iowait

[Tonatiuh]

Red Hat Enterprise Linux 3.0

The "top" command shows a column named 'iowait' which has values about 50%. I have seen that the addition of the values of columns 'user','system' and 'iowait', plus the 'idle' is about 100% so that I think this value (of 'iowait') is involved in a performance problem of the overal server.

Can anyone say me more about the meanning of 'iowait' values?

[Gopi Sekar · Accepted solution]

I believe iowait describes the percentage of cpu resources spent in waiting for IO request to complete (like hard disk reading/writing, IO devices reading/writing etc).

check your harddisk and type of harddisk. this may also come up if you have lesser amount of RAM than required and lot of swapping is happening.

Hope this helps,
Gopi

[Ivan Ferreira]

What Gopi said is right, the iowait may indicate an I/O (disks,network) bottleneck.

You should investigate your paging activity and disk access, 50% iowait is high.

A poor tuned application can cause this behaveour.

[Sudeesh]

If a particular disk hardware is having issues, that will cause more i/o wait. Use sar -d to identify any disk bottlenecks.

See sar man page or below link for command details.

http://www.computerhope.com/unix/usar.htm

Sudeesh