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System Monitor Process (SMON) : Oracle Background Process.

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SMON Performs critical tasks such as crash recovery when the instance is started following a failure, dead transaction recovery, and maintenance tasks such as temporary space reclamation, data dictionary cleanup, and undo tablespace management.
The system monitor process (SMON) performs many database maintenance tasks, including the following:

*Creates and manages the temporary tablespace metadata and reclaims space from orphaned temporary segments.

* Maintains the undo tablespace by onlining, offlining, and shrinking the undo segments based on undo space usage Statistics.

* Cleans up the data dictionary when it is in a transient and inconsistent state.

* Maintains the system change number (SCN) to time mapping table that is used to support Oracle Flashback features.
SMON is resilient to internal and external errors that occur during background activities. SMON can run as a thread or an operating system process. In an Oracle Real Application Clusters(Oracle RAC) database, the SMON process of one instance can perform instance recovery for other instances that have failed.