512 /* * [OUT] Number of input extents that were successfully exchanged: * 1. The first @nr_exchanged input extents were successfully * deallocated. * 2. The corresponding first entries in the output extent list correctly * indicate the GMFNs that were successfully exchanged. * 3. All other input and output extents are untouched. * 4. If not all input exents are exchanged then the return code of this * command will be non-zero. * 5. THIS FIELD MUST BE INITIALISED TO ZERO BY THE CALLER! */