Rollback Segments

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Rollback segments manage all transactions in your Oracle databases. They maintain read consistency among concurrent users in a database and can rollback transactions. They are transaction logs that maintain discrete records of changes to data. By maintaining a history of data changes, rollback segments can rollback uncommitted transactions so that data is rolled back to their prior state.

Tip: Databases with large user bases and intensive data manipulation characteristics can benefit from having multiple rollback tablespaces that are placed on different server drives or file systems. This strategy aids in the reduction of I/O and rollback segment contention.

Because all changes are logged to a rollback segment, they can become a performance bottleneck as the number of concurrent users and transaction activity increases. You can tune rollback segment performance by creating additional ones to handle increased traffic. To determine whether rollback segment performance is satisfactory, you should monitor the ratio of gets to waits. If you see that the number of waits is climbing, create another rollback segment to further spread concurrency.

Note: The analysis for determining when to add a rollback segment can be quite complicated. As a rule-of-thumb, however, many DBAs simply plan on allocating a rollback segment for every four concurrent users.

The proper sizing of rollback segments is critical to their overall performance. Performance degrades whenever a rollback segment must extend, wrap or shrink in response to transaction loads. Ideally, you want to make their extents as small as possible while ensuring that each transaction fits into a single extent, which in practice is very difficult to achieve.

Note: You should carefully monitor each rollback segments for the occurrence of extends, wraps and shrinks. If you see a high level of extends and wraps, then the rollback segment is probably too small. If you see a high level of shrinks, then you should increase the rollback segment's optimal size.

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