Adaptec 1891300 - RAID Controller User's Guide download pdf (Page 69)

Appendix A: Introduction to Serial Attached SCSI ● 69

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 73.)

Note:

Although you can use both SAS and SATA disk drives in the same SAS domain (see page

73), 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

page 70). 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 74.

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Adaptec 1891300 - RAID Controller User's Guide Page 69