostat Command

Purpose

Reports Central Processing Unit (CPU) statistics and input/output statistics for the entire system, adapters, tty devices, disks and CD-ROMs.

Syntax

iostat [ -s ] [ -a ] [ -d | -t ] [ PhysicalVolume ... ] [ Interval [ Count ] ]

The iostat command is used for monitoring system input/output device loading by observing the time the physical disks 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 and adapters.

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 tty and CPU header row followed by a row of tty and CPU statistics. On multiprocessor systems, CPU statistics are calculated system-wide as averages among all processors.

If the -s flag is specified, a system-header row is displayed followed by a line of statistics for the entire system. The hostname of the system is printed in the system-header row.

If the -a flag is specified, an adapter-header row is displayed followed by a line of statistics for the adapter. This will be followed by a disk-header row and the statistics of all the disks/CD-ROMs connected to the adapter. Such reports are generated for all the disk adapters connected to the system.

A disks header row is displayed followed by a line of statistics for each disk that is configured. If the PhysicalVolume parameter is specified, only those names specified are displayed.

If the PhysicalVolume parameter is specified, one or more alphabetic or alphanumeric physical volumes can be specified. If the PhysicalVolume parameter is specified, the tty and CPU reports are displayed and the disk report contains statistics for the specified drives. If a specified logical drive name is not found, the report lists the specified name and displays the message Drive Not Found. If no Logical Drive Names are specified, the report contains statistics for all configured disks and CD-ROMs. If no drives are configured on the system, no disk report is generated. The first character in the PhysicalVolume parameter cannot be numeric.

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.

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.

Note: Some system resource is consumed in maintaining disk I/O history for the iostat command. Use the sysconfig subroutine, or the System Management Interface Tool (SMIT) to stop history accounting.

Reports

The iostat command generates four types of reports, the tty and CPU Utilization report, the Disk 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:

Column	Description
`tin`	Shows the total number of characters read by the system for all ttys.
`tout`	Shows the total number of characters written by the system to all ttys.
`% user`	Shows the percentage of CPU utilization that occurred while executing at the user level (application).
`% sys`	Shows the percentage of CPU utilization that occurred while executing at the system level (kernel).
`% idle`	Shows the percentage of time that the CPU or CPUs were idle and the system did not have an outstanding disk I/O request.
`% iowait`	Shows the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request.

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 Utilization Report

The second report generated by the iostat command is the Disk Utilization Report. The disk report provides statistics on a per physical disk basis. The report has a format similar to the following:

`% tm_act`	Indicates the percentage of time the physical disk was active (bandwidth utilization for the drive).
`Kbps`	Indicates the amount of data transferred (read or written) to the drive in KB per second.
`tps`	Indicates the number of transfers per second that were issued to the physical disk. A transfer is an I/O request to the physical disk. Multiple logical requests can be combined into a single I/O request to the disk. A transfer is of indeterminate size.
`Kb_read`	The total number of KB read.
`Kb_wrtn`	The total number of KB written.

Statistics for CD-ROM devices are also reported.

For large system configurations where a large number of disks are configured, the system can be configured to avoid collecting physical disk input/output statistics when the iostat command is not executing. If the system is configured in the above manner, the first Disk report displays the message Disk History Since Boot Not Available instead of the disk statistics. Subsequent interval reports generated by the iostat command contain disk statistics collected during the report interval. Any tty and CPU statistics after boot are unaffected. If a system management command is used to re-enable disk statistics keeping, the first iostat command report displays activity from the interval starting at the point that disk input/output statistics were enabled.

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:

`Kbps`	Indicates the amount of data transferred (read or written) in the entire system in KB per second.
`tps`	Indicates the number of transfers per second issued to the entire system.
`Kb_read`	The total number of KB read from the entire system.
`Kb_wrtn`	The total number of KB written to the entire system.

Adapter Throughput Report

This report is generated if the -a flag is specified. This report provides statistics on an adapter-by-adapter basis.This report has the following format:

`Kbps`	Indicates the amount of data transferred (read or written) in the adapter in KB per second.
`tps`	Indicates the number of transfers per second issued to the adapter.
`Kb_read`	The total number of KB read from the adapter.
`Kb_wrtn`	The total number of KB written to the adapter.

Disk Input/Output History

To improve performance on, the collection of disk input/output statistics has been disabled. To enable the collection of this data, enter:

chdev -l sys0 -a iostat=true

To display the current settings, enter:

lsattr -E -l sys0 -a iostat

If the collection of disk input/output history is disabled, the first disk report of iostat output displays the message Disk History Since Boot Not Available instead of disk statistics. As before, subsequent interval reports generated by the iostat command contain disk statistics collected during the report interval.

Flags

-a	Displays the adapter throughput report.
-d	The -d flag is exclusive of the -t flag and displays only the disk utilization report.
-s	Displays the system throughput report.
-t	The -t flag is exclusive of the -d flag and displays only the tty and cpu usage reports.

Notes:

The -s and -a flags can both be specified to display both the system and adapter throughput reports.
If the -a flag is specified with the -t flag, the tty and CPU report is displayed, followed by the adapter throughput report. Disk Utilization reports of the disks connected to the adapters, will not be displayed after the Adapter throughput report.
If the -a flag is specified with the -d flag, tty and CPU report will not be displayed. If Physical Volume parameter is specified, the Disk Utilization Report of the specified Physical volume will be printed under the corresponding adapter to which it belongs.

Examples

To display a single history since boot report for all tty, CPU, and Disks, enter:
```
iostat
```
To display a continuous disk report at two second intervals for the disk with the logical name disk1, enter:
```
iostat -d disk1 2
```
To display six reports at two second intervals for the disk with the logical name disk1, enter:
```
iostat disk1 2 6
```
To display six reports at two second intervals for all disks, enter:
```
iostat -d 2 6
```
To display six reports at two second intervals for three disks named disk1, disk2, disk3, enter:
```
iostat disk1 disk2 disk3 2 6
```
To print the System throughput report, enter:
```
 iostat -s
```
To print the Adapter throughput report, enter:
```
iostat -a
```
To print the System and Adapter throughput reports, with only the tty and CPU report (no disk reports), enter:
```
  iostat -sat
```
To print the System and Adapter throughput reports with the Disk Utilization reports of hdisk0 and hdisk7, enter
```
iostat -sad hdisk0 hdisk7
```

File

/usr/bin/iostat

Contains the iostat command.

Related Information

The vmstat command.

The /dev/kmem special file.

The knlist subroutine, sysconfig subroutine.

Monitoring and Tuning Disk I/O in AIX 5L Version 5.1 Performance Management Guide

The Input and Output Handling Programmer's Overview in AIX 5L Version 5.1 General Programming Concepts: Writing and Debugging Programs describes the files, commands, and subroutines used for low-level, stream, terminal, and asynchronous I/O interfaces.