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Currently we create a rescue instance by creating a new VM with the
original instance's image, then adding the original instance's first
disk to it, and booting. This means we have 2 VMs, which we need to be
careful of when cleaning up. Also when suspending, and probably other
edge cases. We also don't support:

* Rescue images other than the instance's creation image
* Rescue of an instance which wasn't created from an image
* Access to cinder volumes from a rescue instance

I've created a dirty hack which, instead of creating a new VM, attaches
the given rescue image to the VM and boots from it:

https://review.openstack.org/#/c/106078/

It works for me. It supports all of the above, doesn't require special
handling on destroy, and works with suspend[1]. It also doesn't trigger
the spurious warning message about unknown VMs on the cluster which,
while unimportant in itself, is an example of an edge case caused by
having 2 VMs.

Does this seem a reasonable way to go? It would be dependent on a
refactoring of the image cache code so we could cache the rescue image.

Matt

[1] If suspend of a rescued image wasn't broken at the api level,
anyway. I have a patch for that: https://review.openstack.org/#/c/106082/
-- 
Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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