Block locations

During the registering of datanode on namenode(after namenode is restarted) if one data node is dead, how the name node will come to know, the block locations on that node and will distribute them to the other data nodes if required.

Through heartbeat. All live datanodes will send heartbeat to namenode. If namenode does not get heartbeat for 10 minutes by default, then datanode will be blacklisted by namenode.

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