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Selecting Disk Counters

The number of disk-related counters that Performance Monitor provides can be overwhelming. A good counter to watch is %Disk Time, which is available under Performance Monitor's LogicalDisk object. The %Disk Time counter reports the percentage of elapsed time that the selected disk is busy servicing read or write requests. If %Disk Time averages 60 to 80 percent, the disk is not causing a bottleneck. However, this level of performance warrants taking a closer look at the disk in question. When %Disk Time exceeds 80 percent, the disk is getting busy. At this level of performance, the time that the disk requires to service each request increases, and you need to closely monitor several other disk-related counters that are also available under Performance Monitor's LogicalDisk object.

The first of these additional counters is Avg. Disk Queue Length. This counter measures the average number of read and write requests that NT queued for the selected disk during a sample interval. A hard disk becomes a serious bottleneck when the Avg. Disk Queue Length exceeds 2 for a sustained period. When this delay occurs, applications are waiting to access the disk.

Another counter to watch when the %Disk Time exceeds 60 to 80 percent is the Avg. Disk sec/Transfer. This counter measures the time in seconds of the average disk transfer (the time that the disk needs to service each request). A disk can complete only so much work before its service begins to degrade. When disk performance begins to degrade, the Avg. Disk sec/Transfer increases dramatically. This increase affects NT's overall performance.

You will want to review the Disk Transfers/sec counter to determine the amount of work that a disk is completing. This counter measures the rate of read and write operations (also known as the rate of input/output per second) on the selected disk. The amount of work that a disk can support depends on the disk technology and the I/O workload that the disk encounters. In my experience, an Ultra Fast/Wide SCSI 7200rpm disk encountering a mixed I/O workload (random, sequential, write, and read operations) supports approximately 50 to 100 disk transfers per second before its performance degrades. Monitoring the Avg. Disk sec/Transfer counter lets you observe this performance degradation.

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