Riak’s replication scheme ensures that you can still read, write, and update data if nodes go down. Riak allows you to set a replication variable, N (also known as the n_val), that specifies the number of nodes on which a value will be replicated.
An n_val value of 3 (the default) means that each object is replicated 3 times. When an object’s key is mapped onto a given node, Riak will continue on and automatically replicate the data onto two more nodes. This parameter enables you to replicate values to 7 nodes in a 10-node cluster, 10 nodes in a 15-node cluster, and so on.