[libvirt] [RFC] live migration of VMs with internal snapshots

[Dmitry Mishin]

On 26/04/16 16:34, "Maxim Nestratov" <mnestratov at virtuozzo.com> wrote:

>26.04.2016 13:00, Kashyap Chamarthy пишет:
>
>> On Tue, Apr 26, 2016 at 06:26:59AM +0300, Maxim Nestratov wrote:
>>> Hi,
>>>
>>> As far as I understand, currently there is no way to live migrate qemu
>>> VMs that have internal snapshots, because live migration works via
>>> qemu drive mirroring, which in turns mirrors only shallow block layer,
>>> effectively losing existing embedded snapshots. The problem could be
>>> fixed if we created an external delta for all disks with internal
>>> snapshots, live migrate such VMs and then auto merge them back into
>>> original images. That said, I would like to know very much your
>>> opinion on the matter and would also like to know if this approach is
>>> affordable. If so, I or one of my colleagues will send a pathset to
>>> fix this, otherwise what solution for this problem you will recommend?
>> There seems to be an old discussion here[1] on this topic:
>>
>> "You are not the first to request this - libvirt would
>> also like the ability to have read-only access into the contents of an
>> internal snapshot while the rest of qemu continues to write into the
>> image." (Eric Blake)
>>
>> When I asked if this was possible with today's QEMU, Kevin Wolf (QEMU
>> block layer maintainer) on #qemu IRC said:
>>
>>      "No, this requires some non-trivial code design changes and was
>>      never considered important enough.  This would also require writes
>>      to those snapshots on the destination, by the way (which wouldn't
>>be
>>      impossible if the infrastructure for having multiple active
>>      snapshots at the same time were there)"
>>
>>
>> [1]
>> http://lists.nongnu.org/archive/html/qemu-devel/2012-11/msg00402.html --
>> live migration which includes previos snapshot
>>
>
>Hmm. Interesting. But what I'm talking about doesn't involve qemu much.
>We create
>an external temporarily delta for images with internal snapshots on
>libvirt side,
>then merge it on libvirt side thus, qemu shouldn't be affected at all as
>far as I
>understand.

Right now, we are trying the following workaround of this issue:
1) create external snapshot (without metadata)
2) undefine internal snapshots' metadata (because libvirt prohibits
migration if detects snapshots)
3) copy metadata & backing file (primary data qcow2 file with internal
snapshots) to the destination
4) migrate VM using libvirt
5) define metadata back
6) block-commit - to merge delta to original qcow2 image.

Things would be much simpler, if they were in libvirt rather than in
management app on top of it.

Thank you,
Dmitry.

>
>Copying my colleagues to the thread.
>