It is true that it's not a DOS attack and it won't bring any of the OpenStack services down (glance/nova). But any user can call backup instance passing rotation as 0 consuming CPU/memory/network/disk resources of glance and nova hosts at the expense of service provider. In this situation, user cannot be charged a cent for backing up instance even though it utilized valuable resources of service provider.
It is true that it's not a DOS attack and it won't bring any of the OpenStack services down (glance/nova). But any user can call backup instance passing rotation as 0 consuming CPU/memory/ network/ disk resources of glance and nova hosts at the expense of service provider. In this situation, user cannot be charged a cent for backing up instance even though it utilized valuable resources of service provider.