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Serial ATA takes on SCSI

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 back-up 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 (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 per cent faster. It also supports hot-swappable drives and advanced features such as native command queuing, that lets a disk drive make multiple requests for data from the processor and rearrange the order of the data to maximise 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 45 metres to 1m.

Serial ATA’s biggest potential benefit lies in its price/_performance. Analyst company IDC estimates that about 87 p ercent of all drives today use ATA. Economies of scale have made ATA disk arrays, at $US0.1c to $US0.2c per megabyte (MB), much cheaper than SCSI, $US0.3c to $US0.5c/MB. 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,” IDC analyst, Robert Grey, said.

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 would be about 10 per cent higher than for ATA drives, chairman of the IEEE’s Serial ATA Working Group, Jason Ziller, said.

But manufacturers will benefit. Serial ATA’s seven-pin connector cable, which is about 8mm wide, replaces ATA’s 40-pin ribbon cable, which is 50mm 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’s Symmetrix and Hitachi’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. The mortgage banking firm uses Network Appliance Inc.’s ATA-based NearStore R100 near-line storage array for disk-to-disk backup of its email, 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 versus ATA,” he said. “Then it really just becomes a cost issue for me.”

Several vendors are already shipping limited quantities of Serial ATA devices.

Serial ATA vendors are beginning to ship disk drives in small numbers, but bulk shipments are expected by mid-2004.


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