Craig Shallahamer's Blog
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The text below is an except from the book,
Oracle Performance Firefighting, written by
Craig Shallahamer of
OraPub, Inc.
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©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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<p>Based on Figure 4-18, IO subsystem vendors have a difficult problem. Their goal is to somehow, with all their caching and advanced algorithms, to transform the dotted-line situation into the solid-line situation. To transform one fundamentally different queuing system into another is expensive and very difficult. This is one reason IO subsystems seem so expensive.
</p><p>Using Oracle's wait interface, it is extremely simple to detect an IO bottleneck. If you want to know how long it takes the IO subsystem to respond to a single or multiblock read request, or how long it takes the IO subsystem to complete a multiblock database writer or log writer request, you need not go any further than a simple wait event report. Of course, you can also use standard operating system reports like iostat and sar. You can even get a bit tricky and use operating system process tracing to remove Oracle from the equation. But I'm getting ahead of myself!
</p><p>My general IO subsystem performance rule of thumb is read requests must complete in 10 ms or less and write requests must complete in 5 ms or less. Only the most arrogant IO vendors would argue their 20 ms response-time IO subsystem is performing acceptably. When I look at device busyness via iostat or sar, unless the device is at least 5% busy, I don't bother to check the response time. Low-activity devices, when they are active, can result in some crazy-looking response times. Since the low-activity devices are not doing any real work for the system, we can just ignore them.
</p>
©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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