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Oracle Parallel Processing by Sanjay Mishra, Tushar Mahapatra

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Rollback Segments in OPS

All OPS instances need to use rollback segments to store undo information. The SYSTEM rollback segment is shared and is used by all instances for transactions involving system objects. In addition to the shared SYSTEM rollback segment, each instance needs at least one rollback segment of its own. You won’t be able to successfully start an instance and open a database unless that instance can acquire at least one rollback segment. If an instance cannot acquire even a single rollback segment, the instance will start and the database will be mounted, but the database won’t be opened. A rollback segment can be acquired by one instance only. Therefore, you must have at least as many rollback segments in your database as the number of concurrent instances that open that database.

A rollback segment acquired by an instance remains exclusive to that instance. No other instance can write undo information to this rollback segment. However, all instances can read from this rollback segment to maintain read consistency.

Public and Private Rollback Segments

Rollback segments can be public or private. A public rollback segment is created by using the PUBLIC clause with the CREATE ROLLBACK SEGMENT command. Any rollback segments created without using the PUBLIC clause are considered private. Public rollback segments can be acquired by any instance opening a database. A private rollback segment can be acquired by an instance only when the rollback segment’s name has been ...

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