Instance won't start with message ORA-01089: Immediate shutdown in progress - no operations are permitted

Sandeep Nagpal called me yesterday and told me that he was having issues with an Oracle 8.1.7 instance. An INSERT operation was taking a very long time, so he had done a shutdown immediate. That also took time. So he did shutdown abort. That didn't work, so he killed all oracle processes from OS level with kill -9 command. Inspite of this, when he tries to start the instance now, the following error is displayed:

ORA-01089: Immediate shutdown in progress - no operations are permitted

This is usually due to orphaned shared memory segments and semaphores. Metalink Note 1010214.6 describes how to remove these.

To remove the orphaned shared memory segments and semaphores, use the
following commands:

 ipcrm -m       - for shared memory
 ipcrm -s       - for semaphores

Once Sandeep identified and removed the shared memory segments and semaphores, he was able to startup the instance.