[Hadoop Wiki] Update of "NameNode" by SteveLoughran

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The following page has been changed by SteveLoughran:
http://wiki.apache.org/hadoop/NameNode

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The NameNode is the centerpiece of an HDFS filesystem. It keeps the directory tree of all files in the filesystem, and tracks where across the cluster the files are kept. It does not store any 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. There is support for NameNodeFailover, with a SecondaryNameNode hosted on a separate machine being able to stand in for the original NameNode if it goes down. However, HDFS is not currently a HighAvailability filesystem. When the NameNode goes down, the filesystem goes offline.

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

 * Use a good server with lots (15GB+) of RAM.
 * Use fast RAID5 storage for keeping the index.
 * Configure the name node to store one set of transaction logs on a separate disk from the index. 
 * Configure the name node to store another set of transaction logs to a network mounted disk.  
 * Monitor the disk space available to the NameNode. If is getting low, add more storage.
 * Do not host DataNode, JobTracker or TaskTracker services on the same system.

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