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6-Gig SAS Coming to an Array Near You

Disk and component manufacturers are getting excited about 6Gb/s Serial Attached SCSCI (SAS), but storage users will have to wait a while to see what the excitement is all about.

"Going to 6G SAS is the next natural evolutionary step for SAS," said Henry Fabian, executive director for enterprise product marketing at Seagate. "6 Gig SAS offers a 2X increase in speed."

As the next-generation storage interface to Ultra320 parallel SCSI, SAS was originally intended for internal and external direct-attached storage (DAS) environments. But its ability to support SAS and SATA drives has led to its rapid adoption as a drive interface in external storage. That led to a whole new world of storage tiering — SAS for speed and higher availability, and SATA for high capacity and bulk storage.

"While parallel SCSI was limited to DAS environments, SAS brings new features that are opening up the possibility of SAS in SAN-like deployments, which you might call scalable, sharable DAS," said David So, product marketing manager for the storage components group at LSI Corp. "Such architectures are already being shipped in some blade systems today."

LSI and other SAS vendors have already introduced several SAS-to-SAS external RAID subsystems, for instance, as well as SAS switches. Current 3Gb/s SAS also created the market for 2.5-inch small form factor (SFF) enterprise drives. The trend toward 2.5-inch drives is expected to continue with 6Gb/s SAS. As well as the obvious sizing gains, this provides the advantage of lower power consumption and less heat generation.

"Further enhancements in 6Gb/s SAS will make it even more attractive as a host interface for external storage moving forward," said So. "This includes a standardized zoning scheme, and the removal of limitations in SAS configurations."

What's New?

6Gb/s SAS is the marketing term the SCSI Trade Association (STA) gave to the industry standard based on the SAS-2 specification. This next-generation SAS protocol defines a short list of features that include 6Gb/s data transfer rates and self-configuring expanders. This feature allows the expanders to discover topology changes rather than having to rely on the host controllers, which leads to an increase in device scalability, as well as gains in system performance — as the expanders can perform discovery in parallel.

Other upgrades in the standard include new zoning arrangements to ensure interoperability between vendors, a means of increasing cable length from 6-8 meters up to around 10 meters, and the necessary signal and data integrity features to perform in high-end network applications.

"Zoning may be used to hide disks or volumes from servers or computers that are not deliberately given access," said James Rankin, a technology specialist at CDW Corp. "This is a security feature, and is similar to the LUN masking concept in Fibre Channel."

Another useful feature is multiplexing. This offers the ability to combine 3Gp/s SAS links with a 6Gb/s SAS link — it lets you attach an existing 3Gp/s storage system into a larger 6Gp/s backbone.

6Gb/s SAS, then, will replace parallel SCSI is most applications. While that aging standard has been a workhorse for some time, it cannot compete with the speed, features and ease of use present in 6Gb/s SAS. Although SAS uses the SCSI command set, it is not bound to the same limitations as parallel SCSI. For example, SAS can support several thousand hosts per channel, where parallel SCSI supports only 32.

"6Gb/s SAS should also be able to quickly replace parallel SCSI as the preferred host attachment protocol for DAS," said Rankin. "With the speed, price and feature set offered by 6Gb/s SAS, this technology will become an attractive alternative to FC."

He concedes, however, that Fibre Channel host attachment and disks will remain the preferred choices for Tier 1 storage. But he views SAS disks and SAS host attachment as the major contenders for Tier 2 storage needs.

So adds that the external storage market is also adopting 3Gb/s SAS, especially as the drive (or back-end) interface. This applies both to JBODs and external RAID subsystems. This is due in large part to its ability to support both SAS and SATA disk drives.

"3Gb/s SAS is just now beginning to penetrate the host (or front-end) interface of external RAID subsystems in the lower price bands," said So. "I fully expect SAS to continue to grow as both the host and drive interfaces of external storage."

Fabian takes a bolder stance. He predicted that while Fibre Channel will continue to exist, SAS will easily surpass its usage in data centers. With SCSI going away and being replaced by SAS, the latter will become the dominant interface, he said.

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