Serial Attached SCSI Standard Moves Forward

The history of computing is littered with efforts by different consortia to set standards and interoperability protocols that failed for one reason or another. Not infrequently, the process for setting a standard is delayed and the window of opportunity for a specific technology is missed. In other cases, the standard doesn't work as anticipated. And sometimes, major players just don't adhere to a specific standard, opting instead to let the market make a de facto decision. Consequently, announcements from trade associations and other groups heralding initiatives to establish new standards in this space or that are often justifiably greeted with skepticism or a big yawn.

But sometimes the industry standards process works. The effort to establish a standard for Serial Attached SCSI (SAS) interfaces that also incorporates Serial ATA (SATA) hard disk drives appears to be moving along right on schedule and might actually come to fruition.

More than 2 years ago, the SCSI Trade Association (STA) launched an effort to establish standards for the next generation of the SCSI technology. In January 2003, the STA announced that it would work with the Serial ATA II Working Group, an industry consortium promoting the SATA interface, to enable system-level compatibility with SATA hard disk drives. This spring, in what its leaders are calling a major milestone, the STA conducted its first plugfest for vendors to demonstrate multivendor interoperability of their products using SAS. As Harry Mason, president of STA and director of industry marketing at LSI Logic, told me, the plugfest took place almost exactly on schedule. "It was planned for February 2004 and began on March 1," he said.

SAS promises to provide several benefits over parallel SCSI, which, through several iterations, has been a dominant storage protocol for nearly 20 years. Because the cables and connectors for a serial protocol can be smaller than those for a parallel protocol, manufacturers can develop disk drives that have smaller form factors and can increase the number of devices in each bay of a storage array. As a result, vendors will be able to increase the storage density of arrays.

Moreover, serial cables can run much farther than parallel cables. And the SAS standards will allow for 128 devices to be connected to a single bus, as opposed to the 15 devices that can be connected to a parallel bus. In practice, observers noted, parallel technology didn't provide sufficient throughput to make connecting even 15 devices to a single bus practical.

But perhaps the biggest benefit SAS might offer is the interoperability with SATA. Since the two are plug compatible and use the same commands to move data on and off disks, storage administrators will be able to mix and match devices in their storage subsystems according to whatever criteria they choose. Moreover, as systems and the data on them age and lose value, administrators will have a wide range of options for hard disk size, form factor, and cost.

SAS was finalized at the end of last year, and since January, a slew of companies have announced products that incorporate the new standard. According to Mason, the plugfest was notable for several reasons. Sixteen companies participated, and all vendors that wanted to join in had to come with real products to test. "There were no voyeurs. This was not an event to watch," he said.

Participants included disk-drive, controller, test-equipment, cable, connector, and systems suppliers and such high-profile companies as Adaptec, Hitachi Global Storage Technologies, HP, Maxtor, and Seagate Technology. The technical focus was on initiator and target devices, and the plugfest included tests in a wide range of areas, including the physical layer, cables, backplanes, and rate matching, as well as testing SATA devices and SAS expanders on an ad hoc basis.

However, the plugfest wasn't aimed at testing compliance. Indeed, Mason noted, compliance can be tricky for organizations such as the STA to manage. Instead, he suggested, major OEMs, such as HP and IBM, would be in a better position to ultimately decide whether technology is in compliance with the standard.

Several other plugfests are scheduled during the year. Based on the results of the March event, products should be available in third or fourth quarter 2004 and begin making their way into end users' storage infrastructure by first quarter 2005. When they do, companies will have a more efficient way to provision their storage resources across the enterprise from the most demanding to the lowest priority applications.