Rotating path selection
The CSS tries to initiate each new I/O request for an I/O device on a rotating channel path basis within the paths in the control unit header. The CSS tries the next channel path in the rotation order following the last channel path initially selected for an I/O request for the control unit header. If that channel path is busy or unavailable, the CSS chooses the channel path in the control unit header that has the greatest likelihood of availability. Therefore, the rotation order is only used for the initial attempt for an I/O request.