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Donald K. Burleson

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Number and Placement of Rollback Segments

Another item controlled by the number of data manipulation language users (INSERT, UPDATE, and DELETE commands) and the transaction load on the system is the number of rollback segments. The formula, as stated before, is:
NUMBER OF DML TRANSACTIONS / NUMBER OF TRANSACTIONS PER ROLLBACK SEGMENT
This will yield the number of rollback segments needed. They should be sized to handle the maximum expected data manipulation language (DML) transaction.

The placement of rollback segments is decided based upon resource contention prevention. Put them where they won’t cause contention with other Oracle files. Transactions are spread across all active rollback segments. Usually, it is a good idea to locate the rollback segments in a tablespace or tablespaces dedicated to rollback segments. This allows the DBA to easily manage these resources.

Oracle rollback segments

The size of rollback segments is based upon three factors:

* Average number of simultaneous active DML transactions.
* Average number of bytes modified per transaction.
* Average duration of each transaction.


This is an excerpt by Mike Ault’s book “Oracle Administration & Management”.  If you want more current Oracle tips by Mike Ault, check out his new book “Mike Ault’s Oracle Internals Monitoring & Tuning Scripts” or Ault’s Oracle Scripts Download.

 

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