By default, the replication factor is 3. There are no two copies that will be on the same data node. Usually, the first two copies will be on the same rack, and the third copy will be off the shelf. It is advised to set the replication factor to at least three so that one copy is always safe, even if something happens to the rack.
We can set the default replication factor of the file system as well as of each file and directory exclusively. For files that are not essential, we can lower the replication factor, and critical files should have a high replication factor.
