SAS-2 spec to challenge Fibre Channel in enterprise SANs

The coming 6 Gbps SAS spec may help SAS drives challenge not just Fibre Channel drives in enterprise SANs but solid state drives as well.

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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 most obvious upgrade in the SAS-2 spec, which is in the final approval phrase with a committee in the SCSI Trade Association (STA), is a doubling in throughput to 6 Gbps, which will make SAS drives more competitive with 8 Gbps Fibre Channel drives at a lower price. But the new SAS drives will include two other features that enhance their value in enterprise arrays: standardized expander zoning and expander self-discovery.

STA officials as well as SAS device vendors predict that 6 Gbps SAS will be available in products shipped next year.

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

With the SAS-2 spec, edge expanders and fan-out expanders will be able to support secure zones, similar to Fibre Channel SAN zones. This 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.

6 Gbps is going to take the fight to Fibre Channel with support for fabrics up to 20 meters apart.
Tory Skyers
senior infrastructure engineer
With 3 Gbps SAS, connecting devices to the expanders is a host-based process. The 6 Gbps spec will allow expanders to perform this kind of discovery themselves, automatically and in parallel, which, according to Czekalski, will make provisioning large systems of SAS drives more practical.

SAS interconnects will standardize on the mini-SAS connector, as opposed to the previous larger InfiniBand-style connector, which was 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.

Plugfest for 6 Gbps SAS

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 that 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 the new SAS spec 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.

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However, Schulz predicts that Fibre Channel will remain the protocol of choice for replication and distance networking, even for server-to-storage interfaces. He said, "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."

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 not only Fibre Channel, but also enterprise solid-state drives. "6 Gbps is going to take the fight to Fibre Channel with support for fabrics up to 20 meters apart," he said. "The high throughput on relatively inexpensive spinning disk could also defer purchases of solid-state drives for the near term."

However, everything that 6 Gbps SAS can achieve emains hypothetical until it is proven not only in plugfests but in enterprise environments. Storage protocols often take longer than expected to get the kinks out. As Schulz notes, "The devil's in the details."

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

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