It seems that this is just a structured way to formally keep more copies of your data, when what you're trying to avoid is a rack level event removing availabil

It seems that this is just a structured way to formally keep more copies of your data, when what you're trying to avoid is a rack level event removing availability of your replicas. Ceph, described here http://ceph.com/papers/weil-thesis.pdf http://ceph.com/papers/weil-thesis.pdf does just that by letting you include the structure of your datacenter in the pseudorandom, deterministic placement algorithm it uses for placing reads and writes.