System Monitor - Definition,Features - Managing Oracle Database Processes

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• Nirja Shah   -Posted on 23 Sep 15
  - Log writer (LGWR) - The log writer process writes redo log entries to disk.
  - Redo log entries are generated in the redo log buffer of the system global area (SGA).
  - LGWR writes the redo log entries sequentially into a redo log file.
  - If the database has a multiplexed redo log, then LGWR writes the redo log entries to a group of redo log files.
  
  - Checkpoint (CKPT) - At specific times, all modified database buffers in the system global area are written to the datafiles by DBWn.
  - This event is called a checkpoint. The checkpoint process is responsible for signalling DBWn at checkpoints and updating all the datafiles and control files of the database to indicate the most recent checkpoint.
  
  - System monitor (SMON) - The system monitor performs recovery when a failed instance starts up again.
  - In an Oracle Real Application Clusters database, the SMON process of one instance can perform instance recovery for other instances that have failed.
  - SMON also cleans up temporary segments that are no longer in use and recovers dead transactions skipped during system failure and instance recovery because of file-read or offline errors.
  - These transactions are eventually recovered by SMON when the tablespace or file is brought back online.
  
  - Process monitor (PMON) - The process monitor performs process recovery when a user process fails.
  - PMON is responsible for cleaning up the cache and freeing resources that the process was using.
  - PMON also checks on the dispatcher processes (described later in this table) and server processes and restarts them if they have failed.

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