The rack awareness that Ambari maintains only propagates the /etc/hadoop/conf/topology_mappings.data file for Datanodes. If I have a node with only Nodemanager Ambari does not add it to the topology_mappings.data file. These seems to indicate that a Compute only nodes does not get rack awareness in Ambari.
Second issue is that when adding a Datanode, Ambari asks for a restart, I'm assuming just to propogate the topology_mappings.data file. Adding a datanode ideally shouldn't need a restart.