Oracle Database Background Processes:
1.Database writer (DBWn)
The database writer writes modified blocks from the database buffer cache to the data files. Oracle Database allows a maximum of 36 database writer processes (DBW0-DBW9 and DBWa-DBWj). The DB_WRITER_PROCESSES initialization parameter specifies the number of DBWn 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, see the Oracle Database Performance Tuning Guide.
2.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. See Chapter 12, "Managing the Redo Log" for information about the log writer process.
3.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.
4.System monitor (SMON)
The system monitor performs database 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.
5.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.Free the resource used by a user process when it fals.Dynamically register database services with listeners. Monitor sessions for idle session timeout.
6.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 Chapter 13, "Managing Archived Redo Logs".
7.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, see Chapter 35, "Managing Distributed Transactions".
8.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".
9.MMON(Manageability Monitor process(MMON):
Issuing alerts when the threshold is crossed,发出告警。收集AWR必须的统计数据，把统计数据写入磁盘。10g中保存在SYSAUX表空间中。生成server--generated报警。每小时把shared pool中的统计信息写入磁盘，或者shared pool占用超过15%。
中文参照：http://www.cnblogs.com/rusking/p/4341664.html
《FROM:http://docs.oracle.com/cd/E11882_01/server.112/e25494/manproc.htm#ADMIN11186》