The Promise of Serial Attached SCSI Has Been Realized

Author: David So
LSI Logic

The many features and benefits of Serial Attached SCSI (SAS) have been touted in previous editions of Serial Storage Wire. As the next evolution of SCSI technology, and with the support of numerous industry powerhouses, SAS is poised to be the dominant I/O interface in the enterprise market, gradually replacing Ultra320 SCSI in that space and extending the industry’s most dominant interface for many years to come. In fact, because of its scalability, greater performance, high availability, and flexibility in supporting both SAS and Serial ATA (SATA) disk drives, SAS is predicted to make great inroads into the storage systems market as well, where parallel SCSI penetration, although successful, never quite reached its full potential. The promised transition to SAS has officially begun, as many companies have recently announced (or soon will announce) production availability of various SAS components. Q1 2005 will see SAS move beyond the prototype phase and into customer applications.

Controllers and Expanders Announced
The first public demonstration of SAS prototype devices occurred in mid-2003, and this new interface has only gained more and more momentum in the ensuing months. In Q4 2004, a few member companies of the SCSI Trade Association (STA) announced availability of the industry’s first SAS components. LSI Logic, for example, announced production of PCI-X to 4-port and 8-port SAS controller ICs. Although there is no limitation in the SAS specification regarding the number of SAS/SATA ports in a design, most companies seem to have settled on eight as the maximum port count for both controllers and single-chip RAID devices. Due to the interface’s serial, point-to-point architecture, host bus adapter (HBA) vendors must carefully consider various SAS/SATA topologies to determine which connector combinations to use, including single-phy (x1) internal connections as well as multi-phy (x4) internal and external connections. Rest assured that both PCI-X and PCI Express SAS HBAs will be available with a multitude of internal and external connector combinations to meet every system design need.

Expander devices are also critical to the success of SAS, as they provide the greater scalability and performance benefits that have already been well publicized. For the most part, the initial SAS expanders that are available are 12-port devices, including LSI Logic’s SASx12, a 12-port SAS edge expander with table routing capabilities. Smaller port-count expanders for specific platform needs have also recently been announced. Expander vendors, including LSI Logic, are planning higher port count expanders (e.g., 24 and 36 ports) for availability later this year to support more extensive topologies (up to 16,384 devices).

Small Form Factor Disk Drives Launched
SAS offers unmatched flexibility in its support of both SAS and SATA disk drives, allowing OEMs to service two market/storage application segments and enhance customer choice for storage solutions with a single SAS design. Timed with the introduction of the SAS interface is the introduction of 2.5″ small form factor (SFF) disk drives for the enterprise market, as announced by several of the disk drive vendors. “Standard” 3.5″ SAS disk drives have also been announced, providing IT professionals yet another degree of flexibility for their data center needs. The 2.5″ SFF disk drives will allow for greater drive counts in the same space, as well as improved performance in terms of IOPs/U.

End-to-End Interoperability Verified
Successful adoption of any new I/O technology depends a great deal on the interoperability of products from various vendors. In addition to ICs, HBAs, and disk drives as mentioned above, there are cables and connectors, storage systems, and protocol analyzers to consider. To ensure such interoperability, STA has sponsored a series of SAS Plugfests, which will continue through at least 2005, at the University of New Hampshire’s Interoperability Lab. These plugfests allow SAS component vendors to verify compliance and test functionality with other vendors in any number of different topologies. Ultimately, the result is seamless integration of SAS infrastructure in the enterprise storage market, regardless of the SAS components chosen.

Promotion of SAS has been on-going for a couple of years now, and the promise of what this new technology has to offer has arrived. With the recent announcements of several SAS components, and with many more to come in the next few months, SAS infrastructure is beginning to take shape. SAS-enabled servers and storage systems will soon be populating data centers worldwide, providing IT professionals with unprecedented deployment flexibility, scalability, reliability, and performance.

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