Daily Archives: April 12, 2014

Rack Awareness in Hadoop 6

Usually Hadoop clusters of more than 30-40 nodes are configured in multiple racks.┬áCommunication between two data nodes on the same rack is efficient than the same between two nodes on different racks. In large clusters of Hadoop, in order to improve network traffic while reading/writing HDFS files, NameNode chooses data nodes which are on the same rack or a near by rack to read/write request (client node). NameNode achieves this […]

Safe Mode in Hadoop 6

Safe Mode Safe Mode in hadoop is a maintenance state of NameNode during which NameNode doesn’t allow any changes to the file system. During Safe Mode, HDFS cluster is read-only and doesn’t replicate or delete blocks. At the start up of NameNode It loads the file system namespace from the last saved fsimage into its main memory and the edits log file. Applies/merges edits log file on fsimage and results […]