File requests to Namenode

How and where to check how many datanode block address requests a namenode gets when running a map reduce job. - Diwakar

Get Hadoop cluster topology

I understand that when Namenode starts up it reads fsimage to get the state of HDFS and applies the edits file to complete it. But how about the cluster topology ? Does the namenode read the config files like core-site.xml/slaves/... etc to determine its cluster topology or uses an API to build