There are three major steps the Oracle instance goes through when you start the instance.
- NOMOUNT stage
- MOUNT stage
- OPEN stage
In NOMOUNT stage, Oracle just reads the parameter file. This stage is normally used to create a control file or to create a database.
In MOUNT stage, Oracle reads the control file for the first time. Oracle also allocates the shared memory for the System Global Area (SGA) and also starts the background processes (SMON, PMON, LGWR, DBWR, etc.). This stage is typically used when performing recovery operations.
In the OPEN stage, Oracle accesses the database datafiles. At this point, users can connect to the instance and interact with the database. This is the normal stage of operations.
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