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Appendix A: Introduction to SAS ● 62
You can connect up to 128 edge expanders to a fanout expander.
You can use only one fanout expander in any single SAS domain (a topology of SAS—and
possibly SATA—end devices and expander devices). A single SAS domain can therefore
comprise up to 16,384 SAS ports (and therefore up to 16,384 SAS addresses).
The following figure illustrates (in very basic terms) a SAS domain and shows how SAS
controllers, SAS and SATA disk drives, and expander devices can fit together in a large data
storage topology.
How is SAS Different from Parallel SCSI?
In summary, although SAS and parallel SCSI use the SCSI command set, how they move data
from one place to another is very different. To support point-to-point serial data transport,
SAS introduces new types of connectors, cables, connection options, and terminology.
Generally speaking, SAS is faster and more flexible than parallel SCSI, and provides more
options for building your storage space. SAS lets you mix SAS and SATA disk drives together,
and lets you connect many, many more devices.
The following table describes many of the main differences between the two interfaces.
SAS Card
SAS Card
Disk Drives
Disk Drives
Disk Drives
Disk Drives
Disk Drives
Disk Drives
Fanout Expander
Disk Drives
Edge
Expander
Edge
Expander
SATA
SATA
SATA
SATA
SAS
SAS
SAS
SATA
Edge
Expander
SATA
SATA
SAS
SAS
Edge
Expander
Edge
Expander
SAS Card
SAS DomainSAS Domain
SATA SAS
Parallel SCSI Serial Attached SCSI
Parallel interface Serial interface
Maximum speed 320 MB/sec
shared by all devices on the bus
Maximum speed 600 MB/sec per phy
when in full-duplex mode
Supports SCSI devices only Supports SATA and SAS disk drives
simultaneously
Up to 16 devices per SCSI channel
More than 128 disk drives per SAS
controller, using an expander (see page
61)
Supports single-port devices only Supports single- and dual-port devices
Uses SCSI IDs to differentiate
between devices connected to the
same adapter
Uses unique SAS addresses to
differentiate between devices