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Channels to the disk (remote) can also have redundant paths.The disk chassis can have redundant power supplies which goto different UPS's on different circuit breakers.
When you introduce the objective of having off-site redundancy the costs and issues become huge.
Two big picture objectives for "redundancy" are "data redundancy" for DR situations and "application" redundancy for service availability.
Our "cluster" situation is an active-passive "redundancy" for the purposes of the mailstore application layer.
Redundancy isn't magical and all choices have a "time" for the operation to failover and the application being used can restrict what can be done or increase the length of "time" of the operation.
Timing of the operation is where the service/server has a client side impact of being "unavailable".
It would take a book to fully explore all the issues and possibilities but it still comes down to - how much money you have to spend on the project.