Appendix A: Introduction to Serial Attached SCSI (SAS)
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77
SAS Expander Connections
A SAS expander device literally expands the number of end devices that
you can connect together. Expander devices, typically embedded into a
system backplane (see
), support large configurations of SAS
end devices, including SAS cards and SAS and SATA disk drives. With
expander devices, you can build large and complex storage topologies.
There are two types of SAS expanders: fanout expanders and edge
expanders. Each performs a different role in a storage system. (For more
information about how SAS expanders work, refer to the STA Web site
at
www.scsita.org
.)
You can connect up to 128 SAS ports to an edge expander. (A single
edge expander can therefore support up to 128 SAS addresses.)
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 figure below illustrates (in very basic terms) a SAS domain and
shows how SAS cards, SAS and SATA disk drives, and expander devices
can fit together in a large data storage topology.
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 Domain
SAS Domain
SATA
SAS
