I'm trying to set up a cascaded SnapMirror relationship between B-C where A-B is already in place. When I try and initialize the B-C mirror, the A-B SnapMirror becomes busy and won't complete an update, apparently until this completes. This will take the protection outside the desired RPO.
Is there any way around this, do I simply need to ask the source filer admin to create an additional snapshot?
When snapmirror operation is running it sets a software lock on destination volume and because of that lock it is not possible to initilize/update cascade snapmirror. One way could be a script in place which once A-B snapmirror update completes can break A-B snapmirror relation and then make initilize B-C. Once B-C snapmirror complete break B-C snapmirror and re-establish A-B snapmirror.
You may consider initial replication from A to C, then continuing from B to C. Note that it is possible to initialize snapmirror by simply shipping shelves (or small filer) if everything else is impossible.