[libvirt] [PATCH] qemu: Do not override config XML in case of snapshot revert

[Eric Blake]

On 5/14/19 8:37 AM, Michal Privoznik wrote:
> On 5/13/19 8:48 PM, Daniel Henrique Barboza wrote:
>> Tried to reproduce the error using my x86 laptop but got hit by
>> https://bugzilla.redhat.com/show_bug.cgi?id=1689216 when trying
>> to create the snapshot using upstream code:
>>

> 
> Actually, I think that this is not the proper fix. The proper fix would
> be to store both active AND inactive XMLs when creating a snapshot and
> then restore them both on snapshot revert. While this fix may fix one
> use case, it's not dealing with the issue properly IMO. But I'm not
> snapshot expert really.

Sadly, if we were to modify snapshot XML to store both the active and
inactive XML with a snapshot, all existing snapshots of a running guest
would be incomplete (right now, the snapshot XML only allows the storage
of a single <domain> sub-element).  I think the best we can do is store
an active XML with a snapshot of an active domain, and an inactive XML
with a snapshot of an offline domain. Reverting to an offline snapshot
only has to worry about inactive XML; reverting to an online snapshot
needs the active XML to properly restore the domain, but we then have to
decide whether to also update the inactive XML to match the fact that
the active XML has changed. So changing both inactive and active XML to
describe the same state during a revert to a snapshot of an active
domain seems like the best approach to me, short of rewriting snapshot
XML to store two different domain definitions at once.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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