What is an instance failure?
An instance failure is a termination of an Oracle instance due to a hardware problem (e.g., power outage) or software problem (e.g., an operating system crash) that prevents the Oracle instance from continuing its work. An instance failure always occurs when a database is not cleanly shut down (i.e., the database is shut down using the SHUTDOWN ABORT command). When an instance failure occurs, the data in the database buffer cache is not written to data files. Therefore, after an instance failure, the associated database requires instance recovery to recover the lost data. After an instance failure, Oracle automatically performs an instance recovery when the instance is restarted. The system monitor (SMON) background process is responsible for performing the instance recovery. An instance recovery does not require any manual interference. Oracle automatically applies the data stored in online redo log files and undo segments in order to recover the lost data and roll back the uncommitte
