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

[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.

> > >> 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.

> > 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.

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