Feature

SAS challenges Fibre Channel drives

Ezine

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The evolution of SCSI

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SCSI standard

Maximum speed

Bus width (bits)

SCSI-1

5 MBps

8

Fast SCSI

10 MBps

8

Fast Wide SCSI

20 MBps

16

Ultra SCSI

20 MBps

8

Wide Ultra SCSI

40 MBps

16

Ultra2 SCSI

40 MBps

8

Wide Ultra2 SCSI

80 MBps

16

Ultra3 SCSI

160 MBps

16

Ultra320 SCSI

320 MBps

16

3 Gbps SAS

3 Gbps

Serial

6 Gbps SAS

6 Gbps

Serial

12 Gbps SAS
(Production deployment expected in 2012)

12 Gbps

Serial

Contrary to the unanimous vision on the fate of FC as a hard disk drive interface, the storage industry is divided on the impact of SAS on the future of SATA as an enterprise disk interface. SAS and SATA have much in common: Both are serial point-to-point interfaces with advanced features like command queuing, support for hot swapping and commensurate transfer rates (Seagate Technology LLC shipped the first 6 Gbps SATA drives with the Barracuda XT 2 TB in October 2009).

But SATA is impeded by severe shortcomings as a result of its pedigree and original purpose. Conceived to replace parallel ATA in desktop computers and never intended for the enterprise space, SATA is handicapped by its lack of dual porting required for redundant configurations, its limited ATA command set vs. SAS's rich SCSI command set, signal integrity challenges with 6 Gbps (and beyond) SATA, inferior command queuing as a result of its shallow queue depth, and limited error handling and diagnostic capabilities. To mitigate some of these shortcomings, storage array vendors have used SAS-to-SATA bridges and SATA multiplexers (MUX), which are basically interposers that sit between SATA drives and the SAS backplane to enable higher reliability, dual porting and better diagnostics for SATA drives. "We offer a $30 to $40 active-active SATA MUX adapter option for our VTrak RAID and JBOD subsystems to provide dual porting for SATA drives," said Ray Bahar, vice president of the Americas at Promise Technology Inc.

While the interposer approach has been an acceptable workaround to compensate for SATA's shortcomings, it's questionable if SATA will survive in enterprise storage beyond 6 Gbps. Seagate has been offering so-called nearline SAS drives that compete with SATA drives in the enterprise space. Designed for high capacity and lower cost, nearline SAS drives are identical to SATA drives with the exception of the interface; in other words, they peak at 7,200 rpm and 2 TB capacity. Contrary to smaller capacity, mission-critical tier 1 10K to 15K high-performance SAS drives, nearline SAS drives are intended for tier 2 applications and for more static data. Not burdened by the limitations of the SATA interface, nearline SAS drives perform significantly better than comparable SATA drives. "We see at least a 30% performance improvement with identical drives from the same vendor by simply having a SAS interface," said Howard Shoobe, senior marketing manager at Dell Inc. "Although it's the same disk media, SAS can run faster because of better error handling, better command queuing and dual porting."

In comparison to SATA drives in systems with SAS-to-SATA bridges, nearline SAS drives still fare approximately 5% better, according to Ian Williams, executive director, nearline storage solutions at Seagate. On the downside, nearline SAS drives are substantially more expensive than SATA drives; the cost has slowed adoption as many storage system vendors take a wait-and-see position. "At present, nearline SAS drives are 30% to 35% more expensive than equivalent SATA drives and, as a result, it is more cost effective for us to use a MUX adapter with SATA drives to achieve high availability," Promise Technology's Bahar said.

Click here to view a full-size PDF of "Ideal environments for hard disk types."

Nearline SAS drives are likely to eventually force SATA drives out of higher-end enterprise storage systems where performance and features matter more than cost. But for lower-end arrays where cost is king, the price of nearline SAS drives needs to come down to that of SATA drives, and that won't happen until more drive manufacturers offer SAS drives. Furthermore, "the personal computing market would need to shift away from SATA drives and move to USB or other interface options," said Steve Gardner, director of outbound marketing at LSI Logic. "As long as the PC market maintains SATA drive volumes, there will always be some enterprise users integrating the technology to get the absolute lowest cost."

This was first published in January 2010

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