Where hard drives are headed


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That's one reason why hard drive technologists agree that lower-power drives with form factors 2-1/2 inches or smaller represent the next major step in hard drive evolution. "The normal response in the disk drive industry over the last 45 years has been: 'When the mechanics get tough, the tough get smaller'," says Maxtor's Jipson.

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New magnetic technology
Extending the life of the magnetic hard drive requires battling a phenomenon known as superparamagnetism, the spontaneous reversal of magnetic polarity. The problem is this: Hard drive write heads turn tiny particles on the platter into little magnets - and when magnets get small enough, ambient thermal energy can cause them to flip polarity and scramble data. Last year, IBM introduced antiferromagnetically-coupled (AFC) media, which uses a sub-layer of magnetic particles to stabilize polarity. The company has demonstrated AFC drives with areal densities of 100GB per square inch - and that number could go higher.

When AFC runs out of gas - perhaps at 150GB - perpendicular recording will kick in. Also called vertical recording, this scheme requires a special write head that magnetizes particles perpendicularly to the plane of the platter instead of flat against it, as in current drives. In perpendicular orientation, particles are less likely to destabilize others' polarity, combating superparamagnetism in ultra-high-density media. According to Dr. Mark Kryder, senior vice president at Seagate Research, perpendicular recording could take us to one terabit per square inch, resulting in terabyte drives. Products could appear as soon as three years from now.

One successor to perpendicular recording will likely be Heat Assisted Magnetic Recording (HAMR). HAMR, under development by researchers at Seagate, uses a laser to heat the recording media and lower its coercivity - the amount of magnetism required to change a recording bit's polarity. When the media cools, its coercivity rises, reducing the possibility of superparamagnetic reversal. This technique could enable researchers to break the one-terabit-per-square-inch barrier.
The incredible shrinking drive
But hasn't the hard drive already gotten smaller? Laptops typically use 2 1/2-inch models, while IBM's Microdrive for consumer devices sports a tiny, 1-inch platter. Munce says, "Mobile drives today are optimized for high shock and minimum power operation for longer battery life." Not to mention that mobile drives spend most of their time idle - as opposed to server drives that must run flawlessly 24x7. To be viable in server applications, Munce believes that mini drives must be "re-optimized" for reliability and performance, something that IBM is "certainly looking at."

A clear leader in this area is Seagate, Scotts Valley, CA, which already uses 2 1/2-inch platters in its 15,000rpm SCSI drives. "The principal reason for going to smaller discs in enterprise 15K devices is to hold power consumption at a practical level - and also to improve time to data by reducing the physical distances that need to be covered," says Nigel Macleod, senior vice president for Seagate's Advanced Concepts Labs. But Macleod believes Seagate may have already reached the practical limit for rotational speed, noting that pushing drives faster than 15,000rpm produces "diminishing returns" in improved access times. And he offers no road map for drives with platters smaller than 2 1/2 inches.

The path to miniaturization has several obstacles, the immediate one being the finely-etched photolithography required for read/write heads. "Previously, the disk drive industry lagged the semiconductor industry relative to line-width requirements in photolithography by two to three years," says Geenen. "Right now, we're pushing the envelope as to what they can do. And hitting the next generation of recording heads will require tolerances smaller than anything the semiconductor industry can currently provide."

Thanks to IBM, the areal density of magnetic media itself shouldn't be a gating factor in miniaturization for some time. In 2001, the company unveiled its antiferromagnetically-coupled (AFC) media, a multilayer scheme currently used in mainstream drives that will result in drives with areal densities of 100 gigabits per square inch - or desktop drives topping 400GB - by 2003. Technologists speculate that AFC will be pushed even further to 150Gb. Beyond this, perpendicular recording will be required.

ATA meets the back office
While mini server drives may take awhile to reach the mainstream, Serial ATA II - a new drive interface spec that should be finalized in the second half of this year - may have a big impact on server-based storage as early as next year. An extension to the Serial ATA spec - which stipulates higher transfer rates and easier device installation for desktop IDE drives - Serial ATA II will pose a direct challenge to today's parallel SCSI by adding server and networked storage features to ATA drives.

Serial ATA, just like plain old ATA, is a one-to-one controller-to-drive architecture rather than a full-fledged, parallel storage bus like SCSI or Fibre Channel (FC). But Serial ATA reduces the number of leads in the controller-to-drive connection, enabling manufacturers to consolidate perhaps as many as eight ATA controllers on a single die. More to the point, Serial ATA II will at last provide enhanced IDE (EIDE) drives with a switched architecture, enabling multiple servers to be connected together at ATA interface speeds.

The Serial ATA II spec isn't complete yet, but observers believe that chipsets will be priced well below equivalent SCSI chipsets. In part, that's because Serial ATA II in its initial version won't attempt to emulate the sophistication of the SCSI command set. But a second version of the Serial ATA II spec - slated for development in 2003 - may well include command queuing and reordering for routing simultaneous requests among multiple drives - the key SCSI advantage in server I/Ops today. According to the non-profit Serial ATA Working Group, devices compliant with the second version of the Serial ATA II spec should appear on the market by 2004.

But can ATA drives match the reliability of SCSI models? Absolutely, says Maxtor's Jipson, who observes that Network Appliance already employs ATA drives in its NearStore line of storage solutions. "We believe that ATA drives - when properly done - can take reliability off of the table as a concern," Jipson says. However, he says I/Ops-intensive applications will continue to demand high-rpm SCSI drives.

This was first published in August 2002

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