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Oracle: Instance Recovery

Instance recovery ensures that a database is in a consistent state after an instance failure.
It takes place when an instance is re-started after
In these two situations, an instance recovery is necessary because
The data necessary bring the database to a consistent state is found in
No other data is required for instance recovery.
Instance recovery is performed by SMON.

Determining if instance recovery is necessary

When Oracle opens a database, it checks if redo threads are marked open in the control file. If this is the case, it means that the database was not closed properly (because closing a database marks the redo threads as closed when an instance is shut down consistently) and that instance recovery is necessary.
If an instance crashes in a RAC environment, one of the surviving instances performs instance recovery on the dead one.

Recovery phases

An instance recovery takes place in two phases; cache recovery and transaction recovery.

Cache recovery (rolling forward)

All transactions in the online redo log are applied to the data files.
This phase also regenerates the state of undo segments.

Transaction recovery

This phase uses the undo segments to roll back changes stored to data files that were not commited.

See also

MOS Note 1495163.1
recovery

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