Comment 14 for bug 1811235

Per comment 2:

> Dear Brin, I feel so sorry for replying too late.
> I found this issue in private cloud, and also I found Ocata version of nova (15.1.5 release) may
> have this issue after reading its code. The key codes in nova/compute/manager.py --> class
> ComputeManager --> prep_resize --> _error_out_instance_on_exception.

This is the latent bug from Ocata (and probably earlier). My patches above should fix this bug.

> Besides, I have some questions. First, why the latest version of nova not supports migrating
> instance to same host event through some drivers support that function..Second, if there is any
> problem when user migrates instance without migration destination host but there is just one
> compute node.

This looks like a regression with the 2.56 microversion in the API code, which was added in Queens. As we can tell from the vmware driver's supports_migrate_to_same_host=True capability, we know that at least one compute driver supports cold migrating to the same compute service host and the API should probably allow that. It kind of makes sense for vmware since you could have a single nova-compute service hosting a single vcenter cluster which has 1000 ESXi hosts in it, so your only option to cold migrate is to that same compute host (but underlying vcenter would actually cold migrate to another ESXi host in the cluster).

That regression in 2.56 is a separate bug though in my opinion.