Starting Oracle Database

Starting Oracle Database has following stages :
a)nomount
b)mount
c)open

a)nomount:

when the Oracle Database starts it starts in nomount mode first. In this stage it reads the paramter file to read the configuration.By default it looks for spfileSID.ora if this file dosen't exists then it looks for default spfile on the server side.
If default spfile is also not found then it looks for initSID.ora the pfile and starts the instance.
You can also mention the pfile at the startup command to override the use of default spfile to start the instance. for example

STARTUP PFILE=$ORACLE_HOME/DBS/INITDEVEN.ora

After this it allocates the SGA and starts the background processes.After this opening the alert file and trace file is performed.

b)mount :

Generally we start the database in mount mode to perform some maintenance operations.In this stage the database is started but it is not open that is users are not allows to connect to the database.

you can start the database in mount mode to perform operations like renaming data files,enabling and disabling archivelog mode and also for cold backups

In the mount mode following tasks are performed:

a) associating a database with previously started instance.
b)locating and opening control file
c) reading the control file to obtain the information regarding the names and status of data file and online redo log files.

Here in this stage the existence of the datafile and online redo log file is not checked.

c) Open:

Now the database is open means valid user can connect to database and perform data access operations.

In this stage the opening of datafiles and online redo log files are performed.If any of the datafile or online redo log files are not present then the oracle server will return an error.

Oracle server verifies that all the data files and online redo log files are consistence.If necessary the background process SMON perform the instance recovery