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[nova][ironic] need help understanding 'cpu_arch' in nova ironic driver


Hey all,

Recently I'm trying to figure out how to fulfill a use case in the nova 
ironic driver around treating an ironic node's 'cpu_arch' as optional.

This is coming up as a result of a downstream issue [1] and recent 
change on the ironic side [2] to make cpu_arch optional for iscsi 
deployments. The change makes it so that ironic will _not_ include a 
'cpu_arch' attribute as part of a node's properties if iscsi.

On the nova side, we have a filter scheduler ImagePropertiesFilter which 
will only match a node if the architecture, hypervisor_type, and vm_mode 
in the glance image properties match the cpu_arch of the node. We have 
always pulled the cpu_arch of the from the ironic node properties since 
the original ironic driver code was added to nova.

Now, the use case I'm trying to solve today [3] is where an iscsi ironic 
deployment has no cpu_arch specified on the ironic side and the deployer 
wants to use glance images with architecture specified in their image 
properties. Today (and historically always) the request to create an 
instance in this situation will fail with NoValidHost because the nova 
ironic driver could not determine a cpu_arch from the ironic node.

My questions are:

* Is this a valid thing to want to do?

* If if is valid, what is the correct way that we should handle missing 
cpu_arch in the nova ironic driver? Should we guess at a default 
cpu_arch? If so, how? Or should we add a new config option where a 
default cpu_arch can be specified? Or, other?

Thanks in advance for any input you can offer.

Cheers,
-melanie

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1653788
[2] https://review.opendev.org/620634
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1688838