Serial ATA Takes on SCSI

By Lucas Mearian

March 31, 2003 12:00 PM ET

Computerworld - Low-cost Advanced Technology Attachment (ATA) disk arrays are already gaining ground in near-line storage and disk-to-disk backup applications, but a faster class of drive arrays that uses the new Serial ATA interface standard is likely to challenge SCSI for high-performance applications as well.

The Serial ATA standard, approved in November by the Institute of Electrical and Electronics Engineers Inc. (IEEE), has several advantages over the parallel, shared-bus master/slave architecture of ATA.

Serial ATA works at lower voltages, and because it uses just four data lines compared with ATA's 32, it's 50% faster. It also supports hot-swappable drives and advanced features such as native command queuing, which lets a disk drive make multiple requests for data from the processor and rearrange the order of the data to maximize throughput—a feature traditionally available only on SCSI and Fibre Channel drives. Finally, Serial ATA can support up to 128 devices per channel (vs. two for ATA and 15 for SCSI) and extends the maximum supported cable length from 18 inches to 1 meter.


Interface Advantage: While the ATA drive uses a 40-pin interface (bottom), Serial ATA devices use a compact, seven-pin edge connector with a ribbon cable that's just 5/16 in. wide (top). These drives should be faster, require less power and allow for more efficient and reliable array designs.

Serial ATA's biggest potential benefit lies in its price/performance. Analyst company IDC estimates that about 87% of all drives today use ATA. Economies of scale have made ATA disk arrays, at 1 to 2 cents per megabyte, much cheaper than SCSI, at 3 to 5 cents per megabyte. Serial ATA disk arrays should benefit from those same economies and could displace SCSI in small servers and even large storage arrays using the emerging iSCSI storage networking protocol.

"Five to 10 years from now, Serial ATA with iSCSI will be the dominant storage model," predicts IDC analyst Robert Grey.

Serial ATA's first incarnation, available in drives and controllers, won't do much for end users. That's because ATA disk speeds, at a maximum sustained throughput rate of about 75MB/sec., can't use the bandwidth increase that Serial ATA offers. And initial pricing will be about 10% higher than for ATA drives, says Jason Ziller, chairman of the IEEE's Serial ATA Working Group.

But manufacturers will benefit. Serial ATA's seven-pin connector cable, which is 5/16 in. wide, replaces ATA's 40-pin ribbon cable, which is 2 in. wide, restricts airflow and increases heat in an enclosure.

In addition, chips are now smaller, and their voltage needs are lower than ATA's 5-volt requirement. Serial ATA requires just 0.5 volt.

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