Disk Queue Length counters

Windows provide two main counters for the Disk Queue Length, the Average Disk Queue Length and the Current Disk Queue Length. What are the differences with it and how can I use it?

Current Disk Queue length is a number of requests outstanding on the disk at the time the performance data is collected. It includes requests for service at the time of data collection. The value represents an instantaneous length, not an average over a time interval. 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.

The Current Disk Queue Length counter is the actual number of requests that are either in process or waiting to be processed by the Disk at the time the counter is captured.

Average Disk Queue Length, as its name implies, is the average number of both read and write requests that were queued for the selected disk during the sample interval.

The Average Disk Queue Length counter is the “estimated” average number of requests that are either in process or waiting to be processed by the Disk. Average Disk Queue Length is equal to the (Disk Transfers/sec) * ( Disk sec/Transfer). This is based on “Little’s Law” from the mathematical theory of queues. It is important to note this is a derived value and not a direct measurement.

Our Drives Monitor gadget will show you your disk queue length in real time. Watch about it because too high Disk Queue Length is a bottleneck of your drive.