Get a performance boost with Serial Attached SCSI (SAS)

In this article, Scott Lowe takes a detailed look at the specs of SAS and compares its performance with current SATA standards.

In my last article, you learned about a whole lot of SCSI—torrid history and all. As you know, like ATA, SCSI is a parallel technology with multiple devices included on each data channel. Also like ATA, SCSI has been overhauled with its own serial technology, aptly named Serial Attached SCSI (SAS).

In short, SAS is the next generation of SCSI and includes substantial improvements, including higher transfers speeds, better scalability and improved reliability. Some call SAS a melding of the best of SCSI and Serial ATA. In fact, SAS uses the same connector type as SATA, which might make it a little more difficult to identify a drive, but can help reduce costs for manufacturers. Further, it provides administrators with a way to mix-and-match drive types for different applications, as needed.

Why the change from parallel to serial on something as robust as SCSI? Simply put, older parallel storage technologies were beginning to face an end-of-the-road scenario in performance improvements. As such, the significant transition from parallel to serial solutions is taking place. SAS provides storage administrators with a point-to-point serial, manageable storage solution.

SAS specs comparison

It's hard to talk about SAS without comparing it to the current SATA standards. The initial SAS standard provides a transfer rate of 300MB/s or 3Gb/second, while the most prominent SATA standard limits this transfer rate to 150MB/s. SATA-II (also, SATA-IO), increases this transfer rate to 300MB/s, which brings it more in line with the current SAS spec. However, the next SAS specification, for which drives should start appearing next year, brings SAS up to 600MB/s, keeping it ahead of its SATA cousin. SAS is eventually expected to reach a speed of 1,200MB/s. That's fast!

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There are a lot of questions regarding other differences between SATA and SAS. Simply put, all of the things that make SCSI a better choice than ATA drives for the enterprise make SAS drives a better choice than SATA for the enterprise. The SCSI command set is extremely robust and has been in use for decades in mission-critical applications. SCSI includes such features as command queuing, which allows the controller to run commands in the most efficient order possible for increased performance. In SCSI systems, the job of handling the flow of data between the disk system and the computer is handled by a dedicated controller. In most SATA systems, the CPU has to assume this responsibility, meaning that processing cycles that could, for example, go to running a database, need to be used to manage storage.

Like the venerable SCSI technology and SATA, SAS also supports hot plugging of disks, which is critical for maintaining a highly available environment. Further, SAS is a full-duplex system, while SATA continues IDE's tradition of half-duplex communication. Thus, SAS systems can provide roughly twice the throughput of a similarly specified SATA system. Further, while there are a very few SATA drives that spin faster than 7,200 RPM, many vendors are providing or will provide disks that run at 10,000 and 15,000 RPMs, also resulting in a faster disk system.

There is another key different between SATA and SAS: cost. Like ATA and SCSI, SATA and SAS disks have very differences prices. SATA disks are inexpensive, while SAS disks aren't cheap. However, for bulletproof storage, and additional storage features, many enterprises will continue to use SCSI's successor in the data center, and, from the points above, rightfully so.

Since SATA and SAS drive connectors are pin-compatible, it stands to reason that their cables would be similar. However, SAS cables can be up to 6 meters in length, six times longer than SATA's limit of 1 meter. As stated, however, the ends are the same.

When it comes to comparing SCSI and SAS side-by-side, besides speed, SAS has one significant advantage over SCSI. With SCSI, when different types of drives were connected to the chain, all of the drives ran at the speed of the slowest disk. With SAS, this is no longer the case. Even with drives of different types, each drive can run at its native speed. Speaking of multiple drives, SAS allows up to 128 drives to be attached simultaneously. Through the use of expanders, this number jumps to 16,000, making SAS uniquely qualified to handle even the largest data center's needs. Further, SAS disks can handle requests from multiple SAS controllers, further enhancing its expansion capabilities.

While there are similarities between SATA and SAS, when it comes to raw performance in the data center, SAS is the clear winner. However, SATA and SAS are very much complementary technologies. SATA is extremely well-suited to the desktop and to near-line storage requirements, and even enterprise storage requirements for smaller organizations. SAS, on the other hand, is more than capable of picking up where SCSI left off in the enterprise.