I assume i can collect info from SMF through REXX
Surely, not a good starting position. . .
I'd like to know how to use SMF and how i can retrieve the info i need.
Depending on exactly what you are looking for, it may already exist. Suggest you speak with your storage management, system suppoprt, or performance measurement people to learn what is already available.
There is the liklihood that you will not have access to the real smf data. The volume is huge. On the good side, nearly everything that happens on the system is recorded.