Tagged Command Queuing
With the SCSI2 specification, Tagged Command Queuing (TCQ)101 was added as a feature to optimize operations. TCQ allows an initiator to send multiple commands to the same SCSI address or LUN. Tagged queues allow the tar-get to store up to 256 commands per initiator. Without tagged queues, targets could only support one command per device for each initiator on the bus. Because SCSI devices are intelligent, they can each optimize their own specific operations, such as head positioning, based on their own I/O request queue. Commands for different I/O requests are sent to the drive for proc-essing, kept in the command buffer, and sorted locally into an optimal sequence. TCQ cuts down on overhead by not having to complete the first I/O request before working on the second (or third) request.
Sometimes TCQ is known as command queuing and reordering.102
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