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Re: Libvirt Snapshots


Thanks for the background.  My thoughts:

* Telling a user to build from source isn't a great option for them -
it's painful, they don't get updates automatically etc.  Are we going
to start distributing packages again?

* I can't imagine any open source project removing functionality like
the QEMU pass-through - those sort of tactics are confined to
commercial products that are trying to lock you in, in my experience.
I'm sure libvirt would never try that, and if they did I'm sure it
would rapidly be forked.

* I can imagine that libvirt might switch to the JSON QEMU monitor
protocol (I don't understand why it isn't already, I'm probably wrong
here?).  Thankfully the QEMU JSON protocol also has a pass-through,
but we would ideally switch to the better protocol when/if libvirt
switches over.

* It sounds like this is the exact use-case for which the pass-through
functionality was designed - libvirt doesn't support what we need (at
least in commonly-utilized versions), we have some additional
potential roadmap features that may not be supported, so we _should_
use the pass-through.

Once commonly distributed versions of libvirt have the functionality
we need, then we can remove this functionality from OpenStack, but we
do have to fix this for Essex.  Hopefully this can continue long-term:
OpenStack can develop the functionality that clouds need that libvirt
is missing, in Python instead of in C, and then libvirt can
incorporate this functionality where you want to.


> The command line and monitor passthrough capability is there as a means
> to perform short term workarounds/hacks on a specific version of libvirt,
> for functionality not already available via a supported API.
> We make absolutely no guarentee that usage of passthrough capabilities
> will continue to work correctly if you upgrade to a newer libvirt. As
> such it is not something you really want to use as the basis for a
> production release of OpenStack that expects compatibility with future
> libvirt releases
> http://berrange.com/posts/2011/12/19/using-command-line-arg-monitor-command-passthrough-with-libvirt-and-kvm/
> And it possible for people to build any new libvirt release for
> any old Ubuntu release they desire to support. So if someone really
> wants to use new OpenStack on older distros they aren't locked out.
> Regards,
> Daniel

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