pt-table-sync synchronizes data efficiently between MySQL tables.
This tool changes data, so for maximum safety, you should back up your data before you use it. When synchronizing a server that is a replication slave with the –replicate or –sync-to-master methods, it always makes the changes on the replication master, never the replication slave directly. This is in general the only safe way to bring a replica back in sync with its master; changes to the replica are usually the source of the problems in the first place. However, the changes it makes on the master should be no-op changes that set the data to their current values, and actually affect only the replica.