The Physical Disk and Logical Disk objects have a new set of counters designed for multidisk sets, like mirror and stripe sets and RAID (Redundant Array of Inexpensive Disk) systems.
The new disk counters, Avg. Disk Queue Length, Avg. Disk Read Queue Length, and Avg. Disk Write Queue Length use the same data as % Disk Time. However, they display the data as a decimal rather than a percentage, so it can exceed 100%. This is necessary for measuring multiple disks where total disk time often exceeds 100% of a single disk.
For more information on these counters, see "Monitoring Disk Sets" in Chapter 14, "Detecting Disk Bottlenecks."