Fast PCs: Can disk drives keep up?

The new Serial ATA specification will double the bandwidth of the disk drive-PC interface -- a must as computers speed into the gigahertz-plus era

Serial ATA, the new interface standard for fast-talking disk drives, received a considerable boost this week.

The Serial ATA Working Group has published a draft of the 1.0 specification for the new interface for disk drives -- ranging from hard drives to CD-rewriteables -- that would double the bandwidth of the interface between these drives and other PC components.

Serial ATA 1.0 is intended primarily to make sure that the hard-drive interface used by a PC can keep up with the data rates for the drives. However, Serial ATA will also help assure that disk drives can keep up with the performance of the new gigahertz-plus PCs.

"The way chips are being designed today, they're getting smaller and they're getting faster, so Serial ATA helps address a number of performance issues," said Jeff Ravencraft, chairman of the Serial ATA Working Group and also a marketing manager at Intel's Intel Architecture Labs.

"I think we're adding performance that supports both the device [disk drive] side and the host [PC] side," he said.

Serial ATA offers about twice as much bandwidth as the current Parallel ATA standard, known as ATA 100. Serial ATA's first incarnation, dubbed Ultra Serial ATA 1500, would offer a peak bandwidth or transfer rate of 1.5Gbps.

"That basically equates to about 150MB of data per second," Ravencraft said. Additionally, Serial ATA offers point-to-point communications between individual drives and the PC's motherboard, instead of forcing them to share a single interface.

By way of comparison, Parallel ATA's ATA 100 offers a peak transfer rate of 100MB per second.

"It's important that the interface keeps pace and in fact stays in front of transfer rates from hard drives," said Tom Pratt, manager of storage technology for Dell Computer's Peripherals Development Group. "Parallel is running out of runway. The Serial ATA interface is very forward looking. It was created with ten year roadmap in mind."

Intel and Dell are both members of the Serial ATA group.

At the same, Serial ATA will do away with Parallel ATA's familiar ribbon cables. These cables act as arteries for moving data between devices inside the PC. They ribbon cable about two inches wide and can only be made 18-inches long.

The cables' largest drawback, aside from bandwidth, is that they can impede airflow inside some of the smaller form factor PCs, which limits cooling capacity, Ravencraft said.

Serial ATA connectors and cables will cost more initially. However, they are each much smaller, which should eventually translate into lower costs.

The Serial ATA cables can be made up to three feet long, allowing for more elaborate routing, which would aid in creating cooler-running PCs.

The working group expects Serial ATA adoption to begin in late 2001 or early 2002. It will be phased in over a long period of time, possibly up to two years, the group has said.

During this time, Serial ATA will co-exist in the PC market with Parallel ATA. Parallel ATA could be extended for one more generation, but Dell's Pratt and Intel's Ravencraft say it's unlikely.

Due to its higher cost, Serial ATA will be used first in high-end PCs.

Over time, it will migrate downward into lower cost PCs as well as notebook PCS, low-end workstations and low-end servers, Ravencraft said.

The first disk drives that support the technology should appear at the end of 2001.

The first drives will likely utilise an external controller card, allowing for faster time-to-market. However, Serial ATA support will eventually move to PC motherboards and later be integrated directly into PC chipsets.

"Our plan is to incorporate [Serial ATA] support into a future chip set," Ravencraft said.

Future plans call for scaling Ultra Serial ATA 1500 from 1.5Gbps for the first generation to 3Gbps and eventually to 6Gbps.

The seven primary Serial ATA Working Group members, known as Promoters, are APT Technologies, Dell, IBM and Intel as well as drive makers Maxtor Corporation, Quantum Corporation and Seagate Technology. The group also claims about 60 "contributors".

The Serial ATA specification is available via the group's Web site.

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