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.
Why SAS? Answer; Performance, connectivity and scalability.
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 Protocols (STP), SAS is compatible with SATA II
HDDs.
