SCSI Goes Serial

Driven to perform: Serial attached SCSI will push performance far higher than its predecessor as well as the parallell and serial version of the ATA interface.

The high end of the storage world is gearing up for a major advance as the venerable Small Computer System Interface (SCSI) joins the move to serial communications. Disk drives using the recently completed Serial Attached SCSI link are expected to appear next year.

The shift to serial will let SCSI continue advancing beyond the 320 Mbytes/second rate that appeared to be the peak speed for parallel SCSI links. Serial Attached SCSI (SAS) brings many features of the high end Fibre Channel interface, which is designed for far larger enterprise systems. "SAS is scalable. With parallel SCSI you could only get to 16 drives on one adapter, and with SAS you can go to 16,000," says Gordy Lutz, enterprise marketing manager at

Seagate Technology of Scotts Valley, CA (www.seagate.com). He adds that SAS is a point-to-point link with a 300 Mbytes/second speed, so adding more drives effectively adds more bandwidth. Even a single SAS channel exceeds the maximum sustained rate of about 260 Mbytes/second that Ultra320 parallel SCSI could provide.

Additionally, SAS is compatible with the common Serial ATA interface now on desktop disk drives, using identical cabling and connectors which are much smaller and cost less than SCSI products.

The serial interface utilizes software and firmware developed for parallel SCSI, though it isn't backwards compatible on hardware. While the shift to serial is a bigger step than earlier upgrades, standards committees leveraged well-proven existing technologies. "One goal was to eliminate invention, so it's an interface with little risk," says Marty Czekalski, interface architecture initiatives manager for Maxtor's Server Products Group based in SanJose, CA (www.maxtor.com).

Though the standard is complete, products won't appear until next summer. Early next year, the SCSI Trade Association feels there will be enough products available to hold a "plugfest" so engineers can see how all products operate together.

Then design engineers can build complex storage systems or simply gain the benefits of simpler installation and maintenance. In large systems, it's now possible to use low-cost desktop disk drives to speed up tasks that were once delegated to slow tape drives. The reason for that is because SAS can use Serial ATA drives designed for use on desktop PCs.

"Typically, SAS systems will use high performance, high reliability drives, but SATA compatibility lets people use low cost, low performance drives for applications that won't see a lot of accessing," says Dan Reno, server strategy and planning manager, for the San Jose-based Server HDD Business Unit of Hitachi Global Storage Technologies (www.hgst.com).

The small connectors and cabling of SAS make it possible to use several smaller drives in a 1 or 2 U housing, bringing RAID (redundant arrays of independent disks) benefits.

"If you can put in a few more drives, you'll have the benefits of RAID: better performance, improved reliability, and lower heat dissipation," says Mike Chenery, VP for Advance Product Engineering at Fujitsu Computer Products of America Inc. in San Jose (www.fcpa.fujitsu.com).