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1.
Which is the difference between an Oracle database and an Oracle
instance?
The
Oracle instance = SGA + oracle processes. For more information
about SGA and SGA tuning click here.
Oracle
database = Data files + Control Files + Redo log files
Note:
The archive log files, the parameter files and password files are not
part of the database. The archive logs are copies of redo log files,
Init.ora is a configuration file for the Oracle instance and the
password file is a place to store the password for "sysdba"
accounts.
For
more information about the SGA/ tuning the SGA area click here.
2.
Which are the main Oracle background processes?
| Oracle Process |
Tasks performed |
| PMON
(Process Monitor) |
-
when a session is
broken:
-
restart the Dispatcher and server processes if they are stopped
-
registers information about the instance and dispatcher
processes with the network listener
|
| SMON
(System Monitor) |
- when an instance is broken or
restarted:
- a rollback is performed (INSTANCE RECOVERY).
In an Oracle Real Application Clusters database, the SMON
process of one instance can perform instance recovery for
other instances that have failed.
- cleans up temporary segments that are no longer in use
- coalesces contiguous free extents in dictionary managed
tablespaces that have PCTINCREASE set to a non-zero value. |
| DBWn
(Database Writer)
|
- writes modified blocks from the
database buffer cache to the data files.
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. However, the DB_WRITER_PROCESSES
initialization parameter could be used to specifies the number
of DBWn processes. Oracle Database
allows a maximum of 20 database writer processes (DBW0-DBW9 and
DBWa-DBWj). |
| LGWR
(Log Writer) |
- write to disk the content of redo
log buffer |
| CKPT
(Checkpoint) |
- tell to the DBWn to write the buffer
cache information to the disk files |
| CJQ0
(Coordinator Job Queue) |
- check for scheduled jobs create
processes Jnnn processes which will run the particular
jobs. |
| Jnnn
(Job slave process) |
- run different scheduled jobs. For
more information about the
Oracle jobs
follow the link. |
| Dnnn
(Dispatcher process) |
- receive requests from the listener
and acts like a requests coordinator for the Oracle Shared (MTS)
Server processes. For more information about the
Shared (MTS)
Server architecture follow the link. |
| ARCn
(Archiver Process) |
- archive the log files (if database
is in ARCHIVELOG mode). |
3.
Which are the mandatory Oracle background processes?
PMON,
SMON, DBWn, LGWR, CKPT
4.
How could I see which Oracle background processes are running on the OS
level?
On
Solaris, UNIX, Linux systems using ls -ef | grep
<SID> command:
On
Windows, there is one process which has many threads.
More information
about this subject (
Oracle Server Architecture ) you can get from
www.in-oracle.com |