Background Process Description

Process Monitor(PMON)

1. Process Monitor is an Oracle background process created when you start a database instance.
2. The process will free up resources if a user process fails.
3. It normally wakes up every 3 seconds to perform its housekeeping activities.
4. Pmon must always be running else the instance terminates.
5. To speed up the housekeeping the PMON can be manually wokeup as below:

           SQL> oradebug setmypid

           SQL> oradebug wakeup 2

           Assuming, PMON as process 2 here.

System Monitor(SMON)

1. Performs instance recovery, cleans up after dirty shutdowns and coalesces adjacent free extents into larger free extents and also monitors temporary segments.
2. Wakes up every 5 miuntes to perform activities.
3. In a non-failed instance can also perform failed instance recovery for other failed RAC instances.
4. In case of tablespace offline errors or any as such, the transactions are eventually recovered by SMON when taablespace or file is brought back online.
5. To check if the SMON process is running:

                      $ ps -ef | grep smon

                      oracle   31144     1  0 11:10 ?        00:00:00 ora_smon_orcl

 Archiver's Process (ARCH)

1. Created when you start an instance in ARCHIVE LOG MODE.
2. Writes filled redo logs to the archive log locations.
3. An instance can have upto 10 ARCn processes (ARC0 to ARC9).
4. The LGWR starts a new ARCn whenever current number of processes is insufficient. The alert file keeps a track of these.
5. The parameter used is: LOG_ARCHIVE_MAX_PROCESSES.

Log Writer (LGWR)
Created when you start the instance.
Writes the redo log buffers to the on-line redo log files.
If these files are mirrored, all the members of the group will be written out simultaneously.
LGWR writes:
A commit record when a user process commits a transaction.
Redo log buffers
Every three seconds
When the redo log buffer is one-third full.
When a DBWn process writes modified buffers to disk, if necessary.