Monitoring Queue Length

To determine the number of I/O requests queued for service, track Avg. Disk Queue Length for LogicalDisk or PhysicalDisk. Notice that this might overstate the true length of the queue, because the counter includes both queued and in-service requests. If the value of Avg. Disk Queue Length exceeds twice the number of spindles, then you are likely developing a bottleneck. With a volume set, a queue that is never shorter than the number of active physical disks indicates that you are developing a bottleneck.

Figure 8.8 shows a disk bottleneck with high disk usage and a long queue.

Figure 8.8 High Disk Usage and a Long Queue