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System Monitor - SMON


Notes:

The system monitor process has several functions:
SMON performs instance recovery automatically when the instance or server crashes. This process includes applying all changes from the active log to the datafiles (changes that where not already written to the datafiles), then rolling back any changes that were not committed before the instance crashed.
SMON deletes unnecessary temporary segments which are created when disk space is required to support a sort operation. These additional temporary sort area segments may be required for many reasons - for example when an ORDER BY clause is specified for a large result set or the CREATE INDEX command is executed against a large table.
SMON will also periodically and automatically coalesce contiguous free extents into a larger single extent for dictionary-managed files. (You will learn more about dictionary-managed files in the Tablespaces lesson later.) This is beneficial because the larger free extent is more likely to be reused by a subsequent request for a free extent. Note that this only occurs for dictionary-managed tablespaces where the PCTINCREASE setting is > 0.
Finally, SMON will also shrink rollback segments to the size specified in the OPTIMAL parameter, if rollback segments are used (9i provides an Automatic UNDO Management Feature). See the Managing UNDO lesson for more information.