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that if many expansion enclosures are connected, a longer SAS external
cable (e.g., a 120cm cable) may be necessary for connecting a expansion
enclosure from the opposite direction offering high redundancy.
One expansion link connects expansion enclosures from the storage system
to the nearest expansion enclosure, and then to the farthest one. Another
expansion link connects to the farthest expansion enclosure from the
opposite direction and then to the nearest one.
Each expander controller on the SAS expansion enclosure controls a “SAS
Domain” that connects one of the alternative interfaces on all of the disk
drives in the enclosure. For example, one expander unit controls Domain A,
and the other controls Domain B. In a fault-tolerant topology, the SAS
external links always connect to the SAS ports on the same SAS Domain.
The SAS Domains can be identified by the expanders’ locations, i.e., upper
controller slot (Controller A/Slot A) or the lower controller slot (Controller
B/Slot B).
On the storage system, each controller can be considered as managing a
separate SAS Domain. With the help of the port selector mechanism on the
MUX boards, the idea of SAS Domain applies even when SATA drives are
used in a dual-controller expansion enclosure.
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