The counters used for single disks can also be used on disk sets. However, two issues are of particular concern for disk sets:
On Hardware RAID sets, where physical disks are combined into a single logical unit by the disk controller, Performance Monitor cannot see the instances of physical disks. It combines the values for all disks as though the disk activity occurred sequentially, rather than simultaneously.
The % Disk Time counters cannot display the sum of multiple disk activity if it exceeds 100%. The potential maximum disk activity for a RAID cabinet is 100% multiplied by the number of disks in the set.
On software RAID, where Performance Monitor can see instances of the disks, it displays simple sums of values; it does not divide the sum by the number of disks.
Use the new disk counters. Avg. Disk Queue Length, Avg. Disk Read Queue Length, and Avg. Disk Write Queue Length display disk activity as a decimal, not a percentage, so that it can display values over 1.0 (100%). Then, remember to recalculate the values over the whole disk configuration. For more information, see "New Disk Activity Counters" earlier in this chapter.