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Appendix A: Introduction to SAS ● 53
Terminology Used in This Chapter
For convenience, SAS HostRAID controllers are referred to generically in this chapter as SAS
cards. HBAs, HostRAID 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 56.)
Although you can use both SAS and SATA disk drives in the same SAS domain (see page 56),
Adaptec recommends 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
next section). Link speed is 600 MB/sec in full-duplex mode. A SAS card with eight links has a
maximum bandwidth of 4800 MB/sec in full-duplex mode.
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 57.
How Do SAS Devices Communicate?
SAS devices communicate with each other through links. A link is a physical connection
between two phys.