Tagged Command Queuing

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Tagged Command Queueing (TCQ) is a technology that allows to send multiple read and write requests to a hard drive simultaneously. ATA TCQ is not identical in function to the more efficient Native Command Queuing (NCQ) used by SATA drives[1]. SCSI TCQ does not suffer from the same limitations as ATA TCQ.

Before TCQ, an operating system was only able to send one request at a time. In order to boost performance, it had to decide the order of the requests based on its own, possibly incorrect, idea of what the hard drive was doing. With TCQ, the drive can make its own decisions about how to order the requests (and in turn relieve the operating system from having to do so). The result is that TCQ can improve the overall performance of a hard drive if it is implemented correctly.

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For efficiency the sectors should be serviced in order of proximity to the current head position, rather than in the order received. The queue is constantly receiving new requests and fulfilling and removing existing requests, and re-ordering the queue according to the current pending read/write requests and the changing position of the head. The exact reordering algorithm may depend upon the controller and the drive itself, but the host computer simply makes requests as needed, leaving the controller to handle the details.

This queuing mechanism is sometimes referred to as "elevator seeking", as the image of a modern elevator in a building servicing multiple calls and processing them to minimise travel illustrates the idea well.

If the buttons for floors 5, 2, and 4 are pressed in that order with the elevator starting on floor 1, an old elevator would go to the floors in the order requested. A modern elevator processes the requests to stop at floors in the logical order 2, 4, and 5, without unnecessary travel. Non-queueing disk drives service the requests in the order received, like an old elevator; queueing drives service requests in the most efficient order. This may improve performance slightly in a system used by a single user, but really comes into its own in a system with many users, making requests all over the disk surface.

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