Description: Hadoop-0.20.2 SecondaryNameNode only used periodically from the NameNode download mirror (fsimage) the log (edits) merged to form the new mirror then upload to the NameNode This process is called CheckPoint. Memory running SecondaryNameNode node mapping information namespace and data block Block DataNode and so when NameNode downtime, can not be fully restored metadata, metadata can only restore the last CheckPoint. , Hadoop-0.21.0 introduces a BackupNode, relative to memory SecondaryNameNode, BackupNode, namespace NameNode maintain real-time synchronization mechanism to ensure that the meta-data is not lost when the NameNode downtime or lost little data.
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