On Thu, Sep 11, 2014 at 2:18 AM, Daniel P. Berrange <berra...@redhat.com>

> On Thu, Sep 11, 2014 at 09:23:34AM +1000, Michael Still wrote:
> > On Thu, Sep 11, 2014 at 8:11 AM, Jay Pipes <jaypi...@gmail.com> wrote:
> >
> > > a) Sorting out the common code is already accounted for in Dan B's
> original
> > > proposal -- it's a prerequisite for the split.
> >
> > Its a big prerequisite though. I think we're talking about a release
> > worth of work to get that right. I don't object to us doing that work,
> > but I think we need to be honest about how long its going to take. It
> > will also make the core of nova less agile, as we'll find it hard to
> > change the hypervisor driver interface over time. Do we really think
> > its ready to be stable?
> Yes, in my proposal I explicitly said we'd need to have Kilo
> for all the prep work to clean up the virt API, before only
> doing the split in Lxxxxx.
> The actual nova/virt/driver.py has been more stable over the
> past few releases than I thought it would be. In terms of APIs
> we're not really modified existing APIs, mostly added new ones.
> Where we did modify existing APIs, we could have easily taken
> the approach of adding a new API in parallel and deprecating
> the old entry point to maintain compat.
> The big change which isn't visible directly is the conversion
> of internal nova code to use objects. Finishing this conversion
> is clearly a pre-requisite to any such split, since we'd need
> to make sure all data passed into the nova virt APIs as parameters
> is stable & well defined.
> > As an alternative approach...
> >
> > What if we pushed most of the code for a driver into a library?
> > Imagine a library which controls the low level operations of a
> > hypervisor -- create a vm, attach a NIC, etc. Then the driver would
> > become a shim around that which was relatively thin, but owned the
> > interface into the nova core. The driver handles the nova specific
> > things like knowing how to create a config drive, or how to
> > orchestrate with cinder, but hands over all the hypervisor operations
> > to the library. If we found a bug in the library we just pin our
> > dependancy on the version we know works whilst we fix things.
> >
> > In fact, the driver inside nova could be a relatively generic "library
> > driver", and we could have multiple implementations of the library,
> > one for each hypervisor.
> I don't think that particularly solves the problem, particularly
> the ones you are most concerned about above of API stability. The
> naive impl of any "library" for the virt driver would pretty much
> mirror the nova virt API. The virt driver impls would thus have to
> do the job of taking the Nova objects passed in as parameters and
> turning them into something "stable" to pass to the library. Except
> now instead of us only having to figure out a stable API in one
> place, every single driver has to reinvent the wheel defining their
> own stable interface & objects. I'd also be concerned that ongoing
> work on drivers is still going to require alot of patches to Nova
> to update the shims all the time, so we're still going to contend
> on resource fairly highly.
> > > b) The conflict Dan is speaking of is around the current situation
> where we
> > > have a limited core review team bandwidth and we have to pick and
> choose
> > > which virt driver-specific features we will review. This leads to bad
> > > feelings and conflict.
> >
> > The way this worked in the past is we had cores who were subject
> > matter experts in various parts of the code -- there is a clear set of
> > cores who "get" xen or libivrt for example and I feel like those
> > drivers get reasonable review times. What's happened though is that
> > we've added a bunch of drivers without adding subject matter experts
> > to core to cover those drivers. Those newer drivers therefore have a
> > harder time getting things reviewed and approved.
> FYI, for Juno at least I really don't consider that even the libvirt
> driver got acceptable review times in any sense. The pain of waiting
> for reviews in libvirt code I've submitted this cycle is what prompted
> me to start this thread. All the virt drivers are suffering way more
> than they should be, but those without core team representation suffer

Can't you replace the word 'libvirt code' with 'nova code' and this would
still be true? Do you think landing virt driver code is harder then landing
non virt driver code? If so do you have any numbers to back this up?

If the issue here is 'landing code in nova is too painful', then we should
discuss solving that more generalized issue first, and maybe we conclude
that pulling out the virt drivers gets us the most bang for our buck.  But
unless we have that more general discussion, saying the right fix for that
is to spend a large amount of time  working specifically on virt driver
related issues seems premature.

> to an even greater degree.  And this is ignoring the point Jay & I
> were making about how the use of a single team means that there is
> always contention for feature approval, so much work gets cut right
> at the start even if maintainers of that area felt it was valuable
> and worth taking.
> > > c) It's the impact to the CI and testing load that I see being the
> biggest
> > > benefit to the split-out driver repos. Patches proposed to the XenAPI
> driver
> > > shouldn't have the Hyper-V CI tests run against the patch. Likewise,
> running
> > > libvirt unit tests in the VMWare driver repo doesn't make a whole lot
> of
> > > sense, and all of these tests add a not-insignificant load to the
> overall
> > > upstream and external CI systems. The long wait time for tests to come
> back
> > > means contributors get frustrated, since many reviewers tend to wait
> until
> > > Jenkins returns some result before they review. All of this leads to
> > > increased conflict that would be somewhat ameliorated by having
> separate
> > > code repos for the virt drivers.
> >
> > It is already possible to filter CI runs to specific paths in the
> > code. We just didn't choose to do that for policy reasons. We could
> > change that right now with a trivial tweak to each CI system's zuul
> > config.
> We have to jump through far more hoops to do so, even as developers
> running things locally. eg want to run pep8 locally to test your
> work ? You have to wait 3 minutes while it checks the entire of
> the nova codebase. So we had to invent a mode where it only checks
> the files in the current GIT HEAD. Likewise for unit tests - if you
> invoke them you have to pass args to filter to just the area of the
> repo you are working on. These kind of problems simply goes away
> completely if we have separate repos without having to do special
> setup tasks. Smaller modules would be far less daunting for new
> contributors looking to get involved in Nova development too which
> I think is an important factor
> Regards,
> Daniel
> --
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