Database Reference
In-Depth Information
■
Note
For the prior command to work, you need either an
spfile
or
init.ora
file in the
ORACLE_HOME/dbs
directory.
When your instance starts successfully, you should see messages from Oracle indicating that the system global
area (SGA) has been allocated. The database is mounted and then opened:
ORACLE instance started.
Total System Global Area 2137886720 bytes
Fixed Size 2290416 bytes
Variable Size 1207962896 bytes
Database Buffers 922746880 bytes
Redo Buffers 4886528 bytes
Database mounted.
Database opened.
From the prior output the database startup operation goes through three distinct phases in opening an Oracle
database:
1.
Starting the instance
2.
Mounting the database
3.
Opening the database
You can step through these one at a time when you start your database. First, start the Oracle instance
(background processes and memory structures):
SQL> startup nomount;
Next, mount the database. At this point, Oracle reads the control files:
SQL> alter database mount;
Finally, open the data files and online redo log files:
SQL> alter database open;
■
as you'll see later in this topic, it's especially important to understand these startup phases when performing
rMan backup and recovery tasks. For example, in some scenarios you may need your database to be in mount mode. in
that mode, it's important to understand that the control file is open but the data files and online redo logs have not been
opened yet.
Tip
