MicroNet Genesis V User's Guide download pdf (Page 16)

Introduction

1-10

1.3 Serial-Attached SCSI (SAS) Overview

What is SAS? Serial-Attached SCSI (SAS) is the newest storage interface for

Direct-Attached Storage (DAS). SAS is the successor to the highly successful

parallel Ultra 320 SCSI interface. SAS improves and expands upon the parallel

SCSI technology, and is supported by the ANSI T10 Standards Committee. But

why SAS? Performance. Connectivity and scalability. Availability.

One of the major differences between parallel SCSI and SAS is that SAS uses

a serial bus as opposed to a parallel bus. Ultra 320 parallel SCSI has reached

the limit of parallel SCSI performance, and anything beyond Ultra 320 is not

feasible due to the fact that parallel SCSI clock rates are limited by bit-skewing.

This is where SAS comes in. SAS transmission is done serially, and results in

a myriad of advantages over parallel SCSI.

Performance

The serial nature of SAS results in a huge increase in performance over paral-

lel SCSI.

No bit-skewing and is self-clocking

Transfer rates up to 3.0 Gb/s per link

Full duplex which results in a total of 6.0 Gb/s per link.

Link aggregation into 4x wide ports for a total of 24.0 Gb/s

Extensive command queue

Connectivity/Scalability

A new concept introduced by SAS is the expander. Expanders act like minia-

ture switches for routing data from the SAS controller chip to the hard disks.

The connectivity / scalability benefits of SAS are:

Dedicated point-to-point connection between initiator and target

SAS expanders make scalability up to 16K devices in a single SAS domain

possible

Each device is uniquely identified with a World Wide Name (WWN), so there is

no longer a need for SCSI IDs

Using SCSI Tunneling Protocols (STP), SAS is compatible with SATA II HDDs

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MicroNet Genesis V User's Guide Page 16