IMPORTANT:
All current models of WD SATA 6GB/s (SATA III) hard drives come with Native Command Queuing.
Native Command Queuing (NCQ)
NCQ allows multiple commands to:
- Be active at the same time
- Be rescheduled or re-ordered
- Use necessary tracking mechanisms
- Be issued while others are in use
NCQ allows the drive to set up the Direct Memory Access (DMA).
This allows for a data transfer without host software intervention.
This means that the device is capable of complex sequences of operations without CPU intervention.
The drive knows the current angular and rotational position of the drive head.
The drive then selects the next data transfer to minimize both seek and rotational latencies.
Tagged Command Queuing (TCQ)
TCQ is a way to allow hard drives to accept multiple concurrent commands from a host PC.
When commands arrive at the drive's buffer they get tagged with an identifier.
Then it gets reordered by the drive’s microprocessor.
This shrinks the distance the drive's read head needs to move.
This is a different system than IDE/ATA drives use.
They will allow only a single command to be active at a time to any device.
NCQ and TCQ are not interchangeable.
A drive that supports TCQ will not perform queuing when attached to a controller that supports NCQ (and vice versa).
