Oracle DBA

Basic

startup nomount description

+++++++++++++++++++++

when you give startup nomount command parameter file is read from $ORACLE_HOME/dbs location.
and then your oracle instance started.
oracle instance is nothing but sga (system global area) and background processes.
If  it is unable to find prameter file it will through below error.

startup mount

++++++++++++++++
when you give startup mount command it will read the control file .Inside parameter file(init<sid>.ora) you will find control_files which will have controlfile path and numbe of controlfile.
control_files is a static parameter.

Things you can do in mount stage.
================================
1)enable/dis abled archive log
2) datafiles/redo log files can renamed at the mount stage.
3)changing dbid/dbname

After mount your database will open.

====================================
Datafiles and online redo log files are check for consistency while opening the database.
Datafiles and redo logfiles are made available to users at the OPEN stage.