Data storage in HDFS
Whenever any file has to be written in HDFS, it is broken into small pieces of data known as blocks. HDFS has a default block size of 64MB (Hadoop 1.x) or 128 MB (Hadoop 2.x) which can be increased as per the requirements. These blocks are stored in the cluster in distributed manner on different nodes. This provides a mechanism for MapReduce to process the data in parallel in the cluster.
Multiple copies of each block are stored across the cluster on different nodes. This is a replication of data. By default, HDFS has a replication factor of 3. It provides fault tolerance, reliability, and high availability.
A Large file is split into n number of small blocks. These blocks are stored at different nodes in the cluster in a distributed manner. Each block is replicated and stored across different nodes in the cluster.