ORA-27101: shared memory realm does not exist

i am facing this error when i start the oracle. I checked Oracle Home and SID. everything is set properly.

ORA-27101: shared memory realm does not exist

please help me to find out the solution. Thanks in advance

What is shared memory realm does not exist?

it means the SGA you tried to attach to isn't attachable - isn't there. use the wrong oracle_sid and or oracle_home, that'll do it to.. we use the oracle_sid + oracle_home to generate a shared memory segment id, then attach it.

How to fix ORA-01034 Oracle not available?

To resolve ORA-01034, be sure that the ORACLE_HOME and ORACLE_SID properly match within the files /etc/oratab or /var/opt/oracle/oratab . As a last step in solving your ORA-01034 problem and running DBUA, make sure that ORACLE_HOME is set to 'old' home, not 'new' home.

That error generally means there's no Oracle instance (the processes) to connect to. Someone needs to log in and start the instance.