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



On Tue, Apr 19, 2016 at 05:26:50PM -0400, Laine Stump wrote:
(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...


I'm sure you can do live snapshot of the VM and restore to it.  It
must've been some issue with your particular setup or something.  But
even if we sole that, I'm not sure it will help you.  Yes, you would be
able to restore to that state, but running multiple domains at once from
the same saved state...

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