Ceph

What's the motivation for doing that? Is it any better than setting the out interval to be something very short?

It guarantees that you always have the set number of copies on-disk when you get a commit, instead of probably having the correct number of copies.

Not really, a write will still go to N-1 replicas until the new one is backfilled up through the object's position.

In practice, i think this just means backfill may start and then be canceled/discarded if the osd comes back quickly. And there may be one less osdmap published..

Hmm, yeah, I forgot about that.
Somebody was asking about it; I'm not sure if they cared exactly but I'm sure there will be people who want to be able to guarantee a number of on-disk copies.