Do servers such as R720xd R730 have dual port SAS backplanes? What functionality does that provide, is it only for dual controller redundancy, or can it provide double bandwidth to a single card like the H7xx or H3xx?
Many have said it is only for redundancy, Seagate implies it doubles bandwidth (if you had 2 cards??)-
No, dual channel or single channel expander backplanes do not provide two channels to each disk. I think you are getting SAS and SATA differences mixed up with hardware connectors or controller channels. SAS has redundant pathing, SATA does not have redundant pathing. That is a feature of the SAS specification.
If you have fast disks that are highly utilized then a dual controller configuration may be necessary to avoid performance loss. A single controller on a large backplane, like 24 disks, can cause performance loss if all disks are highly utilized. Using two controllers on a split channel 12/12 backplane will allow for higher performance in highly utilized situations.