The serial lives of SCSI and ATA

Both SCSI and ATA (also called EIDE) disk drives are about to reach the end of their useful lives as products, at least in their incarnations as parallel device interfaces. Both are giving way to serially connected implementations.

End-of-life for both of these interfaces has been announced for months now. The Serial ATA Working Group, the marketing body for serial ATA (SATA), has defined a 10-year growth path for ATA technology that takes it from its current 150M bit/sec transfer rate up to 600M bit/sec.

SATA is an industry-accepted standard and disk products are already out the door for both desktop and server implementations. Drives are available from Maxtor, Seagate and Western Digital. In addition, major systems builders (Dell, Gateway, HP and LSI Logic, for example) have signed on and are already shipping products.

The SCSI Trade Association (STA) is an industry-run association that markets SCSI technology for the vendors. Working with the "T10" (SCSI technical) committee, it has developed SCSI from a 5M bit/sec transfer rate in the mid-1980s to a seventh and final generation of product, Ultra320 (which, not surprisingly, moves data at 320M bit/sec).

STA too has announced the end of the road for parallel I/O. The next generation of SCSI will be serial and last May the T10 committee shipped its formal specification for international approval. The current schedule is for serial-attached SCSI (SAS) products to begin entering the pipeline about this time next year.

Both the SATA and serial SCSI specifications are first release specs. Things get much more interesting when we get to SATA II and SAS 2.0, the next generation specifications, because at that point both marketing groups vow to have their requirements aligned to provide system-level compatibility between the two.

This indicates that we can expect both command- and plug-compatibility, which is interesting for those of us who pay attention to the value of our data and may want to assign different levels of hardware support to certain data sets. It is also interesting to consider the possibilities of having both sorts of device in a single cabinet. Some may choose to go with cheaper SATA disks as hot spares for our high end RAID devices, or to swap-out one type of drive for another as pricing or other considerations change.

Another possibility, of course, is that SATA drives will poach the enterprise territory normally thought of as SCSI-only. Time will tell.

ATA and SCSI have been around for a long time - in the case of SCSI, almost 20 years - which is an eternity in the computer business, so these evolutionary changes shouldn't really surprise any of us. What is unexpected however is that both disk drive technologies are changing more-or-less at the same time.

All this can only be good for the IT storage buyer. When it comes to disk products, the next few years may prove to be lively indeed.

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