The NameNode is the centerpiece of an HDFS file system. It keeps the directory tree of all files in the file system, and tracks where across the cluster the file data is kept. It does not store the data of these files itself.

Client applications talk to the NameNode whenever they wish to locate a file, or when they want to add/copy/move/delete a file. The NameNode responds the successful requests by returning a list of relevant DataNode servers where the data lives.

The NameNode is a Single Point of Failure for the HDFS Cluster. HDFS is not currently a High Availability system. When the NameNode goes down, the file system goes offline. There is an optional SecondaryNameNode that can be hosted on a separate machine. It only creates checkpoints of the namespace by merging the edits file into the fsimage file and does not provide any real redundancy. Hadoop 0.21+ has a BackupNameNode that is part of a plan to have an HA name service, but it needs active contributions from the people who want it (i.e. you) to make it Highly Available.

It is essential to look after the NameNode. Here are some recommendations from production use

If a NameNode does not start up, look at the TroubleShooting page.

NameNode (last edited 2011-09-10 05:08:47 by CPE5cff3502d4a6-CM001ceacebae8)