wait event

 select *
from (
select inst_id, event,evt.wait_class, count(*) "SESSION_WAITING_IO"
from gv$active_session_history ash, v$event_name evt
where ash.sample_time > sysdate - 1 and ash.session_state = 'WAITING' and ash.event_id = evt.event_id and evt.wait_class ='System I/O' group by inst_id, evt.wait_class,event order by inst_id, count(*) desc )
where session_waiting_io > 10