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We are graphing various system parameters using Cacti. One of our graphs shows hard drive reads and writes. A question came up: why do we need this graph?

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Hard drives are one of the slowest components of a computer. Why wouldn't you want to know know how it was performing? –  Zoredache Jul 2 '10 at 17:43

4 Answers 4

up vote 10 down vote accepted
  1. You need to know your read/write ratio to optimize RAID and Cache settings.
  2. It helps diagnose bottlenecks.
  3. Helps when planning upgrades; maintenance windows; capacity planning; etc.
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+1 for succintly stating what I was insinuating. –  joeqwerty Jul 2 '10 at 17:40

You only need it if you want to know how your disk susbsystem is performing, or you want to troubleshoot disk related performance problems, or you want to establish a baseline performance metric for the disk susbsystem. If none of those things are your objective, then you don't need it.

Your question is more likely to get quality answers phrased like this "Why would we want to graph HDD reads\writes?" as opposed to "Why do we need to graph HDD reads\writes?" because like I said, you don't need it... but you may want it...

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Like all the rest of the graphs you see, it's primarily relevant in the context of history. If the averages stay around the same for several weeks or months and then quickly spike or drop at the same time as you notice a distinct change in performance, the graph may help you spot what's changed.

If performance goes down and disk IO goes up, then your performance decrease is probably caused by amount of information that must be read/written to perform a specific task. But if performance goes down and disk IO goes down, then your problem probably has to do with something that's preventing or slowing that IO from happening -- perhaps deadlock or disk errors.

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Memory swapping can cause high Disk I/O due to swap in/out. By tracking Disk I/O, you can use it as one of the indicators to determine if your machine has enough memory or not.

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