Create a file called hadoop-metrics.properties, and stick it in your hadoop config directory.
The file should look like this:
dfs.class=org.apache.hadoop.metrics.ganglia.GangliaContext dfs.period=10 dfs.servers=@GMETAD@:8650 mapred.class=org.apache.hadoop.metrics.ganglia.GangliaContext mapred.period=10 mapred.servers=@GMETAD@:8650
I made hadoop talk directly to gmetad. I found that if I have hadoop talk to gmond at localhost, then hadoop metrics get reported from 127.0.0.1, rather than from the more useful host name used to communicate with gmetad.