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SAS-to-SATA II RAID Subsystem
1.4 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, and 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 parallel 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 miniature 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 Protocol (STP), SAS is compatible with SATA II HDDs
Availability
In this fast-paced society, data needs to be available to clients whenever they need it. SAS
was designed with availability in mind. SAS include the following benefits:
Dual ports for redundant controller functionality
Support for multiple initiators
Hot swappable for easy removal / replacement of the SAS drive
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Installation and Configuration Manual
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