When
an Oracle instance starts, the background processes are started after the
memory for the SGA is allocated. The background processes manage system
activity and i/o, monitor other processes and maintain the integrity of
the system.
Mandatory Background Processes
The exact Oracle architecture you have depends
on your database configuration which determines which background processes run
but every Oracle instance will have the following background processes.
- SMON - the system monitor process which performs instance recovery on startup if required, coalesces free space and monitors system activity to ensure the Oracle instance is in a valid state
- PMON - the process monitor which monitors other server processes and performs recovery when a process fails
- LGWR - the log writer process which writes the redo buffers to the online redo log files
- RECO - the recoverer process which cleans up distributed transaction failures in distributed databases
- DBWRn - the database writer process(es) which write changed data blocks from the database buffer cache to the data files
- CKPT - the checkpoint process updates the control file and data file headers with checkpoint information and signals the database writer (DBWRn) processes to write changed data blocks to the data files
- MMON and MMNL - the manageability monitor processes which collect data for the Automatic Workload Repository (AWR)
Optional Background Processes
These
include:
- ARCn - the archiver processes which archive inactive redo logs
- CJQ0 and Jnnn - the job queue coordinator and slave processes which perform scheduled tasks for users
- FBDA - the flashback data archiver process which writes the pre-change image of changed rows of tracked tables into Flashback Data Archives.
- SMC0 - the space management coordinator process which coordinates space management related tasks