Dear person at fudcon who pointed out I should be using DRBD in my cloud talk.  Had I checked my notes I would have noticed that DRBD does, in fact, do what I want it to.  My apologies[1].  I have no experience with it however because its not in Fedora/RHEL  So for now it seems that mdadm --write-mostly is still the plan.

I am now doing some research on why exactly DRBD isn't in Fedora/RHEL though initial reports are that its not in the mainline kernel.  I might be able to get someone to get that stuff done so it can be properly evaulated.

[1] This applies to all of you watching the video online.