I was rebalancing one of the Hadoop clusters I run at work. It was not running very fast, so I modified the appropriate setting:

<property>
  <!-- 100Mbit/s -->
  <name>dfs.balance.bandwidthPerSec</name>
  <value>104857600</value>
</property>

I restarted the namenode and thought that would make the trick. But no, you also need to restart all your datanodes for the setting to take effect. Now I can see some action on my network graphs :-).