If you suspect that your system’s hard disk might be a bottleneck for the system, then there are two counters that you should examine within the Physical Disk performance object. The counters are % Disk Time and Current Disk Queue Length. These are found in Windows Performance Monitor.
The % Disk Time counter works very similarly to the % Processor Time counter that I talked about earlier. It shows you what percentage of the time the hard disk is busy. You want this counter’s average value to be as low as possible (but not zero). An average value of 90% or above indicates that the hard disk can’t keep up with the demand. This could be due to a hard disk that is too slow, or it could be caused by excessive paging. I will talk more about paging in the section on memory.
The Current Disk Queue Length counter tells you how many I/O operations are waiting for the hard disk to become available. Again, this number should be as low as possible. Experts give differing opinions of what is an acceptable value, but my opinion is that the average disk queue length should be three or less.
The content of this blog post was written by Brien M. Posey.