Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 8 - RAC Administration

Undo Management in RAC

Oracle recommends using the automatic undo method for rollback purposes in a RAC system. Each instance in the RAC system can only use one undo tablespace at a time. In other words, instances cannot share undo tablespaces. Each instance in the cluster, being an independent transaction-processing environment, maintains its own UNDO area for undo management. 

We can use either automatic undo management or rollback segment undo to manage undo space. If you are planning to follow the automatic rollback method, set the global parameter undo_management to auto in your server parameter file, and set the undo_tablespace parameter to assign the undo tablespace to the instance.

The RAC system allows you to create and use several undo tablespaces.  When the instance is started, it uses the first available undo tablespace. A second instance will use another undo tablespace. Thus, each instance in a RAC system will have exclusive access to a particular undo tablespace at a given time. The undo tablespace cannot be shared among the instances at the same time. Only once an undo tablespace is released by an instance, it can be assigned to another instance. However, all instances can read blocks from any or all undo tablespaces for the purpose of constructing read-consistency images.

You can switch to an idle undo tablespace if the need arises. This feature is useful if you want to switch to a new undo tablespace located in a different file system, or utilize a new undo tablespace that is larger in size.