Which I/O controller is the fairest of them all?

A higher level software (a daemon or something else) doing dynamic weight adjustments for the groups at physical device level to achieve the bandwidth goal at virtual block device sounds like a good idea. It might be little complicated to implement though. :-). At LSF we sort of had the agreement to go in that direction but dm-ioband developers have to respond to this scheme of things and see if it satisfies their requirements or not. If it does, then probably they can start development on daemon for weight adjustment while we stabilize the IO scheduler based io controller.

I am not sure what kind of storage configurations are common but IO scheduler based solution alone should just work fine for all kind of Hardware RAID solutions and for disks directly attached to system without any software RAID. The only problematic case seems to be software RAID where bandwidth allocation will take place at higher level logical device.

With-in software RAID also, one needs to figure out which configurations are of particularly of more concerns. For striped ones, may be it is fair to assume that IO is evenly distributed across the various disks and if we can provide proportional bandwidth on individual disk, it should also translate into proportional bandwidth division for logical device.