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

[Maxim Nestratov]

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.

Copying my colleagues to the thread.