A closer look at SAS
SCSI (or Small Computer System Interface) drives have brought high-performance storage to servers and storage sub-systems for over two decades. But traditional SCSI drives are attached to their host computer using a bulky ribbon cable (with multiple data and control signals traveling simultaneously across different wires). By comparison, SAS drives attach to their host computer using a much smaller cable based upon a single (serial) data wire. SAS is also an industry-standard interface, defined by ANSI/INCITS 376-2003.
Serial performance improvements
SAS interfaces can support faster data transfers than parallel SCSI cables. Where popular parallel-type Ultra320 SCSI drives can achieve burst data transfers across the interface reaching 320 megabytes per second (MBps) (using a 16-bit data path), initial SAS drives can reach 3 gigabits per second (Gbps) (over 375 MBps) across the interface, with performance expected to scale to 10 Gbps into the future. Greg Schulz, senior analyst with Evaluator Group, explains that SAS technology "is the next evolution -- not generation -- of SCSI disk drives." He says that speed improvements are due to fewer wires and superior cable designs. Fewer wires produce far less electrical noise and can be shielded effectively, thus allowing for higher data rates. SAS cables are also connected using simpler (but more powerful) interface components.
Support for SATA and SAS drives
There are two other key attributes to consider. SAS drives use the existing SCSI command set, offering backward compatibility with existing SCSI software (e.g., device drivers and applications), allowing for SAS hardware implementations and upgrades without major software infrastructure changes. In addition, SAS controllers also interoperate with serial ATA (SATA) drives, supporting both SAS and SATA drives on the same controller. Schulz notes that this cross-compatibility may be an ideal solution for companies seeking to mix midrange SAS and low-end SATA drives in the same storage system -- allowing data to be moved from SAS devices to lower-performance, less-expensive SATA drives as the data ages. According to Brian Garrett, analyst at Enterprise Strategy Group, "It used to be ATA, SCSI and FC, but now it's SATA, SAS and FC."
SAS vendor offerings
It's important to emphasize that SAS is an "interface" -- the circuitry and interconnection used to move data to or from the drive. SAS has virtually no impact on the mechanical design or operation of the drive itself (e.g., the platters, heads or other drive components). According to Schulz, Seagate Technologies is an early adopter and leader in SAS technology, and their Cheetah line of 15K RPM enterprise hard drives are available in 2 Gbps FC, SAS and traditional Ultra320 (parallel) SCSI interfaces. The availability of multiple interface versions allows IT professionals to select the drive to meet particular storage needs: for example, FC for high-end near-line storage, and SAS or Ultra320 for midrange storage and SATA for high-volume archival storage duties.
Both Schulz and Garrett also point to Hitachi Global Storage Technology , Maxtor and Fujitsu as the other major SAS drive supporters. Schulz notes that Western Digital is focusing on SATA and other desktop interface products (but SAS products may appear at some point in the future).
Don't choose by specs alone
Any hard drive selection includes a serious consideration of technical criteria including the following: rotational speed (RPM), storage capacity (in GB), reliability (expressed in mean time between failure or MTBF), read/write time, sustained data transfer speeds, drive size (form factor), power demands and other features like onboard management for better data integrity and reliability. SAS users also must consider features like command tag queuing (CTQ), and grapple with security issues such as zoning (limiting the storage that a given server can access). But Schulz underscores the desperate need to avoid performance envy, "Don't be enticed by drive features or specifications. Pick the drive features to accommodate your storage needs rather than the other way around. You don't pick a car because of its tires." Analysts agree that any move to SAS products should not be made simply because of the desire for new technology, but rather to address serious business concerns such as storage performance needs, data availability or cost.
Implementations starting to trickle out
At the system level, Schulz says that current options are slim, "No major storage or server vendor are shipping SAS disk drives yet in a major storage system. However, HP is shipping SAS disk drives in their MSA50 entry-level storage subsystem. Both HP and IBM are shipping SAS disk drives as part of blade server products as well." Still, analysts expect SAS implementations to expand greatly in the next 6-12 months, and it's important to choose drives from the viewpoint of your overall storage vendor. Although SAS drives from different drive manufacturers should be fully interoperable, experts suggest avoiding any type of mix-and-match scenario. That is, if you select a SAS drive for a particular array, then the SAS drives used in that particular array should all be the same make/model from the same manufacturer. Although this is an extremely conservative viewpoint, it eliminates the possibility of incompatibilities between different SAS drives -- removing one important troubleshooting point if trouble strikes.
However, smart IT professionals also like to establish alternative sources and "standby plans" – just in case a manufacturer has trouble sourcing replacement drives. The solution is often to work with the storage subsystem's manufacturer (such as EMC or HP) to learn what drive makes and models are qualified for use in a specific storage product. Then, if one drive becomes unavailable for any reason, suitable alternative drives can be installed quickly.
SAS technology and trends
There's a tendency to examine the successes and failures of any product or technology, but Schulz and Garrett say that SAS is simply too new -- noting that SAS products have only really been available since early 2005, and the market has yet to fully emerge. And while SAS adoption has been a little slower than expected, it certainly comes as no surprise to Schulz, "Many forecasts suggested that SAS would be more dominant by now, but high-end enterprise storage arrays are managed in a conservative or cautious manner -- often slowing acceptance."
Storage directions for blades
Blade servers are frequent recipients of SAS drives, though the experts diverge on the future role of SAS drives in the blades themselves. Garrett sees SAS technology as a way to eliminate local blade storage, "One of the problems you have in a blade server is that there's not much room for storage." Many blades may include one or two drives, "But the goal is to get the storage off of the blade." He suggests that SAS drives can easily find a role in external rack storage for blade platforms, allowing the elimination of drives from individual blade units, which make them easier to swap or maintain.
Schulz takes the opposite view, expecting SAS drives to expand their role as high-performance storage options in blade and 1U servers, and follow the overall industry trend toward increased storage capacity with smaller form factors, "SAS drives are shipping in 3.5" form factors now, but 2.5" form factor drives are starting to appear for enterprise use." By shrinking a hard drive, it's possible to fit more drives in a given area (increasing the overall storage capacity of an array or other storage platform). Smaller drives typically demand less power, conserving energy and reducing heat.
Dispelling SAS myths
Schulz also seeks to dispel a few myths about SAS technology. The first misconception is that 2.5" form factor drives are SAS-only. He says, "Size does not dictate the interface." Small form factor drives can easily support SAS, FC or SATA interfaces. The choices of form factor and interface are entirely independent -- determined solely by the product offerings of each drive manufacturer.
Some prospective SAS adopters also think that they need to make a choice between SAS or SATA drives. "It's not an 'all or nothing' proposition", says Schulz. There's absolutely nothing wrong with a mix of SAS and SATA drives -- especially where the SAS controller accommodates both drive types. The key is to decide what mix of drive types will best meet the storage needs of your enterprise.
Ultimately, Garrett says that the future for SAS is bright, "In general I see SAS maturity following the same path as all storage interfaces -- make it work, then make it go faster, then make it cheaper -- all while adding high availability and security features as needed on an application-by-application basis." But Schulz reminds us that the emergence of SAS drives does not displace the importance of FC technology for high-performance, mission-critical storage applications into the future, "There are lots of applications for Fibre Channel", says Schulz, "Fibre Channel drives are not going away in the near- or mid-term."