The impact of SAS
Industry experts generally believe serial attached SCSI (SAS) will be adopted fairly quickly, as evidenced by a steady stream of products already employing the emerging technology. It's anticipated that the primary uses of SAS technology will be:

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SCSI replacement. Storage and server OEMs, as well as enterprises, will implement SAS wherever parallel SCSI is currently used.
Tiered storage. Compatibility between serial ATA (SATA) and SAS drives enables tiered storage within a single array.
Blade servers. The 2.5-inch disk form factor will allow OEMs to pack more capacity into a blade unit.
High-performance storage. The 2.5-inch disk allows OEMs and enterprises to squeeze more fast--15K rpm--disks into a single cabinet that boosts performance because of the greater number of spindles.
One thing SAS is unlikely to do immediately is replace Fibre Channel (FC). "This is not about FC replacement," says Arun Taneja, founder, Taneja Group, Hopkinton, MA. "I don't think SAS will replace FC anytime soon--this is about defending SCSI." SCSI needs SAS if it's to have a future. "FC is not at the end of its life," adds Greg Schulz, senior analyst, The Evaluator Group, Greenwood Village, CO. "But if you look five years ahead, the future for FC starts to get cloudy."
With a wave of new serial attached SCSI (SAS) products about to be released, some storage vendors are ratcheting up campaigns that promote SAS as the inevitable next step in the evolution of SCSI. Vendor interest is clear: to drive new sales and provide an alternative to Fibre Channel (FC) for organizations refreshing their disk storage. Some users are also jumping on the SAS bandwagon, although few have actually gotten their hands on SAS products.

SAS brings some FC-class capabilities and other advances to SCSI, including:

  • Faster disk rotation speeds--10K rpm and 15K rpm
  • Dual ports--the ability to connect the drive to two communications links
  • Serial cabling--more flexible; easier to work with cabling to the disk
  • Duplex operational mode--bidirectional access to the disk
  • Greater throughput--3Gb/sec (6Gb/sec duplex) and slated to double quickly
  • 2.5-inch form-factor option--allows more drives to be packed into a storage cabinet
  • Low price, transparent transition--prices expected to be comparable with enterprise SCSI drives

"We want the 10K rpm and 15K rpm disks that SAS will provide. As our databases grow, we want that extra performance," says Josh Wopperer, network engineer at UVEST Financial Services, Charlotte, NC, a provider of third-party brokerage and investment services to financial institutions in 42 states. Currently, the company stores its databases on an EqualLogic Inc. SAN populated with slower (7,200 rpm) serial ATA (SATA) disks.

With 500 people hitting UVEST's databases simultaneously, the SATA drives are capable of handling the traffic. Looking ahead, however, Wopperer can imagine a time when 2,000 users are accessing the databases and running complex queries. At that point, he anticipates performance problems and faster drives could make a noticeable difference. For storage managers, the switch to SAS from SCSI can be a transparent transition, which is the approach most are expected to take, or they can also devise strategies on how best to deploy some of the new features such as dual porting, increased drive density and high disk speeds.

"In the first phase, IT can treat SAS simply as a parallel SCSI replacement," says Harry Mason, president of the SCSI Trade Association (STA) Board of Directors and director of industry marketing at LSI Logic Corp., a Milpitas, CA-based maker of SAS components. In this case, SAS is a no-brainer because vendors are committed to maintaining SAS pricing that's comparable with existing SCSI and ensuring plug-and-play compatibility.

SAS debut
An interchangeable serial interface with SATA opens up even more possibilities. Because SAS controllers are compatible with SATA drives, "we will be able to mix SAS and SATA drives within the array," says Paula Long, co-founder and vice president of products and strategy at Nashua, NH-based EqualLogic. EqualLogic plans to incorporate SAS drives in its products by early 2006. Storage administrators like Wopperer could then implement a tiered storage strategy within the array or SAN, sending high-demand data and large databases requiring top performance to the faster, more costly SAS disks while steering the rest of the data to less-expensive, lower performing SATA disks.

It's been a four-year journey for SAS as the storage industry has scrambled to find a replacement for parallel SCSI, which was hitting the wall in terms of throughput and device connectivity. By switching from a parallel interface to a serial interface, and then making it backward-compatible with the serial interface for ATA drives, storage vendors are able to shoot past the 15-device limit for the parallel SCSI bus and double the 320MB/sec top-end performance of parallel SCSI. They can also make a number of other good things happen.

The smaller, more flexible serial cabling lets manufacturers pack more drives into a cabinet and, at the same time, improve air flow and cooling capabilities. For even greater density, SAS drives will be available in two form factors: 2.5 inches as well as the typical enterprise 3.5-inch disk drive. "The 2.5-inch drive is one-third the overall size of a 3.5-inch disk, which means you can put more disks in the same physical space," says Mike Chenery, vice president of advanced product engineering at Fujitsu Computer Products of America Inc. The availability of 2.5-inch form-factor disk drives creates some interesting opportunities, although most corporate storage managers who opt for SAS as their SCSI replacement will go with the familiar 3.5-inch disks.

"Some of this is a defensive measure by vendors. They need things like dual porting to compete with Fibre Channel disk drives," says Arun Taneja, founder of the Taneja Group, Hopkinton, MA. Dual ports, for example, ensure high availability by providing an alternate connection to the drive.

This was first published in September 2005

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