Serialized, overlapped, and queued operations

The parallel ATA protocols up through ATA-3 require that once a command has been given on an ATA interface, it must complete before any subsequent command may be given. Operations on the devices must be serialized—with only one operation in progress at a time—with respect to the ATA host interface. A useful mental model is that the host ATA interface is busy with the first request for its entire duration, and therefore can not be told about another request until the first one is complete. The function of serializing requests to the interface is usually performed by a device driver in the host operating system.

The ATA-4 and subsequent versions of the specification have included an "overlapped feature set" and a "queued feature set" as optional features, both being given the name "Tagged Command Queuing", a reference to a set of features from SCSI which the ATA version attempts to emulate. However, support for these is extremely rare in actual parallel ATA products and device drivers because these feature sets were implemented in such a way as to maintain software compatibility with its heritage as originally an extension of the ISA bus. This implementation resulted in excessive CPU utilization which largely negated the advantages of command queuing. By contrast, overlapped and queued operations have been common in other storage buses, in particular, SCSI's version of tagged command queuing had no need to be software compatible with ISA's APIs, allowing it to attain high performance with low overhead on buses which supported first party DMA like PCI. This has long been seen as a major advantage of SCSI.

The Serial ATA standard has supported native command queueing since its first release, but it is an optional feature for both host-adapters and target-devices. Many less expensive PC motherboards do not support NCQ. Many SATA/II hard drives sold today support NCQ, while no removable (CD/DVD) drives do because the ATAPI command set used to control them prohibits queued operations.

July 9, 2011

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