In the command documentation for RENAME it states:

If newkey already exists it is overwritten, when this happens RENAME executes an implicit DEL operation, so if the deleted key contains a very big value it may cause high latency even if RENAME itself is usually a constant-time operation.

This implies that RENAME is doing a synchronous delete (i.e. DEL instead of UNLINK). However, internally, it looks like the the overwrite operation is done with dbDelete which does a lazy free in the default configuration.

The above passage should be altered to indicate that in the default configuration does an implicit UNLINK and that big values only cause high latency if configured to do synchronous deletes.