Oracle Database automatically manages undo segments within a specific undo tablespace that is assigned to an instance. Instances can always read all undo blocks throughout the cluster environment for consistent read purposes. Also, any instance can update any undo tablespace during transaction recovery, if that undo tablespace is not currently used by another instance for undo generation or transaction recovery.
You assign undo tablespaces in your Oracle RAC administrator-managed database by specifying a different value for the
UNDO_TABLESPACE parameter for each instance in your SPFILE or individual PFILEs. For policy-managed databases, Oracle automatically allocates the undo tablespace when the instance starts if you have Oracle Managed Files enabled. You cannot simultaneously use automatic undo management and manual undo management in an Oracle RAC database. In other words, all instances of an Oracle RAC database must operate in the same undo mode.