ATA or SCSI? History and Implications

Since the two interfaces were introduced, this question has faced countless computer enthusiasts, quickly joining the ranks of the endless debates among technology buffs: PC or Mac? Ford or GM? CD or vinyl? While neither remains a buzzword in the aisles of Best Buy, IT professionals and savvy SOHO shoppers still peek under the brand names to find the best solutions the tech world has to offer.

As with all the great debates, there can be no definitive answer; each offers its advantages and drawbacks. SCSI has traditionally been the realm of top-end performance and capacity - for those who can afford it. As with any niche market, it commands a price premium that by now has placed it firmly in the workstation and server domain. ATA, meanwhile, has completely dominated the rest of the market with ever more compelling price/performance; long gone are the days when system integrators gave its choice a second thought in their consumer lineups.

Central to the renewed enthusiasm for ATA drives has been their incredible leaps in capacity. For a long time SCSI not only held the performance crown but also established itself as the only ticket to the largest sizes of the day. With areal densities only progressing at a moderate rate, small wonder that SCSI's market alone could justify cramming the tons of platters necessary into giant (read: expensive) half-height drives. (This strategy can still be seen, for example, in Seagate's Barracuda line - 180GB can be yours for the mere sum of $1500.) However, in recent years breakthroughs in magnetic storage from research labs like IBM's have quickly trickled down into the ATA world, yielding an incredible 80GB/platter in the current generation of drives. Among 3.5" form-factor drives there is no longer any doubt: Maxtor's latest 250GB model, for example, nearly doubles the capacity available from any SCSI competitor.

SCSI's dominance in raw spindle speed isn't going away anytime soon, but that also makes it harder for manufacturers to cram more data onto their platters without giving up the blazing access times that define their niche. Meanwhile, ATA standards have (finally) begun to address the architectural differences between the buses: along with its obvious support for removeable drives, ATAPI allowed device disconnect/reconnect; more recently, UDMA-4 brought tagged command queueing. The jump in areal densities has alone produced sustained transfer rates that let anyone stream and capture digital video, to say nothing of storing vast CD and DVD collections. Not that the fileserver is a new idea, but it's well within the reach of today's savvy consumer to elevate it to an artform.

Of course, real-world server admins aren't impressed by the peak MB/s achieved by a drive, nor even by its access time. All that matters is how well many I/O requests it can serve, and how long each takes while under load. In this regard, SCSI is still king, with woefully obsolete drives often besting the latest and greatest from the ATA camp. Thus, for server applications the tradeoff has become much more clear: ATA for size, SCSI for speed. $250 gets you either a 200GB+ behemoth or a 15k RPM scorcher, but not both.

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