What to do when we get  errors  like ORA-01034: ORACLE not available ORA-27101: shared memory realm does not exist.

Solution:

The ORA-01034 is  simple error, it means that your database is down!  Oracle oerr utility notes this about the ORA-01034 error:

ORA-01034: ORACLE not available

Cause:

 Oracle was not started up. Possible causes include the following:

  • The SGA requires more space than was allocated for it.
  • The operating-system variable pointing to the instance is improperly defined.

Action: 

Refer to accompanying messages for possible causes and correct the problem mentioned in the other messages. If Oracle has been initialized, then on some operating systems, verify that Oracle was linked correctly. See the platform specific Oracle documentation.

You may find that you are experiencing ORA-01034 when attempting to upgrade your Oracle database.ORA-01034 is thrown during the attempt to upgrade because there is an invalid entry in one of the following files:

  1. /etc/oratab
  2. /var/opt/oracle/oratab

The ORA-01034 is a result of a discrepancy between ORACLE_HOME andORACLE_SID

To resolve ORA-01034, be sure that the ORACLE_HOME and ORACLE_SIDproperly match within the files /etc/oratab or /var/opt/oracle/oratab .