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ORA-01545:

rollback segment 'string' specified not available
Cause: Either: 1) An attempt was made to bring a rollback segment online that is unavailable during startup; for example, the rollback segment is in an offline tablespace. 2) An attempt was made to bring a rollback segment online that is already online. This is because the rollback segment is specified twice in the ROLLBACK_SEGMENTS parameter in the initialization parameter file or the rollback segment is already online by another instance. 3) An attempt was made to drop a rollback segment that is currently online. 4) An attempt was made to alter a rollback segment that is currently online to use unlimited extents. 5) An attempt was made to online a rollback segment that is corrupted. This is because the rollback is specified in _corrupted_rollback_segments parameter in initialization parameter file.
Action: Either: 1) Make the rollback segment available; for example, bring an offline tablespace online. 2) Remove the name from the ROLLBACK_SEGMENTS parameter if the name is a duplicate or if another instance has already acquired the rollback segment. 3) Bring the rollback segment offline first. This may involve waiting for the active transactions to finish, or, if the rollback segment needs recovery, discover which errors are holding up the rolling back of the transactions and take appropriate actions. 4) Same as 3). 5) Remove the name from the _corrupted_rollback_segments parameter.