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Queuing Schemes: Parallel ATA vs.Serial ATA

Since the inception of hard disc drives, a number of different queuing schemes have been developed as adaptation to the different environments and their requirements from storage media. The simplest form of command queuing is the legacy command queuing scheme enabled in e.g IBM/Hitachi desktop parallel ATA drives. On the other end of the spectrum are the extremely sophisticated ways of SCSI queuing as an adaptation to the highly demanding workloads arising in any multiple host/multiple device environment.

However, also the desktop segment is changing. Multithreaded operating systems along with the necessary software, not to mention 2-way and upcoming 4-way HyperThreading capable processors or, alternatively, the migration of home-computing towards an entry-level multiple host / multiple target conglomerate, made possible by increasing acceptance of gigabit LAN, creates a new demand for command queuing even in the advanced home / office environment.

In Parallel ATA, command queuing never gained much foothold, in fact, the only adopter was IBM with the GXP series that delivered up to twice the performance of any competing drive in industry standard benchmarks such as Winbench99. However, either problems with a correct implementation or else simply an underestimating of the potential benefits compared to the trade-offs turned out to be a major obstacle in the general acceptance of Legacy Command Queuing.

Serial ATA, on the other hand, abides by different rules than Parallel ATA. One major difference is the fact that Serial ATA employs a point-to-point connectivity, which makes disconnects superfluous. This fact by itself opens up an entirely new world of possible benefits from command queuing, which, in this case, is called native command queuing or NCQ.

The only vendor supporting legacy ATA queuing used to be IBM with their GXP series and in I/O intensive benchmarks those drives were lightning-fast.

Mechanical Overhead

We already mentioned that HDDs are hybrids consisting of electronic and mechanical components, and it is easy to see where the mechanical parts are the slower of the two categories. In addition, electronic wear and tear, even though not unheard of is a relatively rare occurrence whereas mechanical failure, especially of HDDs has been in the spotlight over and again. Suffice it to say that there is a mismatch between the mechanical and the electronic performance of any HDD and the difference is what is referred to as mechanical overhead.

The effect of the mechanical overhead can probably best be described by saying that the drive is not capable of executing the commands at the same rate as they are delivered by the host bus adapter, therefore, it will result in a backlog that, in the long run will cripple the system performance. The mechanical overhead is mostly caused by seek times and rotational latencies but keep in mind that these two factors are independent of each other and require different strategies with respect to optimizing the workflow.

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