Re: [libvirt-users] About live migration with snapshots

On 03/20/2013 08:45 PM, Chiang Hubert wrote: > Hello, > > I'd like to live migration with snapshots. > > But it doesn't work. It comes out a message "cannot migrate domain with 1 > snapshots" Unfortunