I have to model the air flow caused by rotating shaft, should I apply reference frame to the whole shaft solid or should I apply moving wall to the shaft's surface.
Typically there is no reason to include the actual solid volume - only the fluid volume. If it's simply a rotating shaft - no blades, etc - applying the reference frame just to the surface of the shaft is typically good enough. You could also see if there's a substantial difference when you include a volume around the shaft and apply the reference frame to that volume as well.
