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.
The table below describes the basic Oracle Database background processes. The use of additional database server features or options can cause more background processes to be present. For example, when you use Advanced Queuing, the queue monitor (QMNn) background process is present.
| Description | |
| The database writer writes modified blocks from the database buffer cache to the datafiles. Oracle Database allows a maximum of 20 database writer processes (DBW0-DBW9 and DBWa-DBWj). The | |
| 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. | |
| 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. | |
| 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. | |
| 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. | |
| One or more archiver processes copy the redo log files to archival storage when they are full or a log switch occurs. | |
| 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. | |
| Dispatchers are optional background processes, present only when the shared server configuration is used. | |
| In an Oracle Real Application Clusters environment, this process manages resources and provides inter-instance resource control. |
V$BGPROCESS displays information about the background processes.
select * from V$BGPROCESS; --Oracle 10gR2ee
| PADDR | PSERIAL# | NAME | DESCRIPTION | ERROR |
| 00 | 0 | ARB0 | ASM Rebalance 0 | 0 |
| 00 | 0 | ARB1 | ASM Rebalance 1 | 0 |
| 00000007B772D4D0 | 1 | ARC0 | Archival Process 0 | 0 |
| 00000007BE71AC18 | 1 | ARC1 | Archival Process 1 | 0 |
| 00 | 0 | ASMB | ASM Background | 0 |
| 00000007B96EA648 | 1 | CJQ0 | Job Queue Coordinator | 0 |
| 00000007B86F0380 | 1 | CKPT | checkpoint | 0 |
| 00 | 0 | CTWR | Change Tracking Writer | 0 |
| 00000007B772BD18 | 1 | DBW0 | db writer process 0 | 0 |
| 00000007BE719460 | 1 | DBW1 | db writer process 1 | 0 |
| 00 | 0 | DBW2 | db writer process 2 | 0 |
| 00 | 0 | DIAG | diagnosibility process | 0 |
| 00 | 0 | DMON | DG Broker Monitor Process | 0 |
| 00 | 0 | EMN0 | Event Monitor Process 0 | 0 |
| 00 | 0 | FMON | File Mapping Monitor Process | 0 |
| 00 | 0 | GMON | diskgroup monitor | 0 |
| 00 | 0 | INSV | Data Guard Broker INstance SlaVe Process | 0 |
| 00 | 0 | LCK0 | Lock Process 0 | 0 |
| 00000007B96E9E60 | 1 | LGWR | Redo etc. | 0 |
| 00 | 0 | LMD0 | global enqueue service daemon 0 | 0 |
| 00 | 0 | LMON | global enqueue service monitor | 0 |
| 00 | 0 | LMS0 | global cache service process 0 | 0 |
| 00 | 0 | LNS0 | Network Server 0 | 0 |
| 00 | 0 | LNS1 | Network Server 1 | 0 |
| 00 | 0 | LSP0 | Logical Standby | 0 |
| 00 | 0 | LSP1 | Dictionary build process for Logical Standby | 0 |
| 00 | 0 | LSP2 | Set Guard Standby Information for Logical Standby | 0 |
| 00000007B86EFB98 | 1 | MMAN | Memory Manager | 0 |
| 00000007B772CCE8 | 1 | MMNL | Manageability Monitor Process 2 | 0 |
| 00000007B86F0B68 | 1 | MMON | Manageability Monitor Process | 0 |
| 00 | 0 | MRP0 | Managed Standby Recovery | 0 |
| 00 | 0 | NSV0 | Data Guard Broker NetSlave Process 0 | 0 |
| 00000007BE718C78 | 1 | PMON | process cleanup | 0 |
| 00000007B96E9678 | 1 | PSP0 | process spawner 0 | 0 |
| 00000007B86F2B08 | 3 | QMNC | AQ Coordinator | 0 |
| 00 | 0 | RBAL | ASM Rebalance master | 0 |
| 00000007BE719C48 | 1 | RECO | distributed recovery | 0 |
| 00 | 0 | RSM0 | Data Guard Broker Resource Guard Process 0 | 0 |
| 00 | 0 | RSM1 | Data Guard Broker Resource Guard Process 1 | 0 |
| 00 | 0 | RVWR | Recovery Writer | 0 |
| 00000007B772C500 | 1 | SMON | System Monitor Process | 0 |
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