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On Mon, Jul 14, 2014 at 10:48:17AM +0100, Matthew Booth wrote:
> 
> That's interesting. I didn't realise that other drivers had the same
> limitations. Does anybody understand the original thinking which lead to
> this design? The single VM approach seems intuitively correct to me, so
> presumably at some point there was a good reason not to choose it.

I imagine it was just done for ease of implementation initially, and/or
the rescue mode hasn't been updated as new features were added, so it
fell behind.

> I'm not sufficiently familiar with the libvirt driver to know what state
> persists in the hypervisor, but in the vmware driver an attached volume
> remains attached until detached. By re-using the original VM for rescue,
> we wouldn't have to concern ourselves with volumes at all, because they
> are all already attached.
> 
> As for following the linked BP, I *think* we would achieve that 'for
> free' with a single VM approach. Is there any semantic subtlety here
> relating to actually attaching the volumes? i.e. Does it matter that we
> wouldn't attach them, because they are already attached?
> 
> I like the USB idea, and I'm fairly sure it should be achievable in the
> VMware driver. Is it worth codifying it in a BP? Perhaps the same BP.

It is in a gray area, you could probably argue both ways as to whether
it could be slipped in as part of the existing BP :-) It has user visible
change though, so might need a bit of bikeshedding in a BP to agree on
best way to deal with the idea. eg perhaps opt-in to use of USB vs existing
disk approach via an image property.

> Incidentally, I hit an obvious problem when testing this with the Cirros
> image, which mounts filesystems by label. If you have an image which
> mounts by label, or has LVM volumes, and you use the same image as a
> rescue disk, you are adding a second disk containing the same filesystem
> labels and/or LVM volumes. Under these circumstances, the behaviour of
> mount during the boot sequence is not well defined afaik. Consequently,
> re-using the original image as a rescue image doesn't sound to me like a
> good idea in general.

I think that I'd probably say there is an expectation that the rescue
image will be different from the primary image the OS was booted from.

Regards,
Daniel
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