What is NCQ (Native Command Queuing)?
NCQ is a powerful interface/disc technology designed to increase performance of serial ATA drives by allowing them to internally optimize the execution order of workloads. Like other mechanical devices the performance of SATA drives are hampered by the inertia of their components which effectively limits the speed of media access and retrieval of data. The effect of this may be minimized using an intelligent internal management of the sequence of mechanical processes. NCQ uses Direct Memory Access (DMA) to improve data transfer performance, and it uses an always-available channel to indicate which commands have been completed.
A SATA drive that supports NCQ has an internal queue in which commands can be dynamically rescheduled and reordered. NCQ includes a tracking mechanism for both outstanding and completed portions of the drive workload. In addition, NCQ can let the host issue additional commands to the drive while the drive looks for another command.
NCQ comprises three main components of functionality:
- Command queue building in the drive: A SATA NCQ drive can either queue commands or execute them immediately. The drive knows what protocol to apply to different commands. It assigns a unique tag to commands.
- Transferring data for each command: NCQ lets the drive set up a direct memory access (DMA) operation for a data transfer without host software intervention. The drive controls the DMA engine, selects transfers to minimize latencies, and optimizes command ordering.
- Returning status for completed commands: The drive returns a status for completed commands. Command status is race-free, which means a status for any command can be communicated at any time, without a handshake between device and host. Host and drive use a 32-bit register to communicate about outstanding commands, and keep this register always accurate.
NCQ generates measurable benefits, especially in the area of performance, producing vastly improved performance with highly transactional workloads. The performance benefit can amount to an addition of as much as a 10K SATA drive's performance. NCQ is complementary to several performance-boosting features that are part of the SATA protocol. Additionally, as a result of its command and data handling, NCQ reduces the mechanical wear and tear on drives and improves their endurance.
For further information read the white paper on NCQ from Seagate available at
http://www.seagate.com/pdf/whitepape...x_sata_ncq.pdf 
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