Don't be surprised if this question pops up as one of the top Hadoop admin technical interview questions in your next interview. DataNode stores data in HDFS; it is a node where actual data is stored in the file system. Each   DataNode sends a heartbeat message to indicate that it is active. If the NameNode does not receive a message from the DataNode for 10 minutes, the NameNode considers the DataNode dead or misplaced and begins replicating blocks hosted on that DataNode so that they are hosted on another DataNode. A BlockReport contains a list of all blocks on a DataNode. Now the system starts replicating what was stored in the dead DataNode.   The NameNode manages the replication of data blocks from one DataNode to another. In this process, replication data is transferred directly between DataNodes so that the data never passes through the NameNode.