ORA-01102: cannot mount database in EXCLUSIVE mode

The ORA-01102 error "cannot mount database in EXCLUSIVE mode" occurs when an Oracle instance tries to mount the database in exclusive mode but another instance has already mounted the database either exclusively or in parallel mode.

1. Shutdown any other instances that may have the database mounted:

SHUTDOWN IMMEDIATE;

2. Kill any remaining Oracle background or shadow processes related to the database using OS commands:

kill -9 <process_ids>

[oracle@muthu ~]$ ps -ef|grep pmon

oracle    1821     1  0 16:40 ?        00:00:00 ora_pmon_oradbsby

oracle    2750     1  0 16:41 ?        00:00:00 ora_pmon_oradb

oracle    2936  2626  0 16:42 pts/0    00:00:00 grep --color=auto pmon

[oracle@muthu ~]$ kill -9 2750

[oracle@muthu ~]$ kill -9 1821

3. Check and remove the stale lock file lk<SID> in the $ORACLE_HOME/dbs directory if it exists.

4. Remove any leftover shared memory segments and semaphores related to the Oracle SID.

5. In RAC, verify the cluster_database parameter is set to TRUE for all instances.

6. After cleaning up, start the database again:

STARTUP MOUNT;

This error is mostly about ensuring no conflicting instance is holding the database open and cleaning up orphaned processes or locks so the instance can mount the database exclusively without conflict.