If the database fails (for example, as the result of a power failure), and the volumes were fully functioning, the database system carries out a restart. This means that the effects that completed transactions had on the data volumes are reproduced (rolled forward), and the effects that uncompleted transactions would have had are cancelled out (rolled back).

If a data volume fails (physical disk error), the last, complete data backup must be imported after the problem has been rectified. However, the last consistent database state can only be recovered if all the required redo log entries can be imported from the log area or the log backup (if the information in the log area is insufficient).