Comment 3 for bug 1792077

Indeed, we only set the initial disk_mapping['unit'] values if the image meta scsi_controller.model is 'virtio-scsi':

https://github.com/openstack/nova/blob/ce520ee789bf6a46f56de86769d74c095ce432cf/nova/virt/libvirt/driver.py#L3920-L3937

But then, we increment the 'unit' value if 'bus' == 'scsi' (NOT only 'virtio-scsi') when attaching a volume:

https://github.com/openstack/nova/blob/ce520ee789bf6a46f56de86769d74c095ce432cf/nova/virt/libvirt/driver.py#L1422-L1423

Or creating a guest config:

https://github.com/openstack/nova/blob/ce520ee789bf6a46f56de86769d74c095ce432cf/nova/virt/libvirt/driver.py#L3892-L3894

I think the intention was to only to control the 'unit' numbering of the disks in nova if 'virtio-scsi' and let other disk type's unit numbering be handled automatically by libvirt. But because the 'unit' number incrementing code only checks generically for 'bus' == 'scsi', it's controlling the disk unit numbering for non-'virtio-scsi' as well.

I'm not sure there's anyway to check if 'virtio-scsi' from the disk_mapping. If there is, it seems like the fix would be to add that check to the disk unit increment conditionals.