The best way to understand how all the diffrent files work together is to examine the Oracle startup proccess. Even though it might seem like a strange place in this book to cover the topic, it makes the most sense from a sysadmin point of view, so lets dive in.
When Oracle starts an instance it reads the spfile or pfile to determine the initialization paramters. It uses these paramters to allocate the SGA and create background proccesses. All this is done without associating a database to the instance! At this point the instance is started but not mounted, or as some say "Started in no mount mode". They say that because you can reach this state by using the SQL*Plus command "startup nomount".
SQL> startup nomount; ORACLE instance started. Total System Global Area 184549376 bytes Fixed Size 1300928 bytes Variable Size 157820480 bytes Database Buffers 25165824 bytes Redo Buffers 262144 bytes SQL> quit # ps -ef | grep -i ora_ oracle 720 1 0 14:14:20 ? 0:00 ora_reco_test oracle 710 1 0 14:14:19 ? 0:00 ora_mman_test oracle 708 1 0 14:14:19 ? 0:00 ora_pmon_test oracle 712 1 0 14:14:19 ? 0:00 ora_dbw0_test oracle 718 1 0 14:14:19 ? 0:00 ora_smon_test oracle 714 1 0 14:14:19 ? 0:00 ora_lgwr_test oracle 716 1 0 14:14:19 ? 0:00 ora_ckpt_test oracle 726 1 0 14:14:20 ? 0:00 ora_s000_test oracle 724 1 0 14:14:20 ? 0:00 ora_d000_test oracle 722 1 0 14:14:20 ? 0:00 ora_cjq0_test #
When a database is mounted, the datafiles are actually associated is the instance. It's somewhat akin to loading the bullets into a gun; the gun is mearly a vehicle to utilize bullets and without them it's utterly useless. As a fun test, you can rename the datafile directory (where the control and datafiles are) and startup the instance without mounting. You won't get an error, you won't get a complaint... because Oracle isn't interested in anything but the parameter files at this point. Even though the pfile specifies the location of the control file, it hasn't tried to open the control files yet so it won't complain!
This last revolation is extremely important. Why? You'll notice that alot of the documenation, particularly reguarding recovery will tell you to connect to an instance even though it's in need of major recovery. At first glance, in some cases, you'll scratch your head trying to figure out why they expect you to start an instance of a database thats been destroyed. Well, now you know.
Moving along to the second phase of database startup, the database is mounted. In this step we associate the control, data, redo, and other database related files to the running instance. If you are missing files this is where you'll get your error. If you started your instance using nomount you can't use the startup command again, but you can use alter database statements to change the state of your instance.
Lets quickly look at what happens when you mount your database with the instance already running but with the all the data and control files missing (renamed data directory):
SQL> alter database mount; alter database mount * ERROR at line 1: ORA-00205: error in identifying controlfile, check alert log for more info SQL> quit # tail /u01/app/oracle/admin/test/bdump/alert_test.log alter database mount Wed Oct 13 14:28:12 2004 ORA-00202: controlfile: '/u02/oradata/test/control01.ctl' ORA-27037: unable to obtain file status SVR4 Error: 2: No such file or directory Additional information: 3 Wed Oct 13 14:28:12 2004 Controlfile identified with block size 0 Wed Oct 13 14:28:12 2004 ORA-205 signalled during: alter database mount... #
Notice that it complains about the first controlfile and not the datafiles. Thats because the parameter file has a listing of all the controlfiles and the controlfile is responsable for storing information about all the other datafiles and resources used by the database. If the controlfile can't be read the database doesn't know what else exists! This is why you should be careful to keep good backups of your controlfiles using plain ol' system backups. This is also why Oracle maintains multiple copies (typically 3) of the control file for safety.
Lets put all the datafiles back and try mounting the database again.
# mv test.HOLD/ test # sqlplus sys/passwd as sysdba SQL*Plus: Release 10.1.0.2.0 - Production on Wed Oct 13 14:36:09 2004 Copyright (c) 1982, 2004, Oracle. All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - 64bit Production With the Partitioning, OLAP and Data Mining options SQL> alter database mount; Database altered.
Looking at the proccesses on the system, you'll notice that after mounting the database no change has occured to the proccesses.
Once an instance is started using a pfile and the mount proccess has used the controlfile(s) to associate the datafiles with the instance we need to open the database. Untill a database is opened it is not accessable. It's equivilent to starting a system in single-user mode. Some ammount of interaction with the databse is avalible at this stage but it's limited to fixed tables and views. The fixed tables and views are those in the data dictionary (Oracle's internal configuration tables).
But here's the confusing part, the normal data dictionary tables (ALL_USERS, for example) will give you an error, but most of the V$ tables, which are supposed to be the dynamic tables, work fine! What exactly "fixed" is supposed to mean in the case I dunno.
SQL> select * from all_users;
select * from all_users
*
ERROR at line 1:
ORA-01219: database not open: queries allowed on fixed
tables/views only
SQL> select status from v$instance;
STATUS
------------
MOUNTED
SQL>
To open the database for normal access, we can alter the database again.
SQL> alter database open; Database altered.
The shutdown proccess is the simply opposite of the startup.
SQL> shutdown immediate; Database closed. Database dismounted. ORACLE instance shut down.