Serial Attached SCSI Ushers in Sea Change

SAS will increase scalability, manageability of external SCSI devices.

Expect the scalability and manageability of external SCSI storage to increase this year with the coming of servers and storage systems equipped with Serial Attached SCSI. SAS will be the eventual successor to Parallel SCSI and is set to become the de facto direct-attached storage interconnect.

Support for Parallel SCSI will not go away any time soon, but now is the time for IT managers to become familiar

with SAS technology. Indeed, according to members of the SCSI Trade Association with whom eWEEK Labs recently spoke, most new servers will come with SAS controllers by the end of next year.

SAS is being positioned in the midmarket—between expensive, high-performance Fibre Channel storage and low-cost, high-capacity SATA (Serial ATA) storage.

SAS—which uses point-to-point connections to link controllers to individual disks—should allow for faster performance and better scalability than Parallel SCSI, where as many as 16 devices can reside on a single cable. Point-to-point connectivity makes troubleshooting easier because IT managers will not have to guess which device is causing problems.

The current generation of SAS runs at 3G bps full-duplex. The new SATA II specification is 3G bps but runs only half-duplex. The next generation of SAS, due late next year or in early 2007, is expected to run at 6G bps full-duplex.

Another major benefit of SAS systems is that they are compatible with SATA drives, providing the ability to mix and match SATA and SAS drives in the same enclosure. This will allow IT managers not only to leverage each technologys benefits but also to improve the cost efficiency of storage units since RAID systems will not have to be filled with more expensive SAS drives.

SAS drives, like Parallel SCSI drives, feature high rotational speeds (up to 15,000 rpm) and will be rated for higher-duty cycles. This makes SAS a good choice for application storage, while SATA is better for workstation and nearline storage. With mixed-drive enclosures, IT managers will be able to effectively maintain a fast storage tier for application performance (running SAS) and inexpensive nearline storage (running SATA).

Unlike Parallel SCSI, where performance defaults to the speed of the slowest device in a chain, SAS has rate-matching capabilities that allow each connected device to run at its optimal speed.

Cable length for SAS will top out at about 6 meters, making it a good choice for external storage but not a challenger to Fibre Channel for long-range storage or for connecting servers to a SAN (storage area network).

/zimages/1/28571.gifTo read more about Serial Attached SCSI, click here.

Vendors including Hewlett-Packard Co., Seagate Technology Inc. and Maxtor Corp. are putting the finishing touches on their products. The SCSI Trade Association is scheduled to host a SAS Plugfest in September at the University of New Hampshire InterOperability Laboratory. At the event, vendors SAS products will be tested for, among other things, SCSI backward compatibility, SATA compatibility and multivendor interoperability.

At an April Plugfest, participating vendors were able to build relatively large storage systems, with as many as 110 drives.

Based on this and other previous events, association members said that they expect to see good interoperability among vendors products, paving the way for the upcoming technology transition.

Senior Analyst Henry Baltazar can be reached at

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