Which direction does `nodetool repair` go?

Question

Let's say I log into a Cassandra node X and issue nodetool repair. Will I only repair the data residing on X? Or will I also automatically repair all other nodes that have replicas of what X holds (that is, reconsile data inconsistencies two-way)?

Answer 1

You repair all replicas.

The nodetool repair command repairs inconsistencies across all of the replicas for a given range of data. Run repair in these situations:

• As a best practice, you should schedule repairs weekly. Note: If deletions never occur, you should still schedule regular repairs. Be aware that setting a column to null is a delete.
• During node recovery. For example, when bringing a node back into the cluster after a failure.
• On nodes containing data that is not read frequently.
• To update data on a node that has been down.

http://www.datastax.com/documentation/cassandra/1.2/cassandra/operations/ops_repair_nodes_c.html