Disclaimer: Cloudera no longer approves of the recommendations in this post. Please see this documentation for configuration recommendations.
One of the things we get a lot of questions about is how to make Hadoop highly available. There is still a lot of work to be done on this front, but we wanted to take a moment and share the best practices from one of our customers. Check out what Paul George has to say about how they keep thier NameNode up at ContextWeb.