I am having problem on restarting Informix after one of the chunks has been marked as DOWN. How do I change the status back to ONLINE?
Depending on which version you have, this may be easy or hard. Versions after (I believe) 7.3 can use
onspaces -s spacename -p pathname -o offset -O
to bring a chunk back online. For earlier versions, you'll need to ask Informix tech support to use their tbup program to bring the chunk back up.