cephfs client periodically report cache utilisation to MDS server
After seen Gregory's talk "What are "caps"? (And Why Won't my Client Drop Them?) he explained that the MDS servers need more cache than all cephfs clients combined. With a lot of clients and a lot of cache this might become a problem. If the client would periodically, or when it's talking to the MDS server for some other reason, tell the MDS server how much cache it is utilizing, the MDS server can provide a metric / notify operators when max cache limits are being reached.