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On 3/11/2015 7:23 PM, Ian Wells wrote:
> On 11 March 2015 at 10:56, Matt Riedemann <mriedem at linux.vnet.ibm.com
> <mailto:mriedem at linux.vnet.ibm.com>> wrote:
>
>     While looking at some other problems yesterday [1][2] I stumbled
>     across this feature change in Juno [3] which adds a config option
>     "allow_duplicate_networks" to the [neutron] group in nova. The
>     default value is False, but according to the spec [4] neutron allows
>     this and it's just exposing a feature available in neutron via nova
>     when creating an instance (create the instance with 2 ports from the
>     same network).
>
>     My question then is why do we have a config option to toggle a
>     feature that is already supported in neutron and is really just
>     turning a failure case into a success case, which is generally
>     considered OK by our API change guidelines [5].
>
>     I'm wondering if there is anything about this use case that breaks
>     other NFV use cases, maybe something with SR-IOV / PCI?  If not, I
>     plan on pushing a change to deprecate the option in Kilo and remove
>     it in Liberty with the default being to allow the operation.
>
>
> This was all down to backward compatibility.
>
> Nova didn't allow two interfaces on the same Neutron network.  We tried
> to change this by filing a bug, and the patches got rejected because the
> original behaviour was claimed to be intentional and desirable.  (It's
> not clear that it was intentional behaviour because it was never
> documented, but the same lack of documented intent meant it's also not
> clear it was a bug, so the situation was ambiguous.)
>
> Eventually it was fixed as new functionality using a spec [1] so that
> the change and reasoning could be clearly described, and because of the
> previous concerns, Racha, who implemented the spec, additionally chose
> to use a config item to preserve the original behaviour unless the new
> one was explicitly requested.
> --
> Ian.
>
> [1]
> https://review.openstack.org/#/c/97716/5/specs/juno/nfv-multiple-if-1-net.rst
>
>
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For anyone following along, we're deprecating the 
allow_duplicated_networks option in Kilo and will remove it in Liberty 
(and just make it work like this by default):

https://review.openstack.org/#/c/163581/

-- 

Thanks,

Matt Riedemann