What is the default replication factor of a block on HDFS?

In Hadoop Distributed File System (HDFS), the default replication factor of a block is set to 3. This means that each block of data is replicated and stored on three different DataNodes within the cluster. The rationale behind this default setting is to ensure fault tolerance and data reliability. If one DataNode goes down or becomes unavailable, the system can still access the data from one of the other two copies. This level of replication helps in balancing the load across DataNodes and enhances the overall performance and robustness of the system.

The replication factor is an important configuration in HDFS because it directly impacts the storage requirements and the overall fault tolerance of the system. A higher replication factor increases data reliability but also consumes more storage space, while a lower factor would reduce space utilization but may compromise data availability in case of node failures. The choice of having a default replication factor of 3 strikes a balance between these considerations, making it suitable for many common use cases in big data environments.