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What action occurs automatically on a cluster when a DataNode is marked as dead?

What action occurs automatically on a cluster when a DataNode is marked as dead?

PrepAway - Latest Free Exam Questions & Answers

A.
The NameNode forces re-replication of all the blocks which were stored on the dead DataNode.

B.
The next time a client submits job that requires blocks from the dead DataNode, the JobTracker
receives no heart beats from the DataNode. The JobTracker tells the NameNode that the
DataNode is dead, which triggers block re-replication on the cluster.

C.
The replication factor of the files which had blocks stored on the dead DataNode is temporarily
reduced, until the dead DataNode is recovered and returned to the cluster.

D.
The NameNode informs the client which write the blocks that are no longer available; the client
then re-writes the blocks to a different DataNode.

Explanation:
How NameNode Handles data node failures?
NameNode periodically receives a Heartbeat and a Blockreport from each of the DataNodes in the
cluster. Receipt of a Heartbeat implies that the DataNode is functioning properly. A Blockreport
contains a list of all blocks on a DataNode. When NameNode notices that it has not recieved a
hearbeat message from a data node after a certain amount of time, the data node is marked as
dead. Since blocks will be under replicated the system begins replicating the blocks that were
stored on the dead datanode. The NameNode Orchestrates the replication of data blocks from one
datanode to another. The replication data transfer happens directly between datanodes and the
data never passes through the namenode.
Note: If the Name Node stops receiving heartbeats from a Data Node it presumes it to be dead
and any data it had to be gone as well. Based on the block reports it had been receiving from the
dead node, the Name Node knows which copies of blocks died along with the node and can make
the decision to re-replicate those blocks to other Data Nodes. It will also consult the Rack
Awareness data in order to maintain thetwo copies in one rack, one copy in another rackreplica
rule when deciding which Data Node should receive a new copy of the blocks.
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