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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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Disks are a bottleneck. You need to know when you are bottlenecking on I/O accesses so that you can do something about it. Examples include:

  • Upgrade to an SSD
  • Upgrade to a RAID solution.
  • Change the I/O patterns of your application / applications.
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Your question title is a little unclear. Is the graph actually reads VS writes? Or is it just reads and writes on the same graph.

Reads vs writes would be rather useless unless you want to see what your applications are doing more of. What you are likely seeing is reads and writes represented on the same graph. This is important, because it can show total disk usage at peak periods. If you are maxing out the disk I/O, this is where you would likely see it.

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Reads and writes on the same graph, yes. I guess it would make sense to find out what the top watermark is and place it on the graph. – user41609 Jul 2 '10 at 15:29

Reads vs writes is a good metric, but if you are looking for something a little more high level to watch as an indicator of a disk IO bottleneck the Avg Disk Queue length performance counter is handy.

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