You can have multiple metadata servers, but then you have multiple volumes - like MapR. When you move a file between two different volumes, the move operation is no longer a simple metadata operation (which is atomic). Instead the move operation involves removing the file from one metadata server, then adding it to the other server - an agreement problem. Except, they don't solve the agreement problem properly. You will notice it more when you move a large directory, as then it will go really slow - and you hope you dont have a failure while the operation is executing.

reference: http://docs.ceph.com/docs/master/cephfs/add-remove-mds/