Tagged Command Queuing
Hard Drive Recovery - Tagged Command Queuing
Tagged Command Queuing refers to a method adopted in certain ATA and SCSI hard drives by which the operating system is able to send multiple read and write requests to a hard drive.
Technology
Prior to the invention of TCQ, an operating system was able to send only one request at a time. The operating system itself had to decide the order of requests based on its idea of what the hard drive was doing. With TCQ, the hard drive can decide its own about ordering the request and thus relieving the operating system from this task. Ultimately, TCQ contributes to improve the overall performance of the hard drives.
SCSI TCQ
SCSI TCQ is the more popular than ATA TCQ. In SCSI TCQ, the tasks are entered into a queue using any of the three different modes: head of queue, ordered, or simple. In head of queue mode, a task is pushed into the front of a queue, ahead of all other tasks including other pending head of queue tasks. In ordered mode, a task must execute after all older tasks and before all newer tasks excluding newer head of queue tasks. In sample mode, the tasks execute in any order that does not violate the constraints on the tasks in the other two modes.
ATA TCQ
ATA TCQ is available in both Parallel and Serial ATA. This technology did not succeed because the ATA bus started out as a reduced-pin-count ISA bus. It caused high CPU utilization without improving performance enough to make this worthwhile. As a result, it did not gain popularity.
