Performance and Compatibility
Serial ATA had two key goals beyond improved signaling integrity: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.
- 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.
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.