What should be the replication factor for HDFS when using locally attached storage?

In the context of HDFS (Hadoop Distributed File System), the replication factor determines how many copies of each data block are stored across the cluster. When using locally attached storage, it's essential to balance data redundancy and storage efficiency.

A replication factor of 3 is typically recommended because it provides a good level of fault tolerance while ensuring that read performance is not significantly compromised. With three copies, even if one or two nodes fail, the data remains accessible, allowing for robust data recovery and high availability. This setup also helps mitigate data loss risks and improves data durability since there are multiple copies distributed across different nodes.

Choosing a replication factor greater than 3, such as 4, may unnecessarily consume additional storage space without providing substantial benefits in terms of data safety, especially when using local storage. Conversely, a replication factor of 2 could be inadequate in scenarios where node failure could lead to a loss of data availability. Therefore, a factor of 3 strikes an ideal balance for most HDFS configurations utilizing locally attached storage.