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The most common performance monitoring metric people quote is usually disk queue length. Disk queue length usually means the length (in number of I/O operations) of the queue of pending operations for a given disk. In other words, how many requests have stacked up waiting for the disk drive to be able to service them. As an example, if I can only process one request a second, but I get 6 requests in a given second, then my queue length will initially be 5 (one request is processed immediately, the five remaining must be queued).

Multispindle disk devices can have multiple requests active at one time, but other concurrent requests await service. This property may reflect a transitory high or low queue length. If the disk drive has a sustained load, the value is consistently high. Requests experience delays proportional to the length of the queue minus the number of spindles on the disks. This difference should average less than two for good performance.

Our Drives Monitor gadget can show you your disk queue length in a real time. The value represents an instantaneous length, not an average over a time interval.