When
the client is ready to load a file into the cluster, the content of the file
will be divided into blocks. Now the client consults the Namenode and gets 3
datanodes for every block of the file which indicates where the block should be
stored. While placing the datanodes, the key rule followed is “for every block
of data, two copies will exist in one rack, third copy in a different rack“.
This rule is known as “Replica Placement Policy“.