mpstat’s wi (wait on I/O) statistic

I have used mpstat over the years to check system performance metrics, and always wanted to know what went on behind the scenes. With the recent release of opensolaris, I can finally find out how mpstat gathers the statistics it displays (it uses the kstat interfaces). One statistic that has always been confusing is “wt.” According to the mpstat man page, the wt column means:

wt percent wait time

This definition is rather vague, so I read Jim Mauro’s article to get additional information on what wi meant. After reading Jim’s article, I started to think that wt was worthless, but decided to verify my hypothesis with the experts in the opensolaris performance forums. 2-minutes after posting my wi question, I received the following reply:

From: johansen
To: perf-discuss@opensolaris.org
Subject: [perf-discuss] Re: Wait I/O Calculations

%wio is a meaningless statistic. On Solaris 9, each thread that was blocked in biowait(), waiting for I/O, would lead to a counter being incremented. When the I/O completed, the counter was decremented. This counter was what was used to generate %wio. The problem is that we attempted to normalize the I/O-wait as portion of CPU time, when it is really the count of threads sleeping for block-I/O. Attempting to decompose the number of threads blocked for I/O into a percentage of CPU-time-used turns out to be both confusing and not particularly useful.

Becuase %wio has lead to so much confusion, and didn’t represent any real quantity of CPU time, we’ve removed the statistic in Solaris 10. %wio will now always return zero. If you’re interested more useful I/O statistics, check out iostat(1M), which will correctly show you the percentage of time each I/O transaction spends waiting to be serviced. This message posted from opensolaris.org

This re-affirmed by beliefs, and comes from the people who wrote the code that runs your servers. :) The opensolaris project is rad!

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