if you already have nagios setup to monitor your servers, it's a good idea to use it to monitor your hadoop data nodes as well. you can use nrpe to call your dfs monitoring plugin. I found this blog post about setting up nrpe on CentOS extremely useful. The only trick to get it to work was to also setup rpmforge repositories for yum, so it finds the nrpe packages (nagios-nrpe and nagios-plugins-nrpe).
The next step was to add monitoring script for hadoop dfs, which I found here. Since I was using hadoop 0.20.2, I had to make some changes to the script to correctly parse values out of the dfs report:More...