% Disk Time may exceed 100 percent in the Performance Monitor MMC
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Your third-party monitoring tool may generate multiple alarm events during times when your disk is very busy. If you monitor the Physical %Disk Time on your Windows 2000-based computer, you may note that the value may go over 100 percent if your computer is very busy. For example, this could occur if you are copying a large amount of files, or you are copying multiple large files, and so on.
This behavior can occur because some controllers allow the operating system to use overlapping input/output operations for multiple outstanding requests. The disk performance counters time the responses by using a 100 nanosecond precision counter, and then report the cumulative statistics for a given sample time. This sample time could go over 100 percent if, for example, you have 10 requests that completed in 2 milliseconds each in a 10 millisecond sampling interval. If you have multiple disks in a Raid arrangement, the overlapped input/output happens because the operating system can read and write to multiple disks, and this could show values that are higher than 100 percent for this counter.
This behavior is by design.
An accurate way to examine or monitor the disk usage (busy) time is to measure disk latencies. Typically, if the value of one of the following counters is larger than 20 ms, the disk is over-loaded:
Avg. Disk sec/Read
Avg. Disk sec/Write
Avg. Disk sec/Transfer
There are several other values you can use to monitor disk performance, and this topic is described in detail in Chapter 30 - Examining and Tuning Disk Performance, of the Windows 2000 Professional Resource Kit, and TechNet monthly CD-ROMs also have this chapter. However, it is important to note that the %Disk Time description there incorrectly states a registry key to clamp the values at %100 exists as that would be misleading.