You might think that the SecondaryNameNode is a hot backup daemon for the NameNode. You’d be wrong. The SecondaryNameNode is a poorly understood component of the HDFS architecture, but one which provides the important function of lowering NameNode restart time. This blog post describes how to configure this daemon in a large-scale environment. The default Hadoop configuration places an instance of the SecondaryNameNode on the same node as the NameNode. A more scalable configuration involves configuring the SecondaryNameNode on a different machine.
We’ve been talking to enterprise users of Hadoop about existing and new projects, and lots of them are asking questions about reliability and data integrity. So we wrote up a short paper entitled HDFS Reliability to summarize the state of the art and provide advice. We’d like to get your feedback, too, so please leave a comment.