Database writer (DBWn)     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 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.        

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 a     
    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.     

Archiver (ARCn)     One or more archiver processes copy the redo log files to archival storage when they     
    are full or a log switch occurs.     

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.       

Dispatcher (Dnnn)     Dispatchers are optional background processes, present only when the shared server      
    configuration is used.
    
Global Cache Service     In a Real Application Clusters environment, this process manages resources and      
(LMS)     provides inter-instance resource control.