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Mixing SAS, SATA prompts caution

Enterprise-class scalability and performance at desktop-like prices is the promise offered by intermixing serial-attached SCSI (SAS) and SATA. But when these drives are mixed in the same enclosure, interoperability issues can arise, forcing vendors to carefully manage their SAS/SATA implementations.

One problem is that SATA and SAS have different voltage levels. SATA was designed for internal use in servers and PCs with cables of up to one meter in length; SAS supports distances of up to 10 meters, says Joel Warford, VP, marketing and business development at SiliconStor, a semiconductor company in Fremont, CA. "Since SATA is not adept at driving a signal over this longer distance, data integrity issues can result," he says.

To boost SATA's voltage level, storage arrays such as Dot Hill's 2730 use SiliconStor interposers. The interposer resides between the SATA drive and the SAS backplane and, notes Warford, "boosts the signal, so it can accept a signal in a degraded form and also send out an amplified signal back to the subsystem."

Further, SAS drives offer enterprise-class features like error detection and logging to proactively predict disk-drive failures, and dual ports in active-active configurations to support multipathing. SATA drives have a single port and offer few features for performing error detection and logging.

Vendors also use interposers to address those SATA deficiencies. For example, they may integrate a chip like SiliconStor's SS1300 directly into their circuit board. The SS1300 chip delivers error logging and drive protection to SATA drives, and includes SiliconStor's Active-Active Multiplexer technology that converts SATA's single port into a dual-port configuration for multipathing.

Paul Vogt, director of product marketing at Adaptec, finds that managing timeouts is another issue. Because SATA drives were initially used in PCs, SATA firmware includes less robust command sets, while SAS drives provide advanced command queuing and sophisticated verification and error correction. While Adaptec lets users intermix SAS and SATA drives on the same controller, it's not recommended. "If users do intermix them, Adaptec's systems generate a prompt to verify that the user really wants to do this," says Vogt.

The SATA timeout problem has been addressed in a couple of ways. First, SATA disk-drive providers have partially resolved it by improving their firmware to alert the storage subsystem to what they're doing. Second, companies like AMCC and LSI Logic include code in their firmware to send quick resets to the disk drive to see if it's working before it marks the drive as failed.

Tony Palmer, a lab engineer at Enterprise Strategy Group in Milford, MA, concludes that while users should still exercise caution when intermixing SAS and SATA, the gap between how they're managed by storage systems is closing. "Vendors are coming out with enterprise-class SATA drives that use many of the same command sets that enterprise-class SAS drives do," he says, "so it's only a matter of time until they're internally managed the same way."

--Jerome M. Wendt

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