Performance Monitor for Windows NT 4.0 includes new counters for monitoring disk activity:
These counters tell how often the disk is busy during the sample interval. Despite their name, they don't count items in a queue. They use the same data as the %Disk Time counters, but they report the result in decimals, rather than percentages. This allows them to report values greater than 100%.
For example, if %Disk Read Time is 96%, then Avg. Disk Read Queue Length is 0.96.
The advantage of these counters is their ability to display values that exceed 100%.
For example, if %Disk Read Time is 90% and %Disk Write Time is 30%, %Disk Time cannot report the sum because it cannot exceed 100%. In this case, %Disk Time is 100% and Avg. Disk Queue Length is 1.2.
Still, you need to be cautious when interpreting these values, especially those that are sums. In a 3-disk set, if one disk is reading for 66% of the sample interval, another is reading for 70% of the interval, and the third is idle, the Avg. Disk Read Queue Length, would be 1.36. This doesn't mean that the set is 136% busy; it means that it is at about 45% (1.36/3) capacity.