[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 13:41:47 UTC 2017

On Sat, Feb 18, 2017 at 01:54:11PM -0500, Artom Lifshitz wrote:
> A few good points were made:
> * the config drive could be VFAT, in which case we can't trust what's
> on it because the guest has write access
> * if the config drive is ISO9660, we can't selectively write to it, we
> need to regenerate the whole thing - but in this case it's actually
> safe to read from (right?)
> * the point about the precedent being set that the config drive
> doesn't change... I'm not sure I 100% agree. There's definitely a
> precedent that information on the config drive will remain present for
> the entire instance lifetime (so the admin_pass won't disappear after
> a reboot, even if using that "feature" in a workflow seems ludicrous),
> but we've made no promises that the information itself will remain
> constant. For example, nothing says the device metadata must remain
> unchanged after a reboot.
> Based on that here's what I propose:
> If the config drive is vfat, we can just update the information on it
> that we need to update. In the device metadata case, we write a new
> JSON file, overwriting the old one.
> If the config drive is ISO9660, we can safely read from it to fill in
> what information isn't persisted anywhere else, then update it with
> the new stuff we want to change. Then write out the new image.

Neither of these really cope with dynamically updating the role device
metdata for a *running* guest during a disk/nic hotplug for example.
You can't have the guest re-write the FS data that's in use by a running

For the CDROM based config drive, you would have to eject the virtual
media and insert new media.

IMHO, I'd just declare config drive readonly no matter what and anything
which requires dynamic data must use a different mechanism. Trying to
make config drive at all dynamic just opens a can of worms.

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