1 HBase Monitoring in the book http://hbase.apache.org/book/ops.monitoring.html
2 HBase provides JMX interface to monitor it and there is a tool can be used http://blog.monitis.com/index.php/2012/03/28/monitoring-hbase/
There is a tool kit for accessing MBeans: https://github.com/larsgeorge/jmxtoolkit
The remote service format in jconsole or JMXTookit: service:jmx:rmi:///jndi/rmi://127.0.0.1:10101/jmxrmi
jmx command line client : http://crawler.archive.org/cmdline-jmxclient/downloads.html
3 Visualize the log of HBase can also be a helper to diagnose the problem and root cause. (This is what an intern in our lab did in the summer 2012 using Timeline. Since it has some performance issue, I will continue to make it workable.)
4 Visualize the Hadoop file system might also be a plus to do performance tuning.
5 how to monitor hadoop with Catci