You can allocate undo space in a single undo tablespace instead of distributing it into a set of statically allocated rollback segments. For each Oracle instance, you only have to allocate enough disk space to the undo tablespace for the workload of that instance. You will no longer have to decide on the number and sizes of rollback segments to create and how to strategically assign transactions (of different sizes) to individual rollback segments.
To create an undo tablespace:
Create a new tablespace and make it an undo tablespace by selecting the Undo radio button in the Create Tablespace dialog. For an undo tablespace, all storage options are disabled because they are handled automatically by the database.
From the All Initialization Parameters page, you must set the following parameters
Undo_Management to AUTO
Undo_Tablespace to the name of the undo tablespace you have previously created.
Restart the database.
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