On starting upan instance in 3 node RAC Database,and got the below error
SQL> startup mount ;
ORA-00304: requested INSTANCE_NUMBER is busy
on checking we found the background sessions are still hanging at the OS level.
Solution is to Kill the hanging sessions at the os level and restart the instance.
ps -ef | grep -i TEST AND
KILL -9 PID1 PID2 PID3
after killing the hanging ssession , wee are able to startup the instance without any issues