Upon the interaction of an ultra intense laser pulse with a solid target, generated fast electrons can produce MeV ions from laser plasmas. These fast ions can be used in different applications ranging from ion implantation to nuclear reaction stimulation. The most important point is the efficiency of this fast ion production. We analyze - with help of an analytical model and PIC code simulations - the different acceleration mechanisms and compare the efficiency of electrostatic ion acceleration, at the front and rear of a foil target, the ponderomotive mechanism and acceleration by the shock wave in detail. The optimal plasma density distribution, shaped by laser pre-pulse, is found.