While serial-attached SCSI (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 SAS) brings 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). Let's take a look at the key enhancements of SAS-2:
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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 serial ATA (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 serial-attached SCSI drives in the data centre. The use of a technique called Decision Feedback Equalization (DFE) to reduce interferences enables SAS-2 to extend cable length to 10 metres 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 metres and support for optical connections up to 100 metres. The SCSI Trade Association (STA) recently said the official release of SAS 2.1 should happen in early 2010.
Standardised expander zoning. While some 3 Gbps SAS expanders have supported proprietary zoning, SAS-2 provides standardised zoning similar to the zoning capabilities in Fibre Channel (FC) switches, enabling enhanced multi-host support and larger configurations.
Expander self-discovery. SAS-2 greatly improves the efficiency and scalability of serial attached SCSI discovery. Combining standardised zoning and moving the discovery task from end devices to SAS expanders greatly reduces the time to discover large topologies.
Serial-attached SCSI technology adoption landscape
While SAS-2 software components have been available from vendors like Seagate, LSI Logic and PMC-Sierra since 2009, SAS-2 storage systems -- with the exception of arrays from small vendors -- aren't expected to be widely available until later in 2010. We'll see some entry-level and midsized storage systems transition to SAS-2 in 2010, but high-end systems based on SAS-2 technology will take much longer. "It will take years for high-end arrays to transition from FC to SAS because it's an extremely risk-averse space," said Greg Schulz, founder and senior analyst at Stillwater, Minn.-based StorageIO Group.
Here are some data storage vendors' SAS-2 products and product plans:
EMC. EMC currently uses 3 Gbps SAS in its Clariion AX4 and Celerra NX4 storage systems with no roadmap for transitioning these or other platforms to 6 Gbps SAS, according to an EMC spokesman.
Dell. Targeted at small and medium size enterprises (SMEs) and branch offices, the Dell PowerVault MD3000 modular array family currently has a 3 Gbps SAS back end. "Although no timeline has been set, it'll be transitioned to 6 Gbps SAS in the near future," said Howard Shoobe, Dell's senior marketing manager.
Hewlett-Packard (HP). With the HP StorageWorks D2000 JBOD connecting to an HP Smart Array controller with 6 Gbps SAS support, HP is one of a few vendors with a shipping end-to-end 6 Gbps storage product targeted for infrastructure applications like Microsoft Exchange and Web 2.0 service providers. The StorageWorks MSA20000 G2 with its current 3 Gbps SAS back end will be available with a 6 Gbps SAS back end in 2010, according to Kyle Fitze, marketing director, HP StorageWorks storage platforms. "As far as the midrange EVA family is concerned, we are looking at SAS as a disk back end, but no availability date has been set," he said.
Hitachi Data Systems (HDS). HDS has been using a 3 Gbps SAS back end in its Adaptable Modular Storage (AMS) 2000 family and will transition it to 6 Gbps SAS in the next technology refresh sometime within the next couple of years. Within the same timeframe, the AMS family and high-end Universal Storage Platform (USP) with its current FC disk back end will share the same SAS-2 back-end arrays, according to Hubert Yoshida, HDS vice president and chief technology officer.
IBM. The IBM System Storage DS3000 series has been shipping with a 3 Gbps SAS back end and will be transitioned to 6 Gbps SAS by mid 2010, according to IBM engineer Andy McNeill. "As far as IBM's higher-end storage systems are concerned, no timeline for transitioning the disk back end from FC to SAS has been set," he said.
LSI Logic. With SAS-to-SATA bridges, expanders, HBAs, I/O and RAID controllers all available for 6 Gbps SAS, LSI covers the SAS component spectrum, with the exception of disk drives. LSI has been offering arrays with a 3 Gbps SAS back end with its Engenio 1000 series for the OEM channel (e.g., IBM DS3000 and Sun StorageTek 2500 series), which it will transition to 6 Gbps SAS by mid 2010, according to Steve Fingerhut, LSI's senior director of marketing.
NetApp. The FAS2040 is currently the only NetApp controller shipping with a built-in SAS host bus adapter (HBA). "We'll add 6 Gbps SAS support to other families as they get refreshed," said Sandra Wu, NetApp's director of solutions marketing. Aiding the transition to 6 Gbps SAS is the DS4243 chassis with Storage Bridge Bay (SBB) support introduced in mid 2009; it allows NetApp to adjust to various disk form factors and interfaces by simply changing SBB containers within the same enclosure.
Xyratex Technology. As a manufacturer of networked storage systems for the OEM channel (e.g., Dell and NetApp), Xyratex has been offering its OneStor family of storage systems with a 3 Gbps SAS back end and plans to offer a 6 Gbps SAS version in the first half of 2010.
This article originally appeared in Storage magazine.