Performance and Compatibility

By Loyd Case  |  Posted 2003-01-09 Print this article Print

Serial ATA had two key goals beyond improved signaling integrity:
  • Better performance: S-ATA achieves improved raw performance by increasing the data rate substantially. The clock rate of Serial ATA is 1.5GHz. The data is encoded with 8b/10b encoding, which reduces the potential bandwidth by 20%. This yields a net bandwidth of 150 megabytes per second of actual data.
  • Software compatibility with parallel ATA: At first blush, it might seem like the operating system will just recognize the drive when you plug it in. In reality, youll still have to install drivers to achieve proper operation. Software compatibility means that the internal protocols are still compatible with parallel ATA, so that software thats dependent on a standard ATA connection (e.g., backup software) should still work. We verified this in our testing. We were able to back up a Serial ATA partition from the DOS prompt using Drive Image 5 with no problems.
The issue of performance is a complex one, due to the fact that the PCI bus lies between todays S-ATA host adapters and the rest of the system. If there is no contention on the PCI bus, then the maximum data rate theoretically possible is 133MB/sec. In practice, this is usually less -- more like 85-90MB/sec on Intel-based systems and as low as 40-50MB/sec in systems using older (pre-8233) Via South Bridge chips.
However, most hard drives -- even fast, high-density, 7200RPM drives -- cant push data out at 150MB/sec. In practice, the best single hard drive data rates for ATA drives max out at around 44MB/sec sustained, from the outer tracks. Substantial PCI traffic from other sources, such as NICs or sound cards can adversely impact the overall PCI throughput. The picture gets worse once you move to RAID devices.
What this means is that were unlikely to get a true picture of Serial ATA performance until we have core logic that places the S-ATA controller north of the PCI bus. Nevertheless, there are a large number of motherboards already shipping with motherboard-down PCI S-ATA host adapters. Serial ATA drives will also be attractive because you can load up additional drives without affecting the number of parallel ATA connections. Want to see how one of the first Serial ATA drives actually performed? Check out our companion story, a first look of the Seagate ST3120023AS.

Loyd Case came to computing by way of physical chemistry. He began modestly on a DEC PDP-11 by learning the intricacies of the TROFF text formatter while working on his master's thesis. After a brief, painful stint as an analytical chemist, he took over a laboratory network at Lockheed in the early 80's and never looked back. His first 'real' computer was an HP 1000 RTE-6/VM system.

In 1988, he figured out that building his own PC was vastly more interesting than buying off-the-shelf systems ad he ditched his aging Compaq portable. The Sony 3.5-inch floppy drive from his first homebrew rig is still running today. Since then, he's done some programming, been a systems engineer for Hewlett-Packard, worked in technical marketing in the workstation biz, and even dabbled in 3-D modeling and Web design during the Web's early years.

Loyd was also bitten by the writing bug at a very early age, and even has dim memories of reading his creative efforts to his third grade class. Later, he wrote for various user group magazines, culminating in a near-career ending incident at his employer when a humor-impaired senior manager took exception at one of his more flippant efforts. In 1994, Loyd took on the task of writing the first roundup of PC graphics cards for Computer Gaming World -- the first ever written specifically for computer gamers. A year later, Mike Weksler, then tech editor at Computer Gaming World, twisted his arm and forced him to start writing CGW's tech column. The gaming world -- and Loyd -- has never quite recovered despite repeated efforts to find a normal job. Now he's busy with the whole fatherhood thing, working hard to turn his two daughters into avid gamers. When he doesn't have his head buried inside a PC, he dabbles in downhill skiing, military history and home theater.

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