In the Hadoop system, if the backup factor of the HDFS file system is 3, then each time MapReduce runs the Task, it must transfer the file segments to be processed from 3 machines with copies.

Question

Options

• ATrue
• BFalse

• A
  21% (7)
• B
  79% (27)

Explanation

Option B (False) is correct because MapReduce leverages data locality - when scheduling a task, Hadoop's scheduler attempts to run the mapper on a DataNode that already holds a local copy of the required block, meaning no network transfer is needed at all. The replication factor of 3 means there are 3 copies stored across different nodes for fault tolerance and availability, not because all 3 must be read simultaneously. When a task does need to fetch data over the network (e.g., no local copy is available), it reads from exactly one replica, chosen by the scheduler - never all three at once.

Option A is wrong because it fundamentally misunderstands the purpose of replication: redundancy ensures data survives node failures, it does not mean every reader must contact all replica holders.

Memory tip: Think of HDFS replicas like 3 identical books distributed across a library system. When a student needs the book, they check out from the nearest branch - they don't visit all three branches simultaneously. The extras exist in case one branch burns down.

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