ORA-01034: ORACLE not available ORA-27101: shared memory realm does not exist – A Comprehensive Troubleshooting Guide

In this article, we present a detailed and comprehensive guide to understanding and resolving the ORA-01034: ORACLE not available and ORA-27101: shared memory realm does not exist errors that often challenge Oracle database administrators and developers. We have meticulously compiled expert insights and troubleshooting strategies to help you quickly diagnose and resolve these issues, ensuring minimal downtime and enhanced system reliability.

Understanding the Oracle Database Architecture

Before diving into the specifics of these errors, it is crucial to understand the underlying Oracle database architecture. The Oracle instance is composed of the memory structures, background processes, and storage mechanisms that work together to manage data efficiently. Key components include:

System Global Area (SGA): A shared memory region that contains data and control information for the instance.
Background Processes: Essential tasks such as process monitor, database writer, and log writer that maintain the instance.
Oracle Listener: Manages incoming client connection requests.

The interdependence of these components is critical. When an error such as ORA-01034 or ORA-27101 occurs, it usually signifies that one or more components are not in sync, often due to configuration issues or an improperly started instance.

ORA-01034: This error code indicates that the Oracle instance is not available or cannot be accessed.
ora-27101: This is not a standard Oracle error code, but it often accompanies ORA-01034. It’s an operating system error code related to shared memory. This code can vary depending on your operating system.

Causes of ORA-01034: ORACLE not available

The ORA-01034: ORACLE not available error is a clear indication that the Oracle instance is either not started or is in a state where it cannot be accessed by client applications. This error is commonly encountered during startup or after an unexpected shutdown. Our approach to resolving this error involves a systematic verification of the database’s operational status and the environment variables.

Key Causes of ORA-01034

Instance Not Started: The most frequent cause is that the Oracle instance has not been initiated.
Incorrect Environment Variables: Misconfigured settings such as ORACLE_HOME or ORACLE_SID can lead to connection failures.
Listener Configuration Issues: A misconfigured Oracle listener can prevent proper communication between the database and client applications.
Incomplete Shutdown or Startup: Abrupt shutdowns or partial startups can leave the instance in an unstable state.

Verifying Steps for ORA-01034

To resolve this issue, you can follow these general troubleshooting steps:

1. Check Oracle Services

Make sure that the Oracle services are running. You can do this using the lsnrctl status command for the listener and sqlplus / as sysdba to check the database instance status.

[oracle@amsora root]$ sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Wed Sep 6 13:00:24 2023
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle. All rights reserved.

Connected to an idle instance.

SQL>

2. Check Oracle Environment Variables

Verify that your Oracle environment variables (e.g., ORACLE_HOME, ORACLE_SID) are correctly set. These variables are essential for Oracle to locate the necessary files and configurations.

[oracle@oractl root]$ echo $ORACLE_HOME
/u01/app/oracle/product/19c/dbhome_1
[oracle@amsora root]$ echo $ORACLE_SID
oractl
[oracle@oractl root]$

3. Review Listener Status:

Use the following commands to verify that the Oracle listener is operational. Restart the listener, first stop then start

lsnrctl status
lsnrctl stop
lsnrctl start

4. Startup the Instance:

If the instance is not running, log in to SQL*Plus as a privileged user (e.g., SYSDBA) and execute:

SQL> STARTUP;

Troubleshooting for Both Errors ORA-01034 and ORA-27101

usually both the errors ORA-01034 and ORA-27101 frequently occur together, our integrated troubleshooting strategy involves a holistic approach:

Resolution: ORA-01034 and ORA-27101

Step-by-Step Integrated Diagnostic Process

Initial Environment Verification:
- Confirm that all necessary environment variables are correctly set.
- Ensure that the Oracle Listener is running and correctly configured.
Instance Status Check:
- Verify the running processes to determine if the Oracle instance has been started.
- If the instance is not running, initiate the startup process using SQL*Plus.
Memory Parameter Review:
- Open the Oracle initialization file (init.ora or spfile) and verify that memory-related parameters are set appropriately.
- Consider temporary adjustments to these parameters to see if the instance can allocate the necessary SGA.
Operating System Compatibility:
- Confirm that the operating system’s shared memory settings are compatible with the memory requirements of the Oracle instance.
- Adjust kernel parameters if there is a discrepancy.
Detailed Log Analysis:
- Analyze the alert log and trace files for any specific error messages related to memory allocation.
- Look for patterns or recurring issues that might indicate deeper configuration problems.
Restart and Monitor:
- Once adjustments have been made, restart the Oracle instance.
- Monitor the startup process closely, checking for the absence of both ORA-01034 and ORA-27101 in the logs.

[oracle@oractl root]$ set oracle_sid=oractl
[oracle@amsora root]$ sqlplus /nolog

SQL*Plus: Release 19.0.0.0.0 - Production on Wed Sep 6 13:03:38 2023
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle. All rights reserved.

SQL> conn sys/sys as sysdba
Connected to an idle instance.
SQL> shutdown abort
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 3741317648 bytes
Fixed Size 8903184 bytes
Variable Size 721420288 bytes
Database Buffers 3003121664 bytes
Redo Buffers 7872512 bytes
Database mounted.
Database opened.
SQL>

An other method to troubleshoot both the errors ORA-01034 and ORA-27101

Sometimes, the database will not be mounted correctly, so we need to mount it manually. For that, shut it down and start it up then mount. Log in as oracle user, then run the following commands:

[oracle@amsora root]$ sqlplus / as sysdba;
SQL*Plus: Release 19.0.0.0.0 - Production on Wed Sep 6 13:02:04 2023
Version 19.3.0.0.0
this is to delete
Copyright (c) 1982, 2019, Oracle. All rights reserved.
this is to delete
Connected to an idle instance.
this is to delete
SQL> shutdown immediate;

SQL> startup nomount;

SQL> alter database mount;
SQL> alter database open;
SQL> exit
…………………………………….
SQL> sqlplus "/ as sysdba"
SQL> startup
this is to delete
Oracle instance started
------
Database mounted.
Database opened.
thi sis to delete
SQL>

In summary, the ORA-01034: ORACLE not available and ORA-27101: shared memory realm does not exist errors are indicative of underlying issues related to the initialization and memory allocation of the Oracle instance. By employing a systematic troubleshooting approach—ranging from verifying instance status and environment variables to reviewing memory parameters and operating system limits—we can resolve these errors efficiently. Furthermore, by adopting preventative measures such as regular maintenance, robust backup plans, and advanced diagnostic techniques, we ensure that our Oracle environments remain stable and resilient.

If you experience these errors, remember that methodical troubleshooting and adherence to best practices are key to restoring system functionality. With careful attention to detail and a focus on proactive system monitoring, we can mitigate the risk of these errors and ensure that our Oracle instances continue to operate at peak efficiency.