The difference between flow-control(global) and pfc is that flow-control stop all traffic, and PFC divides the network pipe in to 8 virtual lanes. This means you can send pause on just one lane, and keep the rest of the traffic flowing. But the thing about CEPH, is that it has different types of traffic, a global flow-control can stop important heartbeats and mark OSD's as failed. Its a big NO NO. I think ECN is the only thing you should enable with CEPH.
