[libvirt-users] [libvirt] vm live storage migration with snapshots

[Edward Young]

On Wed, Feb 11, 2015 at 5:09 PM, Eric Blake <eblake at redhat.com> wrote:

> [dropping multiple lists; let's just use libvirt-users]
>
> On 02/11/2015 02:45 PM, Edward Young wrote:
>
> > I perform a simple test, but failed.
> >
> > In the source, I create:   base <- mid <- active (2 snapshots, the active
> > one is the current one)
> > In order to migrate this vm to the destination, I manually copy both base
> > and mid to the destination, and put them in the sam e directory
> > /var/lib/libvirt/images/
> > Then I call the following commands:
> > 'sudo virsh migrate --live --unsafe --verbose --copy-storage-inc vm1
> > qemu+ssh://192.168.1.3/system tcp://192.168.1.3'
> >
> > The error I got is:
> > error: Requested operation is not valid: cannot migrate domain with 2
> > snapshots.
> >
> > I do not know the reason for this.
>
> That's because libvirt is not (yet) able to migrate snapshots in a way
> that the destination can revert to them.  It is possible to work around
> this in a couple of ways:
>
> 1. Manually dump all the snapshot XML configurations on the source side
> (virsh snapshot-list, then virsh snapshot-dumpxml in a loop), then
> delete the snapshot tracking (virsh snapshot-delete --metadata in a
> loop), then do the migration, then manually recreate the snapshot
> configuration on the destination (virsh snapshot-create --redefine in a
> loop for each of the xml files you saved earlier).
>
> If you don't plan to revert to snapshots on the destination, then you
> can skip the snapshot-dumpxml loop, as well as the snapshot-create
> --redefine loop.  But if that's the case, then you can take a shortcut:
>
> 2. Don't track external snapshot data in libvirt in the first place
> (virsh snapshot-create --no-metadata when taking external snapshots).
> Instead, the backing chain is sufficient to know that external snapshot
> data was saved.
>
It works for me. Thanks!

>
> >
> >
> >> In fact, modern
> >> libvirt/qemu does this for you by setting up an NBD server on the
> >> destination, doing a data mirror from the source into the destination
> >> (so that you DO have shared storage, at least for the duration of the
> >> migration), then doing the live migration, then tearing down the NBD
> >> mirroring link.
> >>
> >
> > Sounds interesting! Can you provide me an example or point me to a link
> > about how to use this function?
>
> It's all done under the hood as part of the implementation of 'virsh
> migrate --copy-source-*', if done as part of migration.  But if you want
> to manually recreate the steps, the source code in
> src/qemu/qemu_migration.c shows the QMP commands used to set up the NBD
> server on the destination, then the mirroring is done with equivalent of
> the 'virsh blockcopy' command pointing to the NBD server as the
> destination.
>
I will take a look at the code.

>
> >
> > Sorry to take you so much time to answer my questions. I appreciate it
> > deeply. After I solve this issue, I will write a blog and share these
> > knowledge with other people.
>
> The libvirt-users list archives has a lot of questions and posts from
> other users on the same topic; it may also help if you peruse the archives.
>
I tried previously, but I can't find them. I search them directly by
google? Is there anyway to find the related posts in the archive?


Thanks a lot!
Ed



>
> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
>
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