Re: IO wait

Hi Jack,

> questions:
>
> 1) is top a valid measure of IO wait?

In my opinion sar is a better tool to look at IO waits. sar -d and sar -b will give you information on how the disk activity and I/O is.

> 2) Is a high io wait an issue to be concerned about?

Here is a nice note from Dave Miller with regards to Solaris though

Just a note on iowait. On any multi-cpu machine, this number is not very
useful, especially before Solaris 8. The algorithm for calculating it was
changed in Solaris 8 but still is not really helpful.

Prior to Solaris 8, iowait was defined as follows. When the scheduler attempted
to schedule a process on a cpu, if there were no tasks that were runnable, but
any task was marked as waiting for I/O, instead of counting as idle it counted
as iowait. The problem on multi-cpu systems is that a single process waiting on
I/O could count as iowait on ALL otherwise idle cpus. With Solaris 8 that was
scaled down a bit (I don't have the exact details in front of me), but still is
a bad gauge of I/O problems.

I nearly always consider iowait to just be idle time and look for I/O problems
elsewhere, like looking at iostat and looking at the %busy and service times
on individual disks. That's much more indicative of a real problem and also
will help you find out if you're hot-spotting on any disks. You also can
monitor your networks using netstat because I believe iowait gets counted on
processes waiting for network I/O, too.

> 3) how else can it be accurately measured?
sar I think gives a good idea

> 4) How can I link IO wait to what is happening inside
> the database?

I think v$filestat will be the starting link.

Hope this helps.
Regards,

Madhavan
http://www.dpapps.com

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Madhavan Amruthur
DecisionPoint Applications

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