I had to amend an earlier article I wrote on DCB, because I was under a misconception about what the priority flow control pause frames are really doing for storage. My thought was that when a link is congested, you would use pause frames to tell the other classes of traffic to shut up for a moment so that the storage traffic could have the link. That’s backwards, for this reason: what if the storage traffic itself is threatening to overwhelm the link? In that case, who really needs to pause to avoid frames being dropped? The storage traffic does. Ah ha!
Ivan Pepelnjak put that out there, and he also refers to this Cisco white paper which explains the math behind the pause frame. Worth a read, as it mentions, “FCoE is the first real application of PFC. In the context of FCoE, PFC enables a switching device to comply with the requirements of traditional Ethernet-based applications, which expect drops as a way of dealing with congestion, as well as the requirements of the Fibre Channel FC-2 layer, which assumes a lossless medium for its payloads. Both types of traffic can coexist, as long as they are classified on different CoSs, with PAUSE enabled on the FCoE CoS.“