Tagged Command Queuing (TCQ) is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. ATA TCQ is not identical in function to the more efficient native command queuing (NCQ) used by SATA drives. 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, the OS had to decide the order of the requests based on its own, possibly incorrect, idea of what the hard drive was doing (otherwise known as I/O Scheduling). 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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