Q What is queue depth?

A The number of simultaneous requests on a drive's request queue.

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When an application requests data from a disk controller, the controller is responsible for fetching the data from the physical drive. Assuming there is just one request for data outstanding the controller will simply fetch the data and return it to the application. If a controller has multiple requests outstanding at any given time it is said to have a queue depth equal to the number of outstanding requests. When there are multiple requests outstanding a disk controller has the option of choosing the sequence in which to service them and it will attempt to optimize the order so as to achieve maximum data throughput. Requests for data blocks that reside "close" to each other on the physical drive will generally be serviced sequentially. Sophisticated algorithms increase data throughput rates by ordering pending requests optimally. Although typical user workloads demand a tiny proportion of their data at queue depths beyond four, heavily loaded database servers often see queue depths in the hundreds.