Serial ATA technology could lead to cheaper server storage

Serial ATA is a new technology that is at last ready for prime time. The question now is which arena it will play in.

SATA is an evolution of the parallel ATA technology (known first as IDE and than as EIDE) that has been with us for over a decade. Likely as not, the disks inside your present PC are EIDE, although your servers' disks are probably either SCSI or Fibre Channel. Why the interest here if this is not a server technology? Because in the near future SATA may well provide cheaper server storage for all of us.

For years now, drive manufacturers have charged a sizable premium for HDDs that use the SCSI interface, and an even larger premium for those using Fibre Channel. The rationale behind this has been that these connectivity methods have been expensive technologies to develop and maintain, and that because of this the controllers continue to be more costly both in terms of their chips and their other onboard technologies.

The HDD vendors have been milking this for a many years now, sometimes charging several multiples of the price for an identically sized EIDE drive. They have enabled this sort of thing - which less polite readers probably call "gouging" - by not producing fast spindle (10K and 15K rpm) drives using the cheaper interfaces. To make matters worse, the array vendors charge an "uplift" when they package these devices in their boxes. Don't buy from them? Sure, but go elsewhere for your maintenance.

ATA, in its incarnation as a parallel interface, has not been a competitive technology at the enterprise level for several reasons. It's cables were short (18 inches), broad (wide enough to contain 80 conductors), and only allowed two devices per cable. Anyone designing an array with a large number of these disks would have had a huge tangle of cabling to contend with, and still could only move data at a rate of 133M bit/sec. By contrast, a SCSI bus could have up to 15 drives on it (although the practical limitation for performance is typically seen as five drives), and could move the data along at a somewhat faster rate. Also, of course, the disks themselves spun faster and were larger.

The new serial version of ATA does away with many of these problems. It doubles the cable length and probably more significantly, reduces the number of wires in the cable to seven, making for a much thinner cable. While it allows only one device per controller, the difference in cable sizes makes this pretty much a non-issue in terms of the system real estate it will require.

Low voltage signal requirements mean SATA will be cheaper to build into a system, and the fact that the specification supports hot plugging means it offers an interesting option for array builders. Most vendors will also like the fact that a large SATA drive is likely to come in at about three-quarters the price of the equivalent SCSI drive.

Two vendors, Maxtor and Western Digital, have announced serial ATA drives in the last week.

SATA will be available on large drives from Maxtor (up to 250GB), running at 7200 rpm, and on 10,000 rpm drives from WD that hold 36G bytes (the lower number is due to the "new physics" of the faster spindle speeds, a challenge that has been known for almost a decade).

Can this play in a RAID box? Will it provide value for you? And what will the response of the SCSI vendors be? More on that next time.

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