[Hadoop Wiki] Update of "NameNode" by AllenWittenauer

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

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  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.
+ The NameNode is a Single Point of Failure for the HDFS Cluster.  HDFS is not currently a HighAvailability filesystem. When the NameNode goes down, the filesystem goes offline.  There is an optional SecondaryNameNode that can be hosted on a separate machine.  It only merges the edits file into the fsimage file and does not provide any real redundancy.
  
  It is essential to look after the NameNode. Here are some recommendations from production use