SAS – To Infinity and Beyond

Author: David So, Strategic Marketing Manager
LSI Logic

So maybe it took a little longer than some of us forecasted or hoped. For others, maybe it happened faster than they expected. But regardless of what any of us thought back then, it is now undeniable that Serial Attached SCSI (SAS) has finally landed, and in a big way.

SAS development time-line
Initial work on the SAS specification began in 2001, as the next evolution of SCSI technology, and it was ratified as an ANSI standard specification in late 2003. The first public demonstration of SAS prototype components actually took place in July 2003. And here we are, just over three years later. During those three years, numerous companies have announced production of SAS components as well as SAS promotional and educational events.

In an effort to ease the industry’s transition to SAS, the SCSI Trade Association (STA) has already sponsored six SAS plugfests at the University of New Hampshire InterOperability Laboratory. These plugfests, which have included SAS controller and expander IC vendors, host bus adapter (HBA) vendors, hard disk drive vendors, server OEMs, disk drive enclosure vendors, cable manufacturers, and protocol test/analysis equipment manufacturers, have been a tremendous cooperative effort in ensuring vendor-to-vendor interoperability before SAS is fully introduced to the market.

Although some companies announced SAS-based systems in 2005, Intel’s announcement in May 2006 of its new dual-core Xeon platform was a major boon to SAS, as most of the server OEMs worldwide had aligned the launch of their SAS-enabled products with this new Intel platform. SAS-based data center solutions are now readily available to enterprise and SMB customers alike, with the promise of improved scalability and greater I/O performance. Not only are there systems with SAS controller ICs integrated on server motherboards, but dozens of other platforms are offered with SAS HBAs or RAID adapters and SAS disk drives as upgrade options from current Ultra320 SCSI solutions.

SAS features and advantages
As has been discussed in previous issues of Serial Storage Wire, SAS enables scalability in direct-attached storage (DAS) solutions far beyond the capabilities of parallel SCSI. With support for over 16,000 end devices, SAS meets the ever-growing storage needs of today’s and tomorrow’s enterprise data centers. Various companies have already announced SAS enclosures with support for 10, 12, 16, and 24 disk drives, with other configurations sure to follow in the coming months. And each of these enclosures can be cascaded through their integrated SAS expanders for seamless scalability, creating topologies once unimaginable for DAS environments.

SAS also supports unprecedented flexibility, capable of connecting to both SAS and Serial ATA (SATA) disk drives. This allows customers to select the best disk drive configuration for their specific needs – SATA disk drives for reference, nearline-type applications where greater capacity and lower cost are primary concerns; and SAS disk drives for those applications requiring higher performance and reliability.

SAS is faster
Without fully understanding SAS technology, many may think that Ultra320 SCSI, at 320MB/s, would be faster than SAS at 3Gb/s (or 300MB/s). But in actuality, SAS performance well exceeds that of Ultra320 SCSI. Unlike parallel SCSI, where all devices on a bus must share its bandwidth, SAS supports point-to-point connections, where each device has 3Gb/s of dedicated bandwidth. And x4 wide ports have become the standard external SAS connection, aggregating the bandwidth of each 3Gb/s link into one 12Gb/s I/O super highway. This results in system throughput that is 200-300% greater than what is achievable with Ultra320 SCSI.

The future of SAS
Because these advantages simply cannot be ignored, SAS will quickly take hold in the enterprise server market, for both internal and external storage attach. Many vendors are beginning to implement SAS infrastructure for the drive-attach side of their external storage systems. This allows them to populate their systems with either SAS or SATA disk drives (or potentially a combination of both) while providing the ability to add additional storage capacity as needed. But these advantages will also ensure that SAS is no longer confined to the realm of DAS solutions. SAS will eventually make its way onto the host interface of some of those same external storage systems. And with some recent industry discussion of potential switched SAS topologies, SAS Storage-Attached Networks (SANs) may eventually encroach on some of the Fibre Channel market, especially in the lower external storage price bands.

As of today, SAS-enabled server platforms are just starting to ramp into full production. One thing is certain; the future of SAS looks very promising, as it will quickly become the dominant storage interface for DAS solutions, ultimately replacing Ultra320 SCSI and signaling the beginning of the end for parallel SCSI technology. And as we are now just beginning to see, SAS will clearly have an impact on SAN environments as well – just how much of an impact remains to be seen.

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