Virtual nodes

Overview of virtual nodes (vnodes).

Virtual nodes, known as Vnodes, distribute data across nodes at a finer granularity than can be easily achieved if calculated tokens are used. Vnodes simplify many tasks in Cassandra:

Tokens are automatically calculated and assigned to each node.
Rebalancing a cluster is automatically accomplished when adding or removing nodes. When a node joins the cluster, it assumes responsibility for an even portion of data from the other nodes in the cluster. If a node fails, the load is spread evenly across other nodes in the cluster.
Rebuilding a dead node is faster because it involves every other node in the cluster.
The proportion of vnodes assigned to each machine in a cluster can be assigned, so smaller and larger computers can be used in building a cluster.

Note: DataStax recommends using 8 vnodes (tokens). Using 8 vnodes distributes the workload between systems with a ~10% variance and has minimal impact on performance.

For more information, see the article Virtual nodes in Cassandra 1.2. To convert an existing cluster to vnodes, see Enabling virtual nodes on an existing production cluster.