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In summary, here is the short and sweet on the different forms of Command Queuing. In Parallel ATA, the merely passive role of the drive, along with the command overhead associated with the disconnect and polling of master and slave devices on the same cable renders the legacy command queuing scheme somewhat ineffective. Therefore, there has been little incentive to move to the more sophisticated albeit more expensive command queuing scheme.

Serial ATA extensions add Native Command Queuing to the FirstPartyDMA engine setup. The point to point topology allows a continuous communication between the device and the controller, which in turn allows to take full advantage of advanced features like for example a so-called non-zero offset DMA engine setup to allow for out of order data delivery, as well as reordering of commands within the queue. SATA NCQ does not allow prioritizing of queues, however, Virtual Head of Queue attributes are possible with the effect that any such command will trash any existing queue. This virtual Head of Queue command will, thus, grant priority to itself in a Last-Man-Standing fashion. Thereafter, the previously uncompleted commands will have to be reissued.

One thing often downplayed in SATA marketing is the fact that the continuous point-to-point topology only grants uninterrupted data transfer from the device to the controller. The latter, however, is still bound by the rules of the system’s backbone, e.g. the arbitration scheme of the PCI bus. Therefore, with multiple SATA devices, the so-called additive performance will run into a bottleneck further downstream. Also keep in mind that the point to point topology prevents any SATA device from directly servicing multiple hosts, on the other hand, multiple host bus adapters can connect through different “networking” strategies to different host systems. Moreover, as outlined earlier, HyperThreading can mimic a multi-host environment on a small scale and in combination with symmetrical multiprocessing, simulate a multi-host-multi-target network within itself.

SCSI Tagged Command Queuing is the most sophisticated version of command queuing by means of different types of queue serving different purposes. The versatility of the SCSI implementation is largely based on the ITLQ nexus tag to keep track of workloads, targets, hosts and anything related to the above. Therefore, it would appear as if SCSI will remain the storage solution of choice for complex systems and new command schemes evolved over the past few years, including simultaneous reads and writes through dual ported drives, have increased its lifespan – at least in the enterprise sector – another few years. On the other hand, within the desktop sector or even in small storage networks, SCSI may not be able to play out its technological superiority.

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