libvirt users - Apr 2016 - Re: Create multiple domains from single saved domain state (is UUID/name fixed?)

On Mon, Apr 18, 2016 at 10:05:02PM +0300, Michael Ravits
wrote:
>Hi Jonas,
>
>I asked a similar question here last week. You could find it by searching
>for this topic in the mailing list archives: "Clone VM with saved
state".
>That being said I'd be very glad if you could update here when you find
a
>complete solution.
>
>- Michael
>
What I *think* Jones wants is to use virt-builder, it could cause some
problems if you started all of them.  What Michael wants is:

  virsh save running_domain saved-image-file.img
  virsh save-image-edit saved-image-file.img

Hope that helped,
Martin
>On Mon, Apr 18, 2016 at 9:49 PM, Jonas Finnemann Jensen
<jonasfj@mozilla.com
>> wrote:
>
>> Hi,
>>
>> I would like to save a running domain (ie. disk + memory) and be able
to
>> restore it multiple times creating duplicates of the orignal domain all
>> starting from the same state.
>> Use case:
>> I'm building a task-processing system for use in a CI flow.
>> I want to run multiple VMs in parallel using the same image (always
>> starting from the same state).
>> And to avoid needlessly booting between each task, I would like to save
>> (and distribute) the domain state, so that I just restore from memory.
>>
>> However, I can't seems to change the UUID or the name of a domain
once it
>> is saved.
>> Nor do I seem able to rename a domain while it is running.
>>
>> I can obviously duplicate both the disks and the file to which I saved
the
>> domain state using "virsh save".
>> But I seem unable to rename before I restore.. Any ideas?
>>
>> Could I do this with snapshots? I suspect not since I see
>> virDomainSnapshotRedefinePrep() calling
>> virDomainDefCheckABIStability which raises the error here:
>>
https://fossies.org/dox/libvirt-1.3.3/domain__conf_8c_source.html#l17991
>>
>> Out of curiosity does anyone know what horrors might befall me if I
were
>> to remove the lines protecting against name and UUID changes? Then
compile
>> my own libvirt...
>> The comment in the code says name can be changed, but I'm guessing
I would
>> have to change the UUID too. Does anyone see how that would create
issues?
>> I'm not sure how libvirt uses the UUID internally.
>>
>> --
>> Regards Jonas Finnemann Jensen.
>>
>> _______________________________________________
>> libvirt-users mailing list
>> libvirt-users@redhat.com
>> https://www.redhat.com/mailman/listinfo/libvirt-users
>>
>_______________________________________________
>libvirt-users mailing list
>libvirt-users@redhat.com
>https://www.redhat.com/mailman/listinfo/libvirt-users

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

Then in parallel I want to do:
  virsh restore saved-domain-A.img
  virsh restore saved-domain-B.img

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).

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)


--
Regards Jonas Finnemann Jensen.

2016-04-19 2:23 GMT-07:00 Martin Kletzander <mkletzan@redhat.com>:
> On Mon, Apr 18, 2016 at 10:05:02PM +0300, Michael Ravits wrote:
>
>> Hi Jonas,
>>
>> I asked a similar question here last week. You could find it by
searching
>> for this topic in the mailing list archives: "Clone VM with saved
state".
>> That being said I'd be very glad if you could update here when you
find a
>> complete solution.
>>
>> - Michael
>>
>>
> What I *think* Jones wants is to use virt-builder, it could cause some
> problems if you started all of them.  What Michael wants is:
>
>  virsh save running_domain saved-image-file.img
>  virsh save-image-edit saved-image-file.img
>
> Hope that helped,
> Martin
>
>
> On Mon, Apr 18, 2016 at 9:49 PM, Jonas Finnemann Jensen <
>> jonasfj@mozilla.com
>>
>>> wrote:
>>>
>>
>> Hi,
>>>
>>> I would like to save a running domain (ie. disk + memory) and be
able to
>>> restore it multiple times creating duplicates of the orignal domain
all
>>> starting from the same state.
>>> Use case:
>>> I'm building a task-processing system for use in a CI flow.
>>> I want to run multiple VMs in parallel using the same image (always
>>> starting from the same state).
>>> And to avoid needlessly booting between each task, I would like to
save
>>> (and distribute) the domain state, so that I just restore from
memory.
>>>
>>> However, I can't seems to change the UUID or the name of a
domain once it
>>> is saved.
>>> Nor do I seem able to rename a domain while it is running.
>>>
>>> I can obviously duplicate both the disks and the file to which I
saved
>>> the
>>> domain state using "virsh save".
>>> But I seem unable to rename before I restore.. Any ideas?
>>>
>>> Could I do this with snapshots? I suspect not since I see
>>> virDomainSnapshotRedefinePrep() calling
>>> virDomainDefCheckABIStability which raises the error here:
>>>
https://fossies.org/dox/libvirt-1.3.3/domain__conf_8c_source.html#l17991
>>>
>>> Out of curiosity does anyone know what horrors might befall me if I
were
>>> to remove the lines protecting against name and UUID changes? Then
>>> compile
>>> my own libvirt...
>>> The comment in the code says name can be changed, but I'm
guessing I
>>> would
>>> have to change the UUID too. Does anyone see how that would create
>>> issues?
>>> I'm not sure how libvirt uses the UUID internally.
>>>
>>> --
>>> Regards Jonas Finnemann Jensen.
>>>
>>> _______________________________________________
>>> libvirt-users mailing list
>>> libvirt-users@redhat.com
>>> https://www.redhat.com/mailman/listinfo/libvirt-users
>>>
>>>
> _______________________________________________
>> libvirt-users mailing list
>> libvirt-users@redhat.com
>> https://www.redhat.com/mailman/listinfo/libvirt-users
>>
>

(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...