However, private rollback segments provide more control over the matching of instances with rollback segments, allowing you to locate the rollback segments for different instances on different disks to improve performance. You can therefore use private rollback segments to reduce disk contention in a high-performance system. Public rollback segments form a pool of rollback segments that can be acquired by any instance needing an additional rollback segment. Using public rollback segments can be disadvantageous when instances are shutdown and started up at the same time. For example, instance X shuts down and releases public rollback segments.
Y starts up and acquires the released rollback segments.
Instance X starts up and cannot acquire its original rollback segments. By default a rollback segment is private and is used by the instance specifying it in the parameter file. Once a public rollback segment is acquired by an instance, it is then used exclusively by that instance. At startup, an instance attempts to acquire rollback segments as follows. If no private rollback segments are specified in the initialization file, the instance attempts to acquire public rollback segments.