Think your disks are slow, but are not really sure?

Testing Spot - Help! I Think My Disk is Slow

Most of what happens here at Testing Services and Labs is determining the performance limit of a customer’s application. To accomplish this, we have to stage the application on systems that we are certain can handle it. After all, we want to know the limit of the application, not the system it’s running on. Pushing the limits typically means testing at volumes at which we can’t be sure what’s going to happen or why it’s happening. As the engineers running the test lab, we are often engaged when a resource constraint is suspected. The number one culprit is typically the disk. This post covers the method that we use to determine if the disk is slow. We will discuss designing a disk for high performance purposes in a later post.

When we are asked if a disk is slow, there are a few steps we take to investigate.

Is the disk really slow?

To find the answer to this question we must measure latency. This is the amount of time that a system waits for a disk I/O request to be satisfied. Specifically, we measure two counters on the physical disk objects:

• Avg. Disk Sec/Read
• Avg. Disk Sec/Write

The Windows Core Team gets into greater depth regarding these measures here:

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A short and sweat guide to diagnosing slow disks and if they are indeed slow, some things you can do about it...