Percent Disk Time from Windows Perfmon can exceed 100 percent

Actually % Disk Time as good as it sounds is not really a good metric to monitor disk performance and really is obsolete.

The explain in Perfmon for %Disk Time has long been a source of confusion. In Perfmon and other third party tools that derive their data from Perfmon the %Disk Tme attribute can have a value greater than 100 percent. This is because the % Disk Time counter includes time for overlapping I/O requests (i.e., queue time - waiting for a busy device). On disks that have SCSI and RAID controllers this counter values can be very high. I actually documented this in the ITM 5.1 Redook:

http://www.redbooks.ibm.com/abstracts/sg245519.html?Open

Some more relative counters/attributes to use for disk performance are % Idle Time and Avg Disk Queue Length. For %Idle Time you can calculate (100 - %Idle Time) to get a better metric for disk busy. Also Avg Disk Queu Length can render values greater than 100% because it uses a decimal format (e.g., .75 would be 75% busy, 1.35 would be 135% busy). If this explaination is not detailed enough for you here is a FAQ by an old friend and ex-coworker of mine that goes into more detail that I usually what to know. Mark literally wrote the book on Windows Disk performance.

http://www.oreillynet.com/pub/a/network/2002/01/18/diskperf.html