Serial Attached SCSI technologies and architectures, 4th edition

Introduction

Serial Attached SCSI (SAS) has become the de facto hard disk drive (HDD) standard for mission-

critical applications. This paper describes SAS devices, the SAS protocol, possible SAS topologies,

zoning, and interoperability with SATA devices and connectors. The Appendix defines technical terms

used in this paper.

SAS devices

There are three types of SAS devices: initiators, targets, and expanders. Initiator devices include host

bus adaptors (HBAs) and controllers. The initiator attaches to one or more targets, forming a SAS

domain. Target devices include SAS hard disk drives (HDDs) or solid state drives (SSDs), SATA HDDs

or SSDs, and SAS tape drives. Using expanders (low-cost, high-speed switches), you can increase the

number of targets attached to an initiator to create a larger SAS domain (Figure 1).

Figure 1: Example of SAS and SATA devices in a single domain

Initiators

SAS initiators have multiple ports for connecting to internal and external targets. Each initiator port

can have a single physical link (a narrow port) or two, four, or eight physical links (a wide port). You

can connect SAS initiator ports to separate domains for fail-over redundancy.