[openstack-dev] Introducing the new OpenStack service for Containers

Daniel P. Berrange berrange at redhat.com
Tue Nov 19 19:09:23 UTC 2013

On Tue, Nov 19, 2013 at 10:02:45AM -0800, James Bottomley wrote:
> On Mon, 2013-11-18 at 14:28 -0800, Stuart Fox wrote:
> > Hey all
> > 
> > Not having been at the summit (maybe the next one), could somebody
> > give a really short explanation as to why it needs to be a separate
> > service?
> > It sounds like it should fit within the Nova area. It is, after all,
> > just another hypervisor type, or so it seems.
> I can take a stab at this:  Firstly, a container is *not* a hypervisor.
> Hypervisor based virtualisation is done at the hardware level (so with
> hypervisors you boot a second kernel on top of the virtual hardware),
> container based virtualisation is done at the OS (kernel) level (so all
> containers share the same kernel ... and sometimes even huge chunks of
> the OS). With recent advances in the Linux Kernel, we can make a
> container behave like a hypervisor (full OS/IaaS virtualisation), but
> quite a bit of the utility of containers is that they can do much more
> than hypervisors, so they shouldn't be constrained by hypervisor APIs
> (which are effectively virtual hardware APIs).
> It is possible to extend the Nova APIs to control containers more fully,
> but there was resistance do doing this on the grounds that it's
> expanding the scope of Nova, hence the new project.

You're focusing on the low level technical differences between containers
and hypervisor here which IMHO is not the right comparison to be making.
Once you look at the high level use cases many of the referenced container
virt apps are trying to address things don't look anywhere near as clear
cut. As mentioned elsewhere libvirt-sandbox which provides a application
sandboxing toolkit is able to leverage either LXC or KVM virt to achieve
its high level goals. Based on my understanding of Docker, I believe it
would actually be possible to run Docker images under KVM without much
difficultly. There will certainly be some setups that aren't possible
todo with hypervisors, but I don't think those will be in the majority,
nor require starting again from scratch, throwing out Nova.

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