5.5 About Oracle Database Background Processes

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To maximize performance and accommodate many users, a multiprocess Oracle Database system uses background processes. Background processes consolidate functions that would otherwise be handled by multiple database programs running for each user process. Background processes asynchronously perform I/O and monitor other Oracle Database processes to provide increased parallelism for better performance and reliability.

Table 5-4 describes the fundamental background processes, many of which are discussed in more detail elsewhere in this book. The use of additional database features or options can cause more background processes to be present. For example:
  • When you use Oracle Database Advanced Queuing, the queue monitor (QMNn) background process is present.
  • When you set the FILE_MAPPING initialization parameter to true for mapping data files to physical devices on a storage subsystem, the FMON process is present.
  • If you use Oracle Automatic Storage Management (Oracle ASM), then additional Oracle ASM–specific background processes are present.
Table 5-4 Oracle Database Background Processes

Process Name Description
Database writer (DBWn or BWnn) The database writer writes modified blocks from the database buffer cache to the data files. Oracle Database allows a maximum of 100 database writer processes. The names of the first 36 database writer processes are DBW0-DBW9 and DBWa-DBWz. The names of the 37th through 100th database writer processes are BW36-BW99.

The DB_WRITER_PROCESSES initialization parameter specifies the number of database writer processes. The database selects an appropriate default setting for this initialization parameter or adjusts a user-specified setting based on the number of CPUs and the number of processor groups.

For more information about setting the DB_WRITER_PROCESSES initialization parameter.
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 data files by DBWn. This event is called a checkpoint. The checkpoint process is responsible for signalling DBWn at checkpoints and updating all the data files 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 terminated 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.
Archiver (ARCn) One or more archiver processes copy the redo log files to archival storage when they are full or a log switch occurs. Archiver processes are the subject of Managing Archived Redo Log Files.
Recoverer (RECO) The recoverer process is used to resolve distributed transactions that are pending because of a network or system failure in a distributed database. At timed intervals, the local RECO attempts to connect to remote databases and automatically complete the commit or rollback of the local portion of any pending distributed transactions. For information about this process and how to start it.
Dispatcher (Dnnn) Dispatchers are optional background processes, present only when the shared server configuration is used. Shared server was discussed previously in "Configuring Oracle Database for Shared Server".

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