Resolving ORA-27101: Shared Memory Realm Does Not Exist
Introduction
Understanding the Cause
Step-by-Step Solutions
Examples
Conclusion
Introduction
The ORA-27101 error is a common issue in Oracle databases that occurs when a shared memory realm does not exist. The error message typically reads:
ORA-27101: shared memory realm does not exist
This error usually indicates a problem with the initialization or configuration of the Oracle instance. Understanding the cause and how to resolve this error is crucial for maintaining database availability and performance.
Understanding the Cause
The ORA-27101 error can be caused by several factors, including:
- Oracle Instance Not Started: The most common cause is that the Oracle instance is not started.
- Invalid ORACLE_SID: The ORACLE_SID environment variable is not set correctly or does not match any running instance.
- Shared Memory Segment Issues: Issues with shared memory segments, such as insufficient memory allocation or operating system limits.
- Incorrect Initialization Parameters: Incorrect or missing initialization parameters in the parameter file (pfile or spfile).
Step-by-Step Solutions
To resolve ORA-27101, follow these troubleshooting steps:
1. Verify the Oracle Instance Status
Check if the Oracle instance is running using the following command:
[oracle@dbdocs ~]$ ps -ef | grep pmon
oracle 2572 1 0 04:36 ? 00:00:04 ora_pmon_dbdocs
oracle 6105 2514 0 09:42 pts/0 00:00:00 grep --color=auto pmon
[oracle@dbdocs ~]$
In the above example dbdocs oracle instance is up and running
If the instance is not running, start it using SQL*Plus:
$ sqlplus / as sysdba
SQL> STARTUP;
2. Check the ORACLE_SID Environment Variable
Ensure that the ORACLE_SID environment variable is set correctly:
[oracle@dbdocs ~]$ echo $ORACLE_SID
dbdocs
[oracle@dbdocs ~]$
If it is not set correctly, set it using the following command:
$ export ORACLE_SID=your_sid
3. Verify Shared Memory Segment Settings
Check the shared memory segment settings on your operating system:
[oracle@dbdocs ~]$ ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x0000020e 0 postgres 600 56 6
0x00000000 3 oracle 600 9691136 134
0x00000000 4 oracle 600 2348810240 67
0x00000000 5 oracle 600 7086080 67
0x20897588 6 oracle 600 28672 67
If there are issues with shared memory segments, you may need to adjust your operating system's shared memory parameters.
4. Review Initialization Parameters
Check the initialization parameters in your parameter file (pfile or spfile). Ensure all necessary parameters are set correctly. For example:
$ sqlplus / as sysdba
SQL> SHOW PARAMETER db_cache_size;
SQL> SHOW PARAMETER shared_pool_size;
If any parameters are missing or incorrect, modify them accordingly.
Examples
Example 1: Oracle Instance Not Started
You attempt to connect to the database and receive the ORA-27101 error. Checking the process list shows that the Oracle instance is not running:
[oracle@dbdocs ~]$ ps -ef | grep pmon
oracle 6154 2514 0 09:46 pts/0 00:00:00 grep --color=auto pmon
[oracle@dbdocs ~]$
Solution: Start the Oracle instance:
$ sqlplus / as sysdba
SQL> STARTUP;
ORACLE instance started.
Total System Global Area 2365585232 bytes
Fixed Size 9688912 bytes
Variable Size 822083584 bytes
Database Buffers 1526726656 bytes
Redo Buffers 7086080 bytes
Database mounted.
Database opened.
SQL>
Example 2: Incorrect ORACLE_SID
You receive the ORA-27101 error due to an incorrect ORACLE_SID environment variable:
$ echo $ORACLE_SID
old_sid
Solution: Set the correct ORACLE_SID:
$ export ORACLE_SID=new_sid
Example 3: Insufficient Shared Memory
The ORA-27101 error occurs due to insufficient shared memory allocation. Check the shared memory settings:
$ ipcs -m
Solution: Adjust the shared memory settings in your operating system configuration files (e.g., /etc/sysctl.conf) and reload the settings:
$ sysctl -p
Conclusion
ORA-27101: Shared Memory Realm Does Not Exist is an Oracle error that can occur due to issues with the Oracle instance, environment variables, shared memory segments, or initialization parameters. By understanding its causes and following the provided solutions, database administrators can effectively troubleshoot and resolve this error, ensuring the smooth operation of the Oracle database. Regular monitoring and proper configuration of database and system parameters are essential to prevent this issue from recurring.
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