[openstack-dev] [nova][placement] Trying to summarize bp/glance-image-traits scheduling alternatives for rebuild

Dan Smith dms at danplanet.com
Thu May 3 20:26:24 UTC 2018

> I'm late to this thread but I finally went through the replies and my
> thought is, we should do a pre-flight check to verify with placement
> whether the image traits requested are 1) supported by the compute
> host the instance is residing on and 2) coincide with the
> already-existing allocations. Instead of making an assumption based on
> "last image" vs "new image" and artificially limiting a rebuild that
> should be valid to go ahead. I can imagine scenarios where a user is
> trying to do a rebuild that their cloud admin says should be perfectly
> valid on their hypervisor, but it's getting rejected because old image
> traits != new image traits. It seems like unnecessary user and admin
> pain.

Yeah, I think we have to do this.

> It doesn't seem correct to reject the request if the current compute
> host can fulfill it, and if I understood correctly, we have placement
> APIs we can call from the conductor to verify the image traits
> requested for the rebuild can be fulfilled. Is there a reason not to
> do that?

Well, it's a little itcky in that it makes a random part of conductor a
bit like the scheduler in its understanding of and iteraction with
placement. I don't love it, but I think it's what we have to do. Trying
to do the trait math with what was used before, or conservatively
rejecting the request and being potentially wrong about that is not
reasonable, IMHO.


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