[libvirt-users] Create multiple domains from single saved domain state (is UUID/name fixed?)

[Jonas Finnemann Jensen]

Fair enough :)
I just wanted to make sure it wasn't supported... I'm probably better off
using QEMU directly. I totally understand that libvirt makes some sane
decisions that makes sense for data center management.

I'm not sure why it couldn't be done. But honestly hacking libvirt to
violate a core invariant is probably asking for trouble :)
Den 19. apr. 2016 11.17 PM skrev "Martin Kletzander" <mkletzan at redhat.com>:

> On Tue, Apr 19, 2016 at 03:22:18PM -0700, Jonas Finnemann Jensen wrote:
>
>>
>>> You'll also need to change the name and uuid of the domain at the very
>>> least.
>>>
>>
>> Agree, but is that possible with libvirt?
>>
>>
> Not in a supported way.  But you can, technically, edit the save file
> (not using virsh), change the name and uuid and restore it.  But don't
> seek help when something doesn't work for you after that =)
>
> If you do that (restore a previously running image with a different MAC
>>
>>> address)
>>>
>>
>> Yeah,  probably I wouldn't change the MAC address. As I want to attach the
>> VMs to different networks.
>> I rely on the IP being in a different subnet to identify the VM in my
>> metadata service.
>> Using IP filters to enforce the subnet seems like the most robust way of
>> being sure which VM I'm talking to.
>>
>> possibly by having the host toggle the interface offline and back on
>>
>> Yeah, I think unplugging the virtual network cable before I save the VM
>> memory, and plugging it back in after I load the VM, then DHCP would run
>> immediately.
>> As an added benefit any guest program I have talking to my meta-data
>> service would be able to detect that the VM has been loaded, by looking
>> for
>> network connection.
>>
>>
>> --
>> Regards Jonas Finnemann Jensen.
>>
>> 2016-04-19 14:26 GMT-07:00 Laine Stump <laine at laine.org>:
>>
>> (please don't top-post. Put your responses inline, in context)
>>>
>>> On 04/19/2016 01:09 PM, Jonas Finnemann Jensen wrote:
>>>
>>> virt-builder looks like some fancy guest/host interaction related to
>>>> building VM images.
>>>>
>>>> What I'm looking for is more like:
>>>>   virsh save running_domain saved-domain-A.img
>>>>   cp saved-domain-A.img saved-domain-B.img
>>>>   virsh save-image-edit saved-domain-B.img   // Change the network,
>>>> possibly MAC, VNC port
>>>>
>>>>
>>> You'll also need to change the name and uuid of the domain at the very
>>> least. And I assume these will all be transient domains, not persistent.
>>>
>>>
>>> Then in parallel I want to do:
>>>>   virsh restore saved-domain-A.img
>>>>   virsh restore saved-domain-B.img
>>>>
>>>>
>>> If you do that (restore a previously running image with a different MAC
>>> address), at the very least the guest OS will be confused about the MAC
>>> address of the network card, and you'll very likely end up with both
>>> guests
>>> responding to ARP requests for the original MAC address. There's likely
>>> other problems that I haven't thought of that will happen as well.
>>>
>>>
>>> So that I have two instances of the same virtual machine starting from
>>>> the same state.
>>>> This way I can reset the VMs without having to reboot them (booting is
>>>> rather slow).
>>>>
>>>> I practice I'll probably have ~16 instances at the same time. Constantly
>>>> being reset to the same state.
>>>> I tried with QEMU, and it's seems totally doable with savevm, copy file,
>>>> then doing loadvm twice in parallel.
>>>> (I'll be using a separate network for each VM, so I can be sure which
>>>> one
>>>> I'm talking to).
>>>>
>>>>
>>> Well, as long as they're completely isolated from each other, you may
>>> have
>>> a better chance of success. However there will still be the issue of the
>>> IP
>>> address of the network interface. You can't have two networks using the
>>> same IP range (since libvirt doesn't use network namespaces for its
>>> networks), so the guest will need to change its IP, which means it will
>>> need to be notified of this need, possibly by having the host toggle the
>>> interface offline and back on - you can use virsh domif-setlink to do
>>> this.
>>>
>>>
>>>
>>> Is this doable with libvirt, or am I better off using QEMU directly? and
>>>> how? I couldn't do internal snapshots with --live, and snapshot-revert
>>>> says
>>>> it can't revert to external snapshots yet :)
>>>> (using QEMU directly would certainly leave me with a lot of manual
>>>> network configuration)
>>>>
>>>>
>>> Someone else will have to talk about the particulars of snapshots...
>>>
>>>
>>>
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