Your cluster has nodes in seven racks, and you have provided a rack topology script. What is
Hadoop’s block placement policy, assuming a block replication factor of three?

A.
One copy of the block is written to a node in each of three racks

B.
One copy of the block is written to a node in one rack; two copies are written to two nodes in a
different rack

C.
All three of the block are written to nodes on the same rack

D.
Because there are seven racks the block is written to a node on each rack

Explanation:
HDFS uses rack-aware replica placement policy. In default configuration there are
total 3 copies of a datablock on HDFS, 2 copies are stored on datanodes on same rack and 3rd
copy on a different rack.
Note: HDFS is designed to reliably store very large files across machines in a large cluster. It
stores each file as a sequence of blocks; all blocks in a file except the last block are the same
size. The blocks of a file are replicated for fault tolerance. The block size and replication factor are
configurable per file. An application can specify the number of replicas of a file. The replication
factor can be specified at file creation time and can be changed later. Files in HDFS are write-once
and have strictly one writer at any time.
24 Interview Questions & Answers for Hadoop MapReduce developers, How the
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