Analysis: Serial ATA takes on SCSI

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 percent 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.

Serial ATA's biggest potential benefit lies in its price/performance. Analyst company IDC estimates that about 87 percent 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 percent 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.

Vendors plan to offer Serial ATA disk arrays that deliver real performance gains. Enhanced specifications are on the way. Serial ATA II, due by mid-2004, is expected to double transfer rates, and a third-generation standard could double them again by 2007.

Best Applications

In the data center, Serial ATA is likely to boost emerging ATA disk-based storage applications such as near-line storage and virtual tape servers, since the faster bus speed works well for streaming data and large file transfers.

As the technology evolves, some analysts say ATA disks could be used in high-end primary storage devices, such as EMC Corp.'s Symmetrix and Hitachi Ltd.'s Lightning arrays.

For the time being, though, SCSI and Fibre Channel disks have the advantage for storing data associated with high-performance, online transaction-processing applications because SCSI's higher spindle speeds, at up to 15,000 rpm, enable faster access to smaller files. And the mean time between failures of ATA drives, at 500,000 to 600,000 hours, still doesn't stack up to SCSI's 1.2 million hours.

That doesn't worry Jamie Riis, CIO at BayView Financial Trading Group LP in Miami. The mortgage banking firm uses Network Appliance Inc.'s ATA-based NearStore R100 near-line storage array for disk-to-disk backup of its e-mail, SQL and Oracle databases.

"As long as I have a RAID 5-capable system . . . so I can get quick access to the information, then I'm kind of agnostic when it comes to SCSI-attached vs. ATA," he says. "Then it really just becomes a cost issue for me."

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