ORA-27101: shared memory realm does not exist tips

MOSC offers valuable information regarding ORA-27101. ORA-27101 is often thrown along with ORA-01034 during database installation.  ORA-27101 is also seen after installation is complete, when attempting to use SQL*Plus.

To resolve this issue with ORA-27101, you should be sure that ORACLE_HOME and ORACLE_SID are correct, and ORACLE_HOME should not have trailing slash.

For checking on trailing trash:

$ echo $ORACLE_HOME
$ /app/oracle/817/ << Incorrect

$ echo $ORACLE_HOME
$ /app/oracle/817 << Correct

For ORACLE_SID:

$ echo $ORACLE_SID
$ ora817

$ ps -ef | grep smon
ora_smon_ORA817

(Here, ORACLE_SID setting has been altered from ora817 to ORA817.)

Basically, to resolve ORA-27101, Once ORACLE_HOME and ORACLE_SID are correct, and then attempt to start SQL*Plus. 

Question:

I keep receiving ORA-27101 with ORA-01034 after I tried installing the Oracle XE on Windows XP.  Unfortunately, when I connect to the database, ORA-27101 is thrown instead.  Any help?

Answer:

ORA-27101 may indicate that the system was not able to find the shared memory realm.  So, you could be receiving ORA-27101 because you tried to modify the database parameters such as SGA size.  If this is something that you have done and feel this is why you are encountering ORA-27101, you can try to bring back the original parameters.  Another cause of ORA-27101 is that the instance pointer, or ORACLE_SID, may be gone.  If this is the case, to resolve ORA-27101 you should first try looking for ORACLE_SID. There are several different methods depending on what server you are using:

• For Unix- try using "echo $ORACLE_SID"
• For Windows- right click on My Computer?properties?Advance Tab?Environment Variable.  You can view here where or not ORACLE_SID is defined.

If you find in either cases that ORACLE_SID is blank or undefined, this may be the cause of ORA_27101.  You can then set the ORACLE_SID appropriate for the database to resolve ORA-27101.

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