SAS challenges Fibre Channel drives

In their 3 Gbps incarnation, SAS drives have proved popular in low-end and nearline data storage systems, but at 6 Gbps, SAS-2 poses a serious threat to Fibre Channel interface drives and could change the landscape of high-end storage arrays.

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In their 3 Gbps incarnation, SAS drives have proved popular in low-end and nearline data storage systems, but at 6 Gbps, SAS-2 poses a serious threat to Fibre Channel interface drives.

Since its introduction in 2004, serial-attached SCSI (SAS) has become the prevailing data storage interface in servers and is on a trajectory to become the same for external storage systems. Those systems are still dominated by Fibre Channel (FC) and serial ATA (SATA) disk drives, but next-generation SAS-2 components, available since early 2009, are hastening this transition.

With 6 Gbps transfer rates and enhancements that neutralize some of the shortcomings of SAS 1.1, SAS-2 has overtaken FC as the most advanced and fastest disk interface. Because FC drives with their 4 Gbps transfer rate and 3.5-inch form factor have reached the end of the road, it's only a question of time as to when SAS will emerge as the drive interface of choice for high-end storage systems and servers. Thanks to its prevalence in servers today, SAS -- mostly 3 Gbps SAS -- already owns approximately 40% of the multiuser disk interface market, trailing SATA's 41.7% share and outpacing the 16% market share of FC-AL/4 Gbps, according to recent analysis by Stamford, Conn.-based Gartner Inc. "SAS will account for 57% and SATA for 43% of the total multiuser HDD [hard disk drive] market by 2013," predicted John Monroe, a Gartner research vice president, who pegged the extinction of FC drives to the year 2013.

The impact of SAS on FC and SATA drives

Despite some disagreement on timing, most data storage analysts agree that the days of FC as a disk interface are numbered. Fibre Channel disks were never able to extend beyond high-end servers and storage arrays and, as result, costs remained high (e.g., a SAS host bus adapter [HBA] costs approximately one-third the price of a comparable FC HBA). Moreover, users have been coping with disparate disk interfaces -- FC for high performance and SATA for large-capacity storage -- requiring conversion chips for the two to coexist in a single storage system. Conversely, SAS and SATA can be supported by a single chip, reducing interface costs, complexity and power requirements. "Enterprise-class SAS drives are on par with high-end FC drives, except SAS has a smaller cooling and power footprint, and enables a single back-end interconnect technology for both SAS and SATA," said Tony Palmer, senior engineer and analyst, ESG Lab at the Milford, Mass.-based Enterprise Strategy Group (ESG).

Contrary to the unanimous vision on the fate of FC as a hard disk drive interface, the storage industry is divided on the impact of SAS on the future of SATA as an enterprise disk interface. SAS and SATA have much in common: Both are serial point-to-point interfaces with advanced features like command queuing, support for hot swapping and commensurate transfer rates (Seagate Technology LLC shipped the first 6 Gbps SATA drives with the Barracuda XT 2 TB in October 2009).

The evolution of SCSI

SCSI standard

Maximum speed

Bus width (bits)

