SAS standard refresh puts Fiber Channel on notice

The SAS-II specification is nearly complete and pundits say that the drives resulting from the new standard - which double throughput - will rival Fiber Channel speeds.

SAS devices are a step closer to gaining the enterprise features that will make SAS a legitimate threat to Fibre Channel in the SAN market.

The SAS-II spec is in the final phases of approval with its T10 Technical Committee, the SCSI Trade Association (STA) said this week. The most obvious upgrade is a bump up to 6 Gbps from 3 Gbps, which makes it more competitive with 8 Gbps Fibre Channel at a lower price. The STA and SAS device vendors predict that 6 Gbps SAS will be available in shipping products next year.

Besides doubling throughput from 3 Gbps, the new SAS drives include two other significant features that enhance their value in enterprise arrays: standardized expander zoning and expander self-discovery.

Expanders are devices that attach the host or array to individual disk drives. Edge expanders plug into SAS ports on a server, and fan-out expanders offer more routing capability and other features at a higher price. For 3 Gpbs SAS, expanders supported up to 128 devices, either disk drives or host bus adapters. The new SAS expanders will support at least 256 devices.

Edge and fan-out expanders will be able to support secure zones with the new SAS, similar to Fibre Channel SAN zones. "It would mean that multiple hosts could talk to the expander but see only a subset of the storage devices," said Marty Czekalski, STA board member and Seagate senior staff program manager for interfaces and emerging architectures.

With 3 Gbps SAS, connecting devices to the expanders is a host-based process. The new spec will allow expanders to do this kind of discovery themselves, automatically and in parallel, which will make provisioning large systems of SAS drives more practical, Czekalski said.

SAS interconnects will standardize on the mini-SAS connector, as opposed to an InfiniBand-style connector used previously that was larger and more cumbersome to operate. A process called spread-spectrum clocking will reduce electromagnetic interference between SAS devices and other equipment in the area.

Cables will also be longer for 6 Gbps SAS – 10 meters instead of six meters for 3 Gbps. New SAS devices will also be backward-compatible with 3 Gbps SAS, and 6 Gbps will support multiplexing so that two 3 Gbps SAS devices can be trunked together over a 6 Gbps connection. SAS drives will remain compatible with SATA, which is also making the jump from 3 Gbps to 6 Gbps.

Industry expects new products next year

A formal plugfest for 6 Gbps SAS will be held Nov. 10 at the University of New Hampshire to test interoperability among SAS devices. If all goes well, more 6 Gbps SAS components will start appearing on the market. Some vendors, including LSI and Adaptec, have already begun rolling out 6 Gbps RAID controllers.

The 10-meter limit on cables and lack of support for optical networks (SAS currently supports copper only) mean SAS will remain an internal protocol for now, rather than running long-distance data transmissions, such as replication. However, StorageIO Group analyst Greg Schulz said that won't stop it from disrupting the market for Fibre Channel.

Because of the lower cost of SAS, picking up share from Fibre Channel is "just a matter of time, similar to how parallel SCSI gave way to Fibre Channel at the high end," he said.

However, Schulz predicts that Fibre Channel, and perhaps eventually FCoE, will remain the protocol of choice for replication and distance networking, even for server-to-storage interfaces. "At the low end, where iSCSI or even NAS is too expensive for, say, a two-server cluster, that's where SAS has [another] market opportunity," he said.

Tory Skyers, senior infrastructure engineer for a financial institution he asked not be named, said he sees 6 Gbps SAS as a lower cost alternative to enterprise solid-state drives, as well as Fibre Channel. "Six Gbps is going to take the fight to Fibre Channel with support for fabrics up to 20 meters apart. The high throughput on relatively inexpensive spinning disk could also defer purchases of solid-state drives for the near term," he said.

However, all of this remains hypothetical until 6 Gbps SAS is proven not only in plugfests but in enterprise environments. Storage protocols often take longer than expected to get the kinks out. "The devil's in the details," Schulz said.

An initial price premium over 3 Gbps may also slow early adoption of SAS, although Harry Mason, STA president and LSI director of industry marketing, predicts that premium won't last. You can expect a 15% premium "short-term" over 3 Gbps SAS, he said. "You generally don't see a sustaining premium on these technologies. These premiums are fairly short-lived, and we tend to move back quickly to the range of the last generation."

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