Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

3.4 Starting Up the Database

Starting
a database is quite simpleon Windows you simply start the Oracle
services, and on Unix you issue the STARTUP
command from Server Manager or SQL*Plus (since
Oracle8i) or through Enterprise Manager. While
starting a database appears to be a single action, it involves an
instance and a database and occurs in several distinct phases. When
you start a database, the following actions are automatically
executed:

Starting the instance. Oracle reads the
instance
initialization parameters from the SPFILE or
INIT.ORA file on the server. Oracle then
allocates memory for the System Global Area and starts the background
processes of the instance. At this point, none of the physical files
in the database have been opened, and the instance is in the NOMOUNT
state. (Note that the number of parameters that must be defined in
the SPFILE in Oracle Database 10g
as part of the initial installation setup is greatly
reduced.)

There are problems that can prevent an instance from starting. For
example, there may be errors in the initialization file, or the
operating system may not be able to allocate the requested amount of
shared memory for the SGA. You also need the special privilege
SYSOPER or SYSDBA, granted through either the operating system or a
password file, to start an instance.

Mounting the database.
The
instance opens the database's control files. The
initialization parameter
CONTROL_files tells the instance where
to find these control files. At this point only the control files are
open. This is called the MOUNT state, and at this time, the
database is accessible only to the database administrator. In this
state, the DBA can perform only certain types of database
administration. For example, the DBA may have moved or renamed one of
the database files. The datafiles are listed in the control file but
aren't open in the MOUNT state. The DBA can issue a
command (ALTER DATABASE) to rename a datafile.
This command will update the control file with the new datafile name.

Opening the database.
The
instance opens the redo log files and datafiles using the information
in the control file. At this point, the database is fully open and
available for user access.