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I know a bit about disk I/O and bottlenecks relating to this especially when relating to databases.

How do I really know what the max I/O numbers will be for my disks? What metric might be available to me for working out roughly (but needs to be a good approximation) of how much capacity (if you will) have I got left available in I/O.

I've seen it before where things are bubbling along nicely and then all of a sudden, everything screams to a halt, and it ends up being an I/O bound problem. Is there a better way to predict when I/O is reaching its limits?

This article was interesting but not giving the answer I desire. So, is my best bet surrounding just looking at 'CPU I/O WAIT'? There must be a more reactive method than this.

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2 Answers 2

The number that's usually the most interesting is the service interval. That's telling you how long it's taking for requests to get completed. What database are you using? A database level tool will usually give you better numbers on how the database is working.

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Service interval? FYI - Tend to use MySQL but i'm asking more of a generalised question rather than having a problem with MySQL. But to be able to work out when to much IO is going on for MySQL would be one of the things i'm interested in knowing in order to track and alert before it causes any problems and slow downs. –  Mark Jan 14 '11 at 17:21

One method would be to simply run a benchmark. Simply perform normal activities and increase the load until performance degrade to a point that isn't acceptable. Once you have reached that point look at the numbers, and log them so you know what to be worried about in the future.

The best data will come from a real-usage test, but you can also use other various tools to stress-test your components of your system. You might use IOmeter to test your storage for example.

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IOmeter - sounds interesting. I've only used iotop and iostat and looked at CPU IO wait before. But if you don't have those benchmark numbers available, then I guess its a stab in the dark. And even with those numbers - that doesn't mean it will be the same for the system a few weeks months down the line. It is a shame that there doesn't appear to be a way to more easily do this task? –  Mark Jan 14 '11 at 17:23

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