I am planning openstack-ansible major upgrade from current version Ussuri to next sequential version Victoria. 

I am checking below link for the same.
https://docs.openstack.org/openstack-ansible/victoria/admin/upgrades/major-upgrades.html

And I need support regarding below points:

1. What are the pre-requisite backups to be taken before the upgrade?

2. What is the impact of the upgrade , are all openstack services upgraded parallel or one by one.

Example: there are 3 control nodes having each openstack control service lxc container (neutron, nova-api, cinder) running, so at a time one service of openstack is upgraded on each control node example nova (control-1)--->nova(control02)--->nova(control03) or all together.

3. What is the downtime expected in this upgrade?

4. How are openstack services upgraded on the compute nodes? What is the impact on VM running on the compute node that is being upgraded?

5. Any suggestion or experience which can be helpful for the upgrade.

6. What should be the rollback plan in case some issue occurs?

Regards

Rambo