Database MOUNT and START--What exactly they mean

I've asked this question to several experts. No one has given me a convinving answer. What exactly happens at these stages:

1)Instance Started
2)Database Mounted
3)Database Opened
4)Database Started.

Can someone explain clearly please.

Hi,

Instance started: memory allocated
database mounted: datafiles available for "maintenance" purposes (very restricted acces, local server acces only)
database opened: users can connect, all datafiles are available for use

anyway, opened/started :?

cheers

Fabien

Well, I think its got to be more than this. Again, to what extent the data files are used is not explained clearly. I am really looking at an absolute unamigious and compelete reply.

What abt the effect on control files, redolog files, rollback segments etc. I would appreciate anyone who can give a detailed explaination.

When a database is just mounted, what is the effect on the instance? etc.

Well, don't think any diff is there between Opened and Started however.

Please folks kindly explain.

I will give it a try:

When the instance is started, the init*.ora file is read by the system and SGA is allocated as per the initialization parameters. No database is associated with this instance.

Once you Mount the database, it is asoociated with the instance. The contol file of the DB is read and db files, redo log files and rollback segments, all are associated with the instance. (these files are not yet 'open') That is why this state is used for moving around the db files.

When the database is started or opened, all these files are open and available for use to the users.

Pick up a book on Oracle Concepts/Architecture and Admin/Backup and Recovery for a more complete answer.

Startup Nomount : -

Read the parameter file and start the instance.
Instance name identified by ORACLE_SID env parameter.
The parameter file controls such things as SGA size, database name that can connect to this instance.
Database not associated with instance yet.
As an example, you may need to do this if you will mount a standby database.

Mount : -

Associates database with the instance.
Open and read control files
Establish physical files associated with database. i.e location, size, amount of redo log files. Physical database file names and locations.

DBA may need to be in mount state for all kinds of DBA activity.
examples :
rename/move system datafile.
put database in archivelog mode.

Database not yet available for user connections.

Opened/started

Ensure physical files exist.
make available for normal operations.
open online datafiles, redo logs.
acquire rollback segments.

These states are useful in many situations to a DBA, main areas where these are useful are ;

Architecture/Admin
Backup/Recovery.

Raminder and Sureshy, thanks a lot. You guys have given some very good explaination. Thanks again. Thanks to Shadow21 too.