Background
When a file is written to HDFS, it is split up into big chucks called data blocks, whose size is controlled by the parameter dfs.block.size in the config file hdfs-site.xml (in my case – left as the default which is 64MB). Each block is stored on one or more nodes, controlled by the parameter dfs.replication in the same file (in most of this post – set to 3, which is the default). Each copy of a block is called a replica.
Hadoop ABCD 