def _sub_quota_values(self, v1, v2):

        if self._is_unlimited_value(v1) or self._is_unlimited_value(v2):
            return self.UNLIMITED_VALUE
        return v1 - v2
A helper method that handles unlimited values when performing
dataset/ETHPy150Open openstack/nova/nova/quota.py/DbQuotaDriver._sub_quota_values