We still have net.topology.impl=org.apache.hadoop.net.NetworkTopology in core-site.default. Actually this property does not effect once dfs.use.dfs.network.topology is true.
in DatanodeManager，networkTopology is initialized as
I think we should still make the NetworkTopology configurable rather than hard code the implementation since we may need another NetworkTopology impl.
I am not sure if there is other consideration. Any thought? vagarychen linyiqun