[openstack-dev] [nova] Proposal for an Experiment

Matt Riedemann mriedem at linux.vnet.ibm.com
Wed Jul 15 14:49:39 UTC 2015

On 7/15/2015 9:18 AM, Ed Leafe wrote:
> Hash: SHA512
> Changing the architecture of a complex system such as Nova is never
> easy, even when we know that the design isn't working as well as we
> need it to. And it's even more frustrating because when the change is
> complete, it's hard to know if the improvement, if any, was worth it.
> So I had an idea: what if we ran a test of that architecture change
> out-of-tree? In other words, create a separate deployment, and rip out
> the parts that don't work well, replacing them with an alternative
> design. There would be no Gerrit reviews or anything that would slow
> down the work or add load to the already overloaded reviewers. Then we
> could see if this modified system is a significant-enough improvement
> to justify investing the time in implementing it in-tree. And, of
> course, if the test doesn't show what was hoped for, it is scrapped
> and we start thinking anew.
> The important part in this process is defining up front what level of
> improvement would be needed to make considering actually making such a
> change worthwhile, and what sort of tests would demonstrate whether or
> not whether this level was met. I'd like to discuss such an experiment
> next week at the Nova mid-cycle.
> What I'd like to investigate is replacing the current design of having
> the compute nodes communicating with the scheduler via message queues.
> This design is overly complex and has several known scalability
> issues. My thought is to replace this with a Cassandra [1] backend.
> Compute nodes would update their state to Cassandra whenever they
> change, and that data would be read by the scheduler to make its host
> selection. When the scheduler chooses a host, it would post the claim
> to Cassandra wrapped in a lightweight transaction, which would ensure
> that no other scheduler has tried to claim those resources. When the
> host has built the requested VM, it will delete the claim and update
> Cassandra with its current state.
> One main motivation for using Cassandra over the current design is
> that it will enable us to run multiple schedulers without increasing
> the raciness of the system. Another is that it will greatly simplify a
> lot of the internal plumbing we've set up to implement in Nova what we
> would get out of the box with Cassandra. A third is that if this
> proves to be a success, it would also be able to be used further down
> the road to simplify inter-cell communication (but this is getting
> ahead of ourselves...). I've worked with Cassandra before and it has
> been rock-solid to run and simple to set up. I've also had preliminary
> technical reviews with the engineers at DataStax [2], the company
> behind Cassandra, and they agreed that this was a good fit.
> At this point I'm sure that most of you are filled with thoughts on
> how this won't work, or how much trouble it will be to switch, or how
> much more of a pain it will be, or how you hate non-relational DBs, or
> any of a zillion other negative thoughts. FWIW, I have them too. But
> instead of ranting, I would ask that we acknowledge for now that:
> a) it will be disruptive and painful to switch something like this at
> this point in Nova's development
> b) it would have to provide *significant* improvement to make such a
> change worthwhile
> So what I'm asking from all of you is to help define the second part:
> what we would want improved, and how to measure those benefits. In
> other words, what results would you have to see in order to make you
> reconsider your initial "nah, this'll never work" reaction, and start
> to think that this is will be a worthwhile change to make to Nova.
> I'm also asking that you refrain from talking about why this can't
> work for now. I know it'll be difficult to do that, since nobody likes
> ranting about stuff more than I do, but right now it won't be helpful.
> There will be plenty of time for that later, assuming that this
> experiment yields anything worthwhile. Instead, think of the current
> pain points in the scheduler design, and what sort of improvement you
> would have to see in order to seriously consider undertaking this
> change to Nova.
> I've gotten the OK from my management to pursue this, and several
> people in the community have expressed support for both the approach
> and the experiment, even though most don't have spare cycles to
> contribute. I'd love to have anyone who is interested become involved.
> I hope that this will be a positive discussion at the Nova mid-cycle
> next week. I know it will be a lively one. :)
> [1] http://cassandra.apache.org/
> [2] http://www.datastax.com/
> - --
> - -- Ed Leafe
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Without reading the whole thread, couldn't you just do a feature branch 
(but that would require reviews in gerrit which we don't want), or fork 
the repo in github and just hack on it there without gerrit?

I'm sure many will say it's not cool to fork the repo, but that's 
essentially what you'd be doing anyway, so meh.  I think you just have 
to have an understanding that whatever you work on in the fork won't 
necessarily be accepted back in the main repo.



Matt Riedemann

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