ANSI Agrees on Serial Attached SCSI Standard

Despite being such a nascent technology, serial-attached SCSI , or SAS, has been bubbling with development among storage vendors anxious to tap into next-generation storage market in the past year.

Now it has earned the blessing of the SCSI
Trade Association (STA) and the International Committee for Information
Technology Standards (INCITS), operating
under the internationally-recognized American National Standards Institute
(ANSI), which approved SAS Wednesday as a
stable standard.

SAS is the natural extension of SCSI, a parallel interface
standard used for hooking peripheral devices to computers. But SCSI has been reaching its limits in terms of the scale and complexity of infrastructure it can handle.

This has become a problem because many customers are seeking more powerful storage systems that can handle more users. Vendors are anxious to supply them and members of the STA, including hardware makers Intel, Adaptec, Maxtor, HP and a number of other vendors, have been working to come through.

SAS is considered far superior due to its more modern software architecture and hardware topology, which meet enterprise-class scales of capacity,
density and scalability, said Martin Czekalski, interface architecture
initiatives manager at hard drive maker Maxtor, one of the STA member
companies.

“SCSI was great for awhile. It carried everything forward and had backward
compatibility, but when vendors wanted to migrate and evolve they found
there were limitations in parallel technology,” Czekalski told
internetnews.com. “The underlying technology in SCSI is similar to
SAS, but SAS offers an increase in ASICS, integration, silicon. It’s more
cost-effective, and provides better performance.”

For example, Czekalski said the cable sizes are much thinner in serial
technology than its parallel brethren, which normally runs a cable with 68
wires, or “pins.” These wires would have to go to every device, or drive.
With SAS, “all you need is 4 wires, two to transmit and two to receive so
you don’t have to don’t daisy chain the same wires into every drive,”
Czekalski said, calling the parallel approach the electrical equivalent of
driving on a crowded freeway.

The architect said wires won’t get confused with one another with serial
technology. Instead of point-to-point connections, the SAS cables are
threaded through an “expander,” which acts as a sort of switch — “sort of
like an Ethernet switch, which handles multiple connections.”

This different topology allows for superior performance, Czekalski said.
While SCSI wires run at 20 megabytes per second, SAS runs each wire at 300
megabytes per second. In addition, the switch architecture of SAS allows
“multiple lanes,’ allowing switches to cascade to thousands of drives. This
makes it possible to install much larger systems and employ more devices.

SAS also allows different kinds of drives into same system, making it
possible for high-performance devices and applications to work in the same
storage pool with low-cost, smaller devices. Accordingly, Czekalski said the
alignment of SAS and Serial ATA technologies to serve
storage users, will provide more flexibility to the storage industry.

The standard is the culmination of a lot of late nights and hard work over
the last year and a half by the STA and its members, which along with Maxtor
includes Intel, Adaptec, LSI Logic, HP and a number of other vendors.

OEMs began preparing products for market some six months ago and several
hardware vendors, particularly hard drive makers, have demonstrated working
SAS products and are now beginning to announce product availability.

For example, Hitachi Data Systems said Wednesday that its Hitachi GST is the
first hard drive vendor to demonstrate SAS interoperability. Last month,
Fujitsu unveiled
its first SAS hard drive interface.

The STA has planned several plugfests throughout the year to assist OEMs in
preparing their SAS products for market