AUM will automatically allocate the undo segments and manage the undospace thus increasing the efficiency and read consistency.

Following are the steps to setup AUM:

1. Configure the following in initialization parameter

   	UNDO_MANAGEMENT = auto

2. Create a dedicated tablespace to hold the undo information.

           SQL> CREATE UNDO TABLESPACE undotbs_01 DATAFILE '/export/home/oracle/oradata/orcldir/undotbs01.dbf' SIZE 500M;

If you have got two undo tablespaces you want to use other undo tablespace (say undotbs_02) then you need to change the                                      UNDO_TABLESPACE value to the new one.

           SQL> ALTER SYSTEM SET UNDO_TABLESPACE=undotbs_02;

This will change the value in the initialization parameter dynamically.