ORA-01102: cannot mount database in EXCLUSIVE mode

What does it mean?

The database cannot be mounted in EXCLUSIVE mode because another instance (or leftover files from an old instance) still appears to hold a lock on it. The instance stops at the MOUNT stage.

Why does it happen?

The previous instance did not shut down cleanly and lock files (lk<SID> under $ORACLE_HOME/dbs, or leftover shared memory/semaphores) remain; another instance is already attached to the same database; or a second instance was started in a non-RAC installation. Leftover processes/shared memory from a previous instance or a stale lk<SID> file are the most common causes.

How to fix it

Verify that no other instance is actually running: ps -ef | grep pmon and ipcs -m (shared memory segments). If remnants exist, ABORT the instance and clean up; if necessary remove leftover segments/semaphores with ipcrm. Delete the lk<SID> lock file under $ORACLE_HOME/dbs (only if the instance is definitely down). Then try STARTUP MOUNT. On RAC, check CLUSTER_DATABASE and node status.