ORA-27100: shared memory realm already exists tips Oracle Error Tips by Stephanie F.

 

The Oracle docs note this on the ora-27100 error:

Also see Startup Fails With ORA-27100 After Restarting The Windows Service [MOSC ID 370469.1]

Typically, ORA-27100 can be resolved by simply restarting Windows.  However, Internet sources offer advice for times when this error is thrown, and not easily resolved, applying to Oracle Enterprise; version 9.2.0.6.0 and Microsoft Server 2003.  The error may be thrown similarly to the example below upon starting up the database:

startup nomount
ORA-27100: shared memory realm already exists

SQL> shutdown immediate
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

Restarted the service , still the same error.
Rebooting the server did not help as well.

This is caused by

• The parameter sga_max_size was changed from 2GB to 3GB through OEM.
• Changing the parameter from OEM will make the change in the spfile.
• ORA-27100 is received as a result of changes made to parameters affecting the SGA size.
• The service was created to start with spfile .
• When the service is restarted , it will use the spfile and the error occurs due to SGA parameter change.

To resolve ORA-27100, you should execute the following instructions:

- Move the spfile from its default location or rename the spfile .

- Restart the service

- Now the service will start the database using pfile , with old SGA settings.  This should start your database successfully .

- Create the new spfile from pfile.