[openstack-dev] [nova] Device tagging: rebuild config drive upon instance reboot to refresh metadata on it

Daniel P. Berrange berrange at redhat.com
Mon Feb 20 15:59:59 UTC 2017

On Mon, Feb 20, 2017 at 10:46:09AM -0500, Artom Lifshitz wrote:
> I don't think we're trying to re-invent configuration management in
> Nova. We have this problem where we want to communicate to the guest,
> from the host, a bunch of dynamic metadata that can change throughout
> the guest's lifetime. We currently have two possible avenues for this
> already in place, and both have problems:
> 1. The metadata service isn't universally deployed by operators for
> security and other reasons.
> 2. The config drive was never designed for dynamic metadata.
> So far in this thread we've mostly been discussing ways to shoehorn a
> solution into the config drive avenue, but that's going to be ugly no
> matter what because it was never designed for what we're trying to do
> in the first place.
> Some folks are saying that we admit that the config drive is only for
> static information and metadata that is known at boot time, and work
> on a third way to communicate dynamic metadata to the guest. I can get
> behind that 100%. I like the virtio-vsock option, but that's only
> supported by libvirt IIUC. We've got device tagging support in hyper-v
> as well, and xenapi hopefully on the way soon [1], so we need
> something a bit more universal. How about fixing up the metadata
> service to be more deployable, both in terms of security, and IPv6
> support?

FYI, virtio-vsock is not actually libvirt specific. the VSOCK sockets
transport was in fact invented by VMWare and first merged into Linux
in 2013 as a vmware guest driver.

A mapping of the VSOCK protocol over virtio was later defined to enable
VSOCK to be used with QEMU, KVM and Xen all of which support virtio.
The intention was explicitly that applications consuming VSOCK in the
guest would be portable between KVM & VMWare.

That said I don't think it is available via XenAPI, and doubt hyperv
will support it any time soon, but it is none the less a portable
standard if HVs decide they want such a feature.

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