Oracle Background Processes

Here are some of the most important Oracle background processes:

 Not all background processes are mandatory for an instance.

 Some are mandatory and some are optional. Mandatory background processes are DBWn, LGWR, CKPT, SMON, PMON.

1.Database Writer (DBWR)  

Database Writer or Dirty Buffer Writer process is responsible for writing dirty buffers from the database block cache to the database data files.
Oracle Database allows a maximum of 20 database writer processes.DBWR only writes blocks back to the data files on commit,
or when the cache is full and space has to be made for more blocks.

2. Log Writer (LGWR)  
 
The Log Writer process (LGWR) writes the redo log buffer to a redo log file on disk. LGWR is an Oracle background process responsible for redo log
buffer management.
LGWR writes all redo entries that have been copied into the buffer since the last time it wrote.In RAC, each RAC instance has its own LGWR process
that maintains that instances thread of redo logs.

3. Checkpoint (CKPT) 

Checkpoint is an internal mechanism of oracle. When a checkpoint occurs the latest SCN is written to the control file and to all datafile headers.
This operation is performed by the checkpoint process.

Main Purposes--
1- To establish a data consistency.
2- Enable faster database recovery.

4.System Monitor (SMON)

The System Monitor process (SMON)performs instance recovery at instance start up. SMON is also responsible for cleaning up temporary segments that
are no longer in use; it also coalesces contiguous free extents to make larger blocks of free space available .It is also responsible for
roll back and roll forward.

5.Process Monitor (PMON)

The Process Monitor (PMON) performs process recovery when a user process fails. PMON is responsible for cleaning up the cache and freeing resources
that the process was using.
For example, it resets the status of the active transaction table, releases locks, and removes the process ID from the list of active processes.

Optional Background Processes

6. Archiver (ARCn)
The optional Archive process writes filled redo logs to the archive log locations. ARCn is present only if the database is running  in archive log mode and automatic archiving is enabled.