Appendix A: Introduction to Serial Attached SCSI (SAS)
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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.
Serial Attached SCSI (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
.)
Note:
Although you can use both SAS and SATA disk drives in the same
SAS domain (see
), 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
). 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
.
