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Serial Attached SCSI (SAS) is becoming the new storage standard in the corporate environment. Being new, SAS brings many questions to the mind of IT Managers and others. This FAQ will attempt to answer the most common of these questions.

What is SAS (Serial Attached SCSI)?

Serial Attached SCSI (SAS) is an evolution of SCSI to a much faster serial interface. This satisfies the enterprise storage requirement of scalability, performance, reliability and manageability. It also provides an infrastructure for both SAS drives and SATA disk drives. Since the SAS interface is compatible with SATA devices, this provides users with almost unlimited choices for server and storage systems structure. SAS drives, with their outstanding speed and reliability can be used for critical online storage while SATA drives, with their higher capacity and significantly lower cost, can be used for less critical storage requirements.

Why was Serial Attached SCSI developed?

SAS was developed to solve future direct attach storage requirements. It provides compatibility with SATA and offers compatibility with SCSI as well as SCSI reliability, performance and manageability.

Aren't parallel interfaces faster than serial interfaces?

Previously, parallel interfaces were preferable to serial because their multiple data paths allowed for greater throughput than the single data path of serial interfaces. New developments in VLSI technology, however, have enabled serial interfaces to make dramatic speed increases. Serial interfaces do not have the complex timing and interference issues that hinder parallel interface development. Serial Attached SCSI features higher throughput and greater potential for advancement in the future as compared to parallel SCSI.

Is parallel SCSI now obsolete?

No. Parallel SCSI has played a fundamental role in enterprise data storage and will continue to do so. Serial Attached SCSI, however, is a strong complement to and matches the excellent reliability and robustness of parallel SCSI, while significantly expanding SCSI storage in terms of speed, scalability and flexibility. For instance, parallel development has stopped at 300GB SCSI hard drives while we already have in stock 450GB SAS drives and 1TB SAS drives are not far off.

Will migrating from parallel SCSI to SAS be difficult?

When Seagate and other companies collaborated to define Serial Attached SCSI standards, ease of migration was a primary consideration. SAS was engineered to be compatible with existing SCSI command sets thus preserving your investment in storage management and enterprise application software.

What is the difference between Serial Attached SCSI (SAS) and Serial ATA (SATA)?

Serial Attached SCSI is an enterprise storage solution that delivers the superior performance, reliability and scalability demanded in mission-critical applications. Serial ATA is primarily intended for desktop applications and suitable for use in low work load non-mission-critical environments where low cost is a high priority. This allows the development of tiered SAS and SATA storage environments where SAS is used for operating the mission-critical applications and SATA is used for providing huge amounts of storage at a low cost.

What are Small Form Factor SAS Drives?

Small form factor (SFF) 2.5 inch SAS enterprise hard drives are a new class of storage solution that feature a 70% smaller physically size and up to 40% less power consumption and heat generation while maintaining true enterprise level performance and reliability.

Because of their smaller size and lower power consumption, SFF drives have found applications in Blade Servers, storage consolidation and data center environments. SFF drives are available in capacities ranging from 36GB to 146GB with higher capacities on the horizon.

Corporate IT departments must store and manage huge amounts of business data. There are a multitude of avenues that the IT department can take to satisfy these requirements. Serial-attached SCSI (or SAS) technology is emerging as a cost effective solution, allowing companies to move their data to drives that offer significant storage capacity at a reasonable cost, while maintaining high performance. SCSI drives (or Small Computer System Interface Drives) have been the standard for high-performance storage for over twenty years. Traditional SCSI drives are attached to their host computer using a bulky ribbon cable. SAS drives, on the other hand, attach to their host computer using a much smaller cable based upon a single (serial) data wire.

Performance Improvements

SAS can support much faster data transfers than parallel SCSI. 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. These speed improvements are due to fewer wires and superior cable designs since fewer wires produce far less electrical noise and can be shielded effectively, thus allowing for higher data rates.

SATA and SAS Drive Support

There are two other advantages to consider when contemplating a switch to SAS hard drives. SAS drives offer backward compatibility with existing SCSI software (e.g., device drivers and applications) since they use the existing SCSI command set allowing for SAS hardware implementations and upgrades without major software changes. Secondly, SAS controllers can also operate with serial ATA (SATA) drives allowing both SAS and SATA drives to be on the same controller. This allows a company to use the higher performance SAS drives for mission critical data where high speed access is indicated and use lower performance but higher capacity SATA hard drives for less critical data requirements.

Interoperability

SAS drives from different drive manufacturers should be fully interoperable. Some users, however, are avoiding any type of mix-and-match scenario and insist that the SAS drives used in that particular array should all be the same make/model from the same manufacturer. This is an extremely conservative position but it does eliminate the possibility of incompatibilities between different SAS drives. This stance can create difficulties if the selected manufacturer has trouble providing replacement drives. The solution is to work with the storage subsystem's manufacturer to determine what drive makes and models are qualified for use in a specific storage product. Then, if one drive becomes unavailable, a suitable alternative drive can be sourced and installed quickly.

In Conclusion

Serial Attached SCSI represents both the next generation in the ongoing evolution of SCSI performance and a significant new advance in the architecture of I/O interfaces. The advantages of SAS are compelling. The high levels of performance and availability inherent in SAS have positioned it as a replacement for parallel SCSI as the predominant interface for both enterprise-class and cost-sensitive applications. SAS storage subsystems have powerful and flexible expansion capabilities that allow for significant future growth and expansion.