[nova] review guide for the bandwidth patches

Bal√°zs Gibizer balazs.gibizer at ericsson.com
Mon Jan 21 10:45:09 UTC 2019

On Fri, Jan 18, 2019 at 7:40 PM, Dan Smith <dms at danplanet.com> wrote:
>>  * 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?

I think in Stein we have time to finish the boot / delete use case of 
the feature but most probably do not have time to finish the move use 
cases. I belive that the boot / delete use case is already useful for 
end users. There are plenty of features in nova that are enabled before 
supporting all the cases, like move operations with NUMA.

> 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.

Why it is an abuse of the microversion to use it to signal that a new 
use case is supported? I'm confused. I was asked to use microversions 
to signal that a feature is ready.  So I'm not sure why in case of a 
feature (feature == one ore more use case(s)) it is OK to use a 
microversion but not OK when a use case (e.g. boot/delete) is completed.

> 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

I think "nothing is changed" would not be true. Some operation (e.g. 
server move) that was rejected before (or even accepted but caused 
unintentional resource overallocation) now works properly. Isn't it the 
"you can do this very obscure thing now" documentation of a 
microversion that makes the new API behavior discoverable?

> 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...

The boot / delete use case would not be experimental, that would be 

403 is a client error but in this case, in Stein, move operations would 
not be implemented yet. So for me that error is not a client error 
(e.g. there is no way a client can fix it) but a server error, like 
HTTP 501.


> --Dan

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