A
Introduction to SAS
This section provides a basic overview of the main features of SAS, introduces some common SAS terms,
and explains how SAS differs from parallel SCSI.
Note: For technical articles and tutorials about SAS, refer to the SCSI Trade Association
(STA
™
) Web site at
www.scsita.org
.
Terminology Used in This Appendix
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
SAS Expander
Connections
on page 72.)
Note: Although you can use both SAS and SATA disk drives in the same SAS domain
(see
SAS Expander Connections
on page 72), we recommend that you do 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
How Do
SAS Devices Communicate?
on page 69). 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 73.
How Do SAS Devices Communicate?
SAS devices communicate with each other through links. A link is a physical connection between two
phys.
As shown in the following figure, SAS devices contain ports (see
What’s a SAS Port?
on page 70), ports
contain phys, and each phy contains one transmitter and one receiver—one transceiver. A phy can
belong to one port only.
69
Proprietary and Confidential to PMC-Sierra, Inc.
Document No.: CDP-00277-02-A Rev. A, Issue:
Serial Attached SCSI RAID Controllers Installation and User's Guide