Rollback Segments

Rollback segments contain undo information for all changes performed by noncommitted transactions. Rollback segments are a shared resource used by all active transactions in the database. When a transaction starts, Oracle binds that transaction to a particular rollback segment. As DMLs in the transaction execute, rollback segment space is used. For large transactions, rollback segments may need to allocate new extents as the transaction continues. When the transaction ends, a properly configured Oracle database releases the additional rollback extents so that they can be used for other rollback segments. To ensure that the additional rollback extents are released, configure the OPTIMAL parameter for each rollback segment.

Use a maximum of five transactions per rollback segment by setting the TRANSACTIONS_PER_ROLLBACK_SEGMENT parameter in the INIT.ora or INIT<SID>.ora file. Use the following rollback segment storage parameters for small, medium, and large models:

Model Type	Initial Extent Size	Next Extent Size	Optimal Size
Small	1 MB	1 MB	~ 2 MB x (# of transactions)
Medium	6 MB	6 MB	~ 12 MB x (# of transactions)
Large	6 MB	6 MB	~ 12 MB x (# of transactions)

Note: The �# of transactions� is the maximum number of simultaneous erwin� Data Modeler Workgroup Edition connections to Oracle.

In Oracle it is common to see the following error statement when the Rollback logs are not set up optimally:

ORA-01562 failed to extend rollback segment number string