SAS cuts a wide data path

Until recently, data centres were populated primarily with storage-area networks that used Fibre Channel connectivity and parallel SCSI interfaces. Such storage solutions deliver high performance and reliability but carry a high price tag. However, serial-attached SCSI technology is poised to keep storage costs down without compromising functionality or performance.

The SAS 1.1 standard is in the final approval phase of the International Committee on Information Technology Standards' (INCITS) T10 committee. Storage vendors plan to introduce SAS products in the next few months that will replace Fibre Channel disks with more-economical SAS disks.

The SAS interface supports SAS and serial-ATA (SATA) drives, the type typically found in PCs and low-end servers. SATA drives cost less and offer less performance than Fibre Channel and SAS drives. But they are ideal candidates for near-line storage of information that is not accessed frequently; a second tier of disk-resident storage.

The compatibility of SAS and SATA drives begins with their low-level interface characteristics. In fact, SAS and SATA physical signalling is set by the same standard and is indistinguishable. The addition of the SATA Tunnelling Protocol to the SAS standard paved the way for allowing a single SAS controller to support first- and second-tier storage.

Traditional storage subsystems begin with a RAID controller that must support two interfaces: one to interconnect to the host SAN (traditionally Fibre Channel) and one to support the connection of the hard-disk drives. Although the Fibre Channel loop can support up to 127 hard-disk drives, performance can be optimized by using far fewer drives per loop plus the use of Fibre Channel switches.

Although the new serial hard-disk drives share the serial nature of Fibre Channel, they do not share the ability to form loops. The SAS/SATA interfaces are strictly defined as point-to-point with unique port addresses. That's where the SAS expander comes in.

RAID controllers designed to support SAS/SATA hard-disk drives have semiconductor expander chips built into them. The Cables and Connectors Addendum to the SATA standard provides for 16 hard-disk drive ports. Implementations today usually provide for 12 to 15 hard-disk drives per enclosure. An SAS expander function is defined in the standard as a chip or printed circuit board that lets one SAS address be expanded to a number of additional ports. RAID internal expanders are chips, while external expanders are boards.

Although the Fibre Channel and SAS expander architectures appear similar, SAS offers an important advantage. The upstream SAS port can be configured as a wide port that lets multiple SAS connections (typically four) be treated as one SAS address. The result is a fat data pipe known as a SAS wide port.

For example, a connection of Fibre Channel drives in a traditional loop or point-to-point interface scenario can't exceed the Fibre Channel bandwidth, which is about 4Gbit/sec. With SAS architecture and the implementation of SAS wide ports, SAS bandwidth of 3Gbit/sec for a single interface can be multiplied to 12Gbit/sec.

Emerging RAID controllers and storage subsystems take advantage of the bandwidth of the SAS wide port and the reduced cost. By providing a way to widen the data path while mixing SAS and SATA drives, SAS goes a long way toward meeting lower-cost, tiered-storage requirements.

Larry Jacob is senior director of product management for Ario Data Networks

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