By default, Hadoop 1x has a block size of 64MB and Hadoop 2x has a block size of 128MB, though let us take the block size to be 100MB, which means there will be 5 blocks replicated 3 times (the default replication factor).   To illustrate how a block is stored in HDFS, let us use a scenario with a file containing 5 blocks (A/B/C/D/E), a client, a NameNode and a DataNode. Initially, the client will ask the NameNode for the locations of the DataNodes where it can store the first block (A) and the replicated copies.   Once the client knows the location of the DataNodes, it will send block A to the DataNodes and the replication process will begin. After block A has been stored and replicated on the DataNodes, the client will be informed, and then it will initiate the same process for the next block (Block B).   In this process, if the first block of 100MB is written to HDFS, and the next block has been started by the client, to store then 1st block will be visible to readers. Only the current block being written will not be visible to the readers.