The Oracle database automatically manages undo segments within a specific undo tablespace that is assigned to an instance. This is called Automatic Undo Management (AUM). Under normal circumstances, only the instance assigned to the undo tablespace can modify the contents of that tablespace. However, all instances can always read all undo blocks for consistent-read purposes. Also, any instance can update any undo tablespace during transaction recovery, as long as that undo tablespace is not currently used by another instance for undo generation or transaction recovery.
You assign undo tablespaces in your Oracle RAC database by specifying a different value for the UNDO_TABLESPACE initialization parameter for each instance in your SPFILE or individual PFILEs. If you do not set the UNDO_TABLESPACE parameter, each instance uses the first available undo tablespace.
You can dynamically switch undo tablespace assignments by executing the ALTER SYSTEM SET UNDO_TABLESPACE statement. You can run this command from any instance. In the example above, the previously used undo tablespace, unodtbs1, assigned to the RAC01 instance remains assigned to it until RAC01's last active transaction commits. The pending offline tablespace may be unavailable for other instances until all transactions against that tablespace are committed. You cannot simultaneously use Automatic Undo Management (AUM) and manual undo management in an Oracle RAC database. It is highly recommended that you use the AUM mode described on this slide.