SAS: Power and Simplicity Become One

Author: Bill Schilling, Marketing Director/Enterprise Segment and Kent Bransford, Sr. Technical Editor
Seagate Technology

After months of speculation and anticipation, Serial Attached SCSI (SAS) storage solutions are now entering the marketplace in force. Given its unparalleled blend of performance, scalability and flexibility, it should come as no surprise that SAS is quickly making converts among IT professionals throughout the enterprise.

Yet some storage managers have been hesitant to adopt SAS, wary of the connection challenges this powerful new serial interface might entail. Having invested years of sweat equity to master the idiosyncrasies of parallel SCSI, they are understandably reluctant to risk a similarly steep learning curve with SAS.

Happily, SAS was specifically designed with a vastly more straightforward and robust connection architecture than parallel SCSI. Gone are the tedious shared bus issues (SCSI ID settings, drive termination, total cable length constraints, etc.) that plague parallel SCSI storage environments; SAS banishes such concerns with the elegant simplicity of point-to-point architecture.

Easier is Better
Point-to-point cabling provides a discrete, dedicated signal path for every SAS device attached. Not only does this boost performance (each attached device is always immediately available on its dedicated bus, minimizing latency), it makes connecting SAS drives a remarkably uncomplicated and intuitive process.

As can be seen in Figure 1, the power and data connectors on a SAS drive are compact and easily accessible. Note the dual data ports; see Figure 4 for the significant benefits this arrangement enables. (Graphics copyrighted by SCSI Trade Association, courtesy of Molex.)


Figure 1


In its most basic form, SAS connectivity entails a single SATA-style SAS cable for each drive, linking to a matching SAS port on the host computer. Figure 2 illustrates cables that are attached to panel-mounted connectors, as well as cables connected directly to the drives. (Drive power is typically handled by a pigtail terminated with a four-pin connector.)


Figure 2


For many IT professionals, the simple type of setup shown in Figure 3 will be all they need to consider. SAS host bus adapters (HBAs) are theoretically capable of carrying dozens of connectors, but in practice most popular HBAs will feature either four or eight SAS ports. Thus an eight-port HBA can directly connect up to eight SAS drives, each port cabled to its respective SAS drive.


Figure 3


As noted earlier, the dual-port drive architecture of SAS provides compelling advantages. Figure 4 illustrates how the extra port on a SAS drive can be connected via a dual-port cable to ensure high availability and greater uptime. If one SAS host controller fails, the extra data port can maintain uninterrupted communication with the second controller. In addition, these two ports can be combined into a single “wide port” for even higher throughput.


Figure 4


And SAS flexibility doesn’t stop there. Small form factor (SFF) “multilane” host connectors can be used to provide multiple-drive connections in a more space-efficient design. As shown in Figure 5, four separate SATA-style SAS ports on the host controller have been replaced by a single SAS 4i multilane connector.


Figure 5




As IT managers face increasing pressure to cut storage costs, the prospect of deploying a new and unfamiliar storage interface may be daunting. Will it prove to be so time consuming and labor intensive to manage that any technical advantages it may offer are negated?

From the outset SAS was architected to deliver unprecedented ease of use while incorporating the most up-to-date serial technologies available. The result is a uniquely accessible blend of performance, scalability and flexibility. Rewriting the rules for enterprise storage value, SAS is simply better.

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