Adaptec 2268300R Terminology Used in This Chapter, What is SAS?, Appendix A Introduction to SAS

Appendix A: Introduction to SAS ● 89

Terminology Used in This Chapter

For convenience, SAS HBAs and SAS RAID controllers are referred to generically in this chapter as SAS cards. HBAs, RAID controllers, disk drives, and external disk drive enclosures are referred to as end devices and expanders are referred to as expander devices.

For convenience, this chapter refers to end devices and expander devices collectively as SAS devices.

What is SAS?

Legacy parallel SCSI is an interface that lets devices such as computers and disk drives communicate with each other. Parallel SCSI moves multiple bits of data in parallel (at the same time), using the SCSI command set.

SAS is an evolution of parallel SCSI to a point-to-point serial interface. SAS also uses the SCSI command set, but moves multiple bits of data one at a time. SAS links end devices through direct-attach connections, or through expander devices.

SAS cards can typically support up to 128 end devices and can communicate with both SAS and SATA devices. (You can add 128 end devices—or even more—with the use of SAS expanders. See page 93.)

Note: Although you can use both SAS and SATA disk drives in the same SAS domain (see page 93), we recommend that you not combine SAS and SATA disk drives within the same array or logical drive. The difference in performance between the two types of disk drives may adversely affect the performance of the array.

Data can move in both directions simultaneously across a SAS connection (called a link—see page 90). Link speed is 300 MB/sec in half-duplex mode. Therefore, a SAS card with eight links has a bandwidth of 2400 MB/sec.

Although they share the SCSI command set, SAS is conceptually different from parallel SCSI physically, and has its own types of connectors, cables, connection options, and terminology, as described in the rest of this chapter.

To compare SAS to parallel SCSI, see How is SAS Different from Parallel SCSI? on page 94.