SCSI-1

5 MBps

8

Fast SCSI

10 MBps

8

Fast Wide SCSI

20 MBps

16

Ultra SCSI

20 MBps

8

Wide Ultra SCSI

40 MBps

16

Ultra2 SCSI

40 MBps

8

Wide Ultra2 SCSI

80 MBps

16

Ultra3 SCSI

160 MBps

16

Ultra320 SCSI

320 MBps

16

3 Gbps SAS

3 Gbps

Serial

6 Gbps SAS

6 Gbps

Serial

12 Gbps SAS
(Production deployment expected in 2012)

12 Gbps

Serial

But SATA is impeded by severe shortcomings as a result of its pedigree and original purpose. Conceived to replace parallel ATA in desktop computers and never intended for the enterprise space, SATA is handicapped by its lack of dual porting required for redundant configurations, its limited ATA command set vs. SAS's rich SCSI command set, signal integrity challenges with 6 Gbps (and beyond) SATA, inferior command queuing as a result of its shallow queue depth, and limited error handling and diagnostic capabilities. To mitigate some of these shortcomings, storage array vendors have used SAS-to-SATA bridges and SATA multiplexers (MUX), which are basically interposers that sit between SATA drives and the SAS backplane to enable higher reliability, dual porting and better diagnostics for SATA drives. "We offer a $30 to $40 active-active SATA MUX adapter option for our VTrak RAID and JBOD subsystems to provide dual porting for SATA drives," said Ray Bahar, vice president of the Americas at Promise Technology Inc.

While the interposer approach has been an acceptable workaround to compensate for SATA's shortcomings, it's questionable if SATA will survive in enterprise storage beyond 6 Gbps. Seagate has been offering so-called nearline SAS drives that compete with SATA drives in the enterprise space. Designed for high capacity and lower cost, nearline SAS drives are identical to SATA drives with the exception of the interface; in other words, they peak at 7,200 rpm and 2 TB capacity. Contrary to smaller capacity, mission-critical tier 1 10K to 15K high-performance SAS drives, nearline SAS drives are intended for tier 2 applications and for more static data. Not burdened by the limitations of the SATA interface, nearline SAS drives perform significantly better than comparable SATA drives. "We see at least a 30% performance improvement with identical drives from the same vendor by simply having a SAS interface," said Howard Shoobe, senior marketing manager at Dell Inc. "Although it's the same disk media, SAS can run faster because of better error handling, better command queuing and dual porting."

In comparison to SATA drives in systems with SAS-to-SATA bridges, nearline SAS drives still fare approximately 5% better, according to Ian Williams, executive director, nearline storage solutions at Seagate. On the downside, nearline SAS drives are substantially more expensive than SATA drives; the cost has slowed adoption as many storage system vendors take a wait-and-see position. "At present, nearline SAS drives are 30% to 35% more expensive than equivalent SATA drives and, as a result, it is more cost effective for us to use a MUX adapter with SATA drives to achieve high availability," Promise Technology's Bahar said.

Nearline SAS drives are likely to eventually force SATA drives out of higher-end enterprise storage systems where performance and features matter more than cost. But for lower-end arrays where cost is king, the price of nearline SAS drives needs to come down to that of SATA drives, and that won't happen until more drive manufacturers offer SAS drives. Furthermore, "the personal computing market would need to shift away from SATA drives and move to USB or other interface options," said Steve Gardner, director of outbound marketing at LSI Logic. "As long as the PC market maintains SATA drive volumes, there will always be some enterprise users integrating the technology to get the absolute lowest cost."

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 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.

SAS adoption landscape

While SAS-2 components from disk drives and expanders to HBAs and RAID controllers 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. The main reason is a rigorous and time-consuming qualification process of the various SAS-2 pieces in enterprise-grade arrays to ensure that all components, firmware and software function properly. We'll see some entry-level and midsized storage systems transition to SAS-2 in 2010, but high-end systems based on SAS-2 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, MN-based StorageIO Group.

Here are some data storage vendors' SAS-2 products and 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-sized businesses (SMBs) 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," Dell's Shoobe said.

Hewlett-Packard. 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's 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 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.

Summarizing SAS-2

With SAS-2, Fibre Channel as a disk interface seems doomed to disappear, but the transition will take some time. We'll see the adoption of 6 Gbps SAS in entry-level arrays pick up speed in 2010, with midrange and high-end arrays following at a more cautious pace. Although the FC disk interface is likely to expire, there's still plenty of life left in SATA, which should continue as the disk interface of choice in entry-level arrays for some time. 2

BIO: Jacob Gsoedl is a freelance writer and a corporate director for business systems. He can be reached at jgsoedl@yahoo.com.

This was first published in January 2010

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