The relative throughput of an instance compared to the backup
        instances.

def instance_throughput_ratio(self, inst_id):
        """
        The relative throughput of an instance compared to the backup
        instances.
        """
        inst_thrp, otherThrp = self.getThroughputs(inst_id)

        # Backup throughput may be 0 so moving ahead only if it is not 0
        r = inst_thrp / otherThrp if otherThrp and inst_thrp is not None \
            else None
        return r