Simply SAS: Power of SCSI, Ease of SATA

Authors: Franco Castaldini, Sr. Product Marketing Manager
and Kent Bransford, Sr. Technical Editor, Seagate Technology

“If you fear change, leave it here!” may be the lighthearted counsel of cash-strapped baristas, but IT professionals know that proactively adopting change is serious business, a key to maximizing system performance and efficiency. Alas, pioneering storage technologies are often seen as double-edged swords, their gains coming at a stiff price in terms of purchase or deployment and integration.

Fibre Channel (FC) is a prime example of this phenomenon, coupling remarkable throughput and scalability with daunting cost. Beyond the substantial investment required for FC infrastructure itself, Fibre Channel solutions also entail complex deployment and management procedures that require highly specialized (and expensive) expertise not found in many IT departments. Hence FC has typically been limited to enterprises with the needs (and deep pockets) to justify its use.

Serial Attached SCSI (SAS) was specifically architected to avoid such pitfalls. By leveraging the best features of two familiar storage interfaces, SCSI and Serial ATA (SATA), SAS makes it surprisingly easy to upgrade existing SCSI servers and storage arrays. While delivering performance and scalability that rival Fibre Channel, SAS is far simpler (and less costly) to buy, install and maintain.

Making the Connection: Easier is Better
A close serial cousin of SATA, SAS ensures users can forget about the quirks and idiosyncrasies of parallel SCSI. No more SCSI address issues, no more termination challenges, no more worries about total cable length on a shared SCSI bus. Simply replace parallel SCSI controllers with SAS host bus adapters and backplanes as needed, attach the new (hot pluggable) SAS drives and go.

Such ease of deployment comes in part from SAS’ automatic drive discovery, which eliminates the need for manual address configuration. By contrast, Fibre Channel storage area networks (SANs) typically require that fabric addresses be specified in FC management software.

Furthermore, SAS makes it possible to consolidate online and nearline storage on a single interface. Before SAS, two separate infrastructures were needed; parallel SCSI or FC for high-performance (online) applications, and SATA for capacity-oriented (nearline) applications. Now SAS compatibility with SATA enables both drive types to reside in the same infrastructure (even the same enclosure). Beyond streamlining purchasing, inventory and maintenance, this also ensures the freedom to choose the optimal drive for any application.

What You Need to Know: You Already Know
Utilizing the same SCSI commands as its parallel predecessor, SAS employs three different protocols to transport information over its serial interface: Serial SCSI Protocol (SSP), SCSI Management Protocol (SMP) and Serial ATA Tunneling Protocol (STP). SSP communicates with SAS devices and existing SCSI software, SMP manages SAS’ point-to-point topology, and STP enables SAS controllers to identify and communicate with SATA devices.

Reflecting a key goal of the SAS standards committee, existing SCSI software and middleware require only minor SMP and STP modifications to function in SAS storage environments. Thus SAS protects current investment in storage management and enterprise application software, integrating the same SCSI command sets that underpin today’s enterprise.

In the critical area of user-friendliness, SAS enables storage managers to continue drawing on their vast SCSI intellectual capital, garnered over many years of configuring, deploying and maintaining SCSI storage solutions. By ensuring IT professionals can tap this wellspring of SCSI knowledge and experience, SAS vastly simplifies its deployment and maintenance.

Bottom line: SAS retains the familiar strengths of parallel SCSI (enterprise-class reliability, mature and robust command set, enormous installed base of SCSI applications), while delivering performance (3.0 Gb/s, w/roadmap to 12 Gb/s) and scalability (up to 16K devices) that rivals costly and complex Fibre Channel solutions. Add its compatibility with SATA and SAS is clearly in a class by itself.

It’s no surprise that many IT professionals view new technologies with a healthy degree of skepticism: the vaunted capabilities of even the most revered storage interface can quickly pale in the harsh light of everyday usability. From the outset SAS was designed to leverage familiar technologies (SCSI, SATA), rather than reinventing the storage wheel. The result is a uniquely accessible enterprise interface with an unprecedented blend of power and ease of use.

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