System Monitor Process (SMON)

The system monitor process (SMON) performs recovery, if necessary, at instance startup. If the Oracle instance fails, any information in the SGA that has not been written to disk is lost. For example, the failure of the operating system causes an instance failure. After the loss of the instance, the background process SMON automatically performs instance recovery when the database is reopened. Instance recovery when the database is reopened. Instance recovery consists of the following steps:

  • Rolling forward to recover data that has not been recorded in the data files but that has been recorded in the online redo log. This data has not been written to disk because of the loss of the SGA during instance failure. During this process, SMON reads the redo log files and applies the changes recorded in the redo log to the data blocks. Because all committed transactions have been written to the redo logs, this process completely recovers these transactions.
  • Opening the database so that users can log on. Any data that is not locked by unrecovered transactions is immediately available.
  • Rolling back uncommitted transactions. They are rolled back by SMON or by the individual server processes as they access locked data.

SMON also performs some space maintenance functions:

  • It combines, or coalesces, adjacent areas of free space in the data files.
  • It deallocates temporary segments to return them as free space in data files
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