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

[Martin Kletzander]

On Tue, Apr 19, 2016 at 11:24:56PM -0700, Jonas Finnemann Jensen wrote:
>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 :)
>

We could add some --i-know-what-i-am-doing flag to allow such things if
there's a use case for such scenario.  Patches are welcome ;)))

>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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