Adaptec ASR-6805Q User Manual

Also see for ASR-6405: Information sheet

Contents

Adaptec ASR-6805Q User Manual Manual pdf 69 page image

A Introduction to SASThis 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 AppendixFor convenience, SAS HBAs and SAS RAID controllers are referred to generically in this chapter as SAScards. HBAs, RAID controllers, disk drives, and external disk drive enclosures are referred to as enddevices 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 communicatewith each other. Parallel SCSI moves multiple bits of data in parallel (at the same time), using the SCSIcommand set.SAS is an evolution of parallel SCSI to a point-to-point serial interface. SAS also uses the SCSI commandset, 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 SATAdevices. (You can add 128 end devices—or even more—with the use of SAS expanders. See SAS ExpanderConnections 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 combineSAS and SATA disk drives within the same array or logical drive. The difference inperformance between the two types of disk drives may adversely affect theperformance of the array.Data can move in both directions simultaneously across a SAS connection (called a link—see How DoSAS Devices Communicate? on page 69). Link speed is 300 MB/sec in half-duplex mode. Therefore, aSAS 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 therest 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 twophys.As shown in the following figure, SAS devices contain ports (see What’s a SAS Port? on page 70), portscontain phys, and each phy contains one transmitter and one receiver—one transceiver. A phy canbelong to one port only.69Proprietary 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