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

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@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...
>>
>>

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@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@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...
>>>
>>>
>>>

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@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@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...
>>>>
>>>>
>>>>