Originally posted by efrijters
You're correct.
When you use "SHUTDOWN IMMEDIATE", Oracle will kill all active sessions and rollback all transactions. If you are shutting down database which has high volumn transaction, it may take more than 15 mins to shut down cleanly.

After you issue "SHUTDOWN IMMEDAITE", try to monitor OS process for checking out what kind of process is running with high CPU time.

For your case, I trust it may be SMON. If so, just wait for it to complete.

Cheers,

Reid Lai
Oracle DBA in HK