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-> Oracle Server Architecture (Q & A)

 

 Oracle Server Architecture (Q & A)

 

 

Questions:

  1. Which is the difference between an Oracle database and an Oracle instance?

  2. Which are the main Oracle background processes?

  3. Which are the mandatory Oracle background processes?

  4. How could I see which Oracle background processes are running on the OS level?

  5. What is the name of the Oracle process that modifies the database buffer cache?

  6. How could I start/ stop the database?

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 SGA and SGA tuning .

Oracle database server architecture

 

Oracle database = Data files + Control Files + Redo log files

 

 

Oracle database server architecture

 

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: Oracle SGA and SGA tuning .

 

 

 

2.  Which are the main Oracle background processes?

 

    Oracle Process     Tasks performed
  PMON (Process Monitor)

- when a session is broken:                                                                           

  • the rollback is performed  (PROCESS  RECOVERY)

  • the memory used became available 

- 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:

 

Oracle database server architecture

 

On Windows, there is one process which has many threads. 

 

 

5.  What is the name of the Oracle process that modifies the database buffer cache?

 

SERVER Process

 

 

6.  How could I start/ stop the database?

 

Supposing the database is down.

 

a) connect to an idle instance as SYS

    set ORACLE_SID = dbr     (dbr is the name of the instance)

    sqlplus /nolog

    SQL> conn / as sysdba

 

b) startup the database

    SQL> startup;

 

c) to stop the database;

    SQL> shutdown immediate;  (roll back applied when database is stopped)  

    SQL> shutdown abort;   (roll back applied when database is started)  

 

Oracle database server architecture

 

 

 

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