[openstack-dev] [nova] key_pair update on rebuild (a whole lot of conversations)

Matt Riedemann mriedemos at gmail.com
Tue Oct 3 22:06:22 UTC 2017

On 10/3/2017 3:16 PM, Sean Dague wrote:
> There is currently a spec up for being able to specify a new key_pair
> name during the rebuild operation in Nova -
> https://review.openstack.org/#/c/375221/
> For those not completely familiar with Nova operations, rebuild triggers
> the "reset this vm to initial state" by throwing out all the disks, and
> rebuilding them from the initial glance images. It does however keep the
> IP address and device models when you do that. So it's useful for
> ephemeral but repeating workloads, where you'd rather not have the
> network information change out from under you.

We also talked quite a bit about rebuild with volume-backed instances 
today, and the fact the root disk isn't replaced during rebuild in that 
case, for which there are many reported bugs...

> The spec is a little vague about when this becomes really useful,
> because this will not save you from "I lost my private key, and I have
> important data on that disk". Because the disk is destroyed. That's the
> point of rebuild. We once added this preserve_ephemeral flag to rebuild
> for trippleo on ironic, but it's so nasty we've scoped it to only work
> with ironic backends. Ephemeral should mean ephemeral.
> Rebuild bypasses the scheduler. A rebuilt server stays on the same host
> as it was before, which means the operation has a good chance of being
> faster than a DELETE + CREATE, as the image cache on that host should
> already have the base image for you instance.

It also means no chances for NoValidHost or resource claim failures.

> A bunch of data was collected today in a lot of different IRC channels
> (#openstack-nova, #openstack-infra, #openstack-operators).
> = OpenStack Operators =
> mnaser said that for their customers this would be useful. Keys get lost
> often, but keeping the IP is actually valuable. They would also like this.
> penick said that for their existing environment, they have a workflow
> where this would be useful. But they are moving away from using nova for
> key distribution because in Nova keys are user owned, which actually
> works poorly given that everything else is project owned. So they are
> building something to do key distribution after boot in the guest not
> using nova's metadata.
> Lots of people said they didn't use nova's keypair interfaces, they just
> did it all in config management after the fact.
> = Also on reboot? =
> Because the reason people said they wanted it was: "I lost my private
> key", the question at PTG was "does that mean you want it on reboot?"
> But as we dive through the constraints of that, people that build "pet"
> VMs typically delete or disable cloud-init (or similar systems) after
> first boot. Without that kind of agent, this isn't going to work anyway.
> So also on reboot seems very fragile and unuseful.
> = Infra =
> We asked the infra team if this is useful to them, the answer was no.
> What would be useful them is if keypairs could be updated. They use a
> symbolic name for a keypair but want to do regular key rotation. Right
> now they do this by deleting then recreating keypairs, but that does
> mean there is a window where there is no keypair with that name, so
> server creates fail.
> It is agreed that something supporting key rotation in the future would
> be handy, that's not in this scope.
> = Barbican =
> In the tradition of making a simple fix a generic one, it does look like
> there is a longer term part of this where Nova should really be able to
> specify a Barbican resource url for a key so that things like rotation
> could be dealt with in a system that specializes in that. It also would
> address the very weird oddity of user vs. project scoping.
> That's a bigger more nebulous thing. Other folks would need to be
> engaged on that one.
> = Where I think we are? =
> I think with all this data we're at the following:
> Q: Should we add this to rebuild
> A: Yes, probably - after some enhancement to the spec *
> * - we really should have much better use cases about the situations it
> is expected to be used in. We spend a lot of time 2 and 3 years out
> trying to figure out how anyone would ever use a feature, and adding
> another one without this doesn't seem good
> Q: should this also be on reboot?
> A: NO - it would be too fragile
> I also think figuring out a way to get Nova out of the key storage
> business (which it really shouldn't be in) would be good. So if anyone
> wants to tackle Nova using Barbican for keys, that would be ++. Rebuild
> doesn't wait on that, but Barbican urls for keys seems like a much
> better world to be in.
> 	-Sean

Sean, thanks for summarizing the various discussions had today. I've 
also included the operators list on this.




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