[nova] review guide for the bandwidth patches

Dan Smith dms at danplanet.com
Fri Jan 18 18:40:23 UTC 2019

> * There will be a second new microversion (probably in Train) that will 
> enable move operations for server having resource aware ports. This 
> microversion split will allow us not to block the create / delete 
> support for the feature in Stein.
> * The new microversions will act as a feature flag in the code. This 
> will allow merging single use cases (e.g.: server create with one ovs 
> backed resource aware port) and functionally verifying it before the 
> whole generic create use case is ready and enabled.
> * A nova-manage command will be provided to heal the port allocations 
> without moving the servers if there is enough resource inventory 
> available for it on the current host. This tool will only work online 
> as it will call neutron and placement APIs.
> * Server move operations with the second new microversion will 
> automatically heal the server allocation.

I wasn't on this call, so apologies if I'm missing something important.

Having a microversion that allows move operations for an instance
configured with one of these ports seems really terrible to me. What
exactly is the point of that? To distinguish between Stein and Train
systems purely because Stein didn't have time to finish the feature?

IMHO, we should really avoid abusing microversions for that sort of
thing. I would tend to err on the side of "if it's not ready, then it's
not ready" for Stein, but I'm sure the desire to get this in (even if
partially) is too strong for that sort of restraint.

Can we not return 403 in Stein, since moving instances is disable-able
anyway, and just make it work in Train? Having a new microversion with a
description of "nothing changed except we finished a feature so you can
do this very obscure thing now" seems like we're just using them as an
experimental feature flag, which was definitely not the intent. I know
returning 403 for "you can't do this right now" isn't *as* discoverable,
but you kinda have to handle 403 for operations that could be disabled
anyway, so...


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