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SAS challenges Fibre Channel drives

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SAS-2 enhancements

While SAS has garnered a significant share of the disk drive market, the limitations of 3 Gbps SAS (SAS 1.1) kept it mostly in low-end to midrange systems. But the enhancements that SAS-2 (6 Gbps) bring to the table have catapulted SAS into a leadership position with the potential of becoming the single enterprise disk interface for high-performance tier 1, high-capacity tier 2 and tier 0 solid-state drives (SSDs). The key enhancements of SAS-2 are:

6 Gbps transfer rate. Doubling the transfer rate from 3 Gbps to 6 Gbps is the most obvious improvement, eclipsing Fibre Channel's 4 Gbps while pushing up the transfer rate in lockstep with SATA. Backward compatible with SAS 1.1, SAS-2 provides for a smooth transition.

Decision Feedback Equalization and longer cable length. Short cable length and the lack of an optical interconnect option hindered the deployment of SAS drives in the data center. The use of a technique called Decision Feedback Equalization (DFE) to reduce interferences enables SAS-2 to extend cable length to 10 meters for improved rack-to-rack connectivity. It's an improvement over SAS 1.1, but it will take SAS 2.1 for copper cable lengths up to 20 meters and support for optical connections up to 100 meters. The SCSI Trade Association (STA) recently said the official release of SAS 2.1 should happen in early 2010. "We don't expect the new advanced connectivity options to have a significant part in the

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adoption of SAS for disk drives, although we do believe that the external storage market will benefit from the improved cabling options," Seagate's Williams said.

Standardized expander zoning. While some 3 Gbps SAS expanders have supported proprietary zoning, SAS-2 provides standardized zoning similar to the zoning capabilities in FC switches, enabling enhanced multi-host support and larger configurations. Zoning is the ability to limit access to drives to certain servers and connections to improve security and enable segregation within the storage topology.

Expander self-discovery. SAS-2 greatly improves the efficiency and scalability of SAS discovery. In SAS 1.1, discovery of SAS components was limited by the lack of standardized zoning and because end devices performed the discovery process. Combining standardized zoning and moving the discovery task from end devices to SAS expanders greatly reduces the time to discover large topologies.

SAS design considerations

Unlike the shared link protocol of parallel SCSI, SAS and SATA are serial point-to-point protocols supporting connectivity between source and destination devices. To go beyond two devices, a SAS expander is required; akin to an FC switch, it provides a switching matrix to interconnect disk drives, controllers and other expanders. Available from LSI Logic and PMC-Sierra Inc. for both 3 Gbps and 6 Gbps SAS, 24- and 36-port SAS expanders are by far the most popular configurations and typically reside within data storage enclosures.

Almost since inception, so-called wide ports have been instrumental to SAS-based storage systems. SAS wide ports aggregate multiple ports -- typically four -- into a single "fat pipe" that allows multiple simultaneous connections to different destination devices. Wide ports are used to connect expanders to storage controllers and other expanders. With 6 Gbps SAS, a typical four-port wide port provides a single 24 Gbps conduit. In addition to the performance benefit, wide ports are resilient against port failures, allowing them to function if at least one port within the assigned port group remains available. "A typical use of wide ports in a redundant array configuration is to connect one four-port wide port to one controller, another four-port wide port to a second controller and an additional four-port wide port to another expander, which leaves 12 ports for drives if a 24-port expander is used," said Paul Vogt, senior director of product management at Xyratex Technology Ltd., a supplier of networked storage products for OEMs.

To reap the benefits of 6 Gbps SAS, all SAS components within a storage system need to operate at 6 Gbps. Simply replacing 3 Gbps SAS drives with 6 Gbps drives won't result in a performance boost unless expanders, controllers and HBAs all operate at 6 Gbps. To foster the adoption of 6 Gbps SAS, SAS-2 has provided for an optional multiplexing feature that allows a 6 Gbps controller to multiplex two 3 Gbps connections into a single 6 Gbps pipe. While the multiplexing features enabled SAS-2 controllers to communicate with 3 Gbps SAS drives at 6 Gbps speed, it has lost its significance with the advent of 6 Gbps SAS drives.

Even though 6 Gbps SAS is 50% faster than 4 Gbps Fibre Channel and twice as fast as SAS 1.1, most data storage systems won't see that kind of performance boost. A storage system is only as fast as its slowest component, so having the interface speed doubled won't change the performance limitation of the underlying disk media. "The fastest SAS hard drives will peak at about 200 MBps for pure sequential access and perform at tens of megabytes per second for random access where mechanical head movements are the limiting factor," said Andy McNeill, distinguished engineer at IBM Corp. Depending on storage system architectures and their particular congestion points, the performance impact of SAS-2 will vary among storage systems. "We are seeing a big performance boost by moving front-end host ports from 3 Gbps to 6 Gbps SAS, but we are seeing little improvement by moving the back-end drives to 6 Gbps, even with 15K drives, and expect even less of an impact with nearline SAS drives because the bottleneck is in the connection to the host not to the drives," explained Scott McClure, senior product manager at Dot Hill Systems Corp.

This was first published in January 2010

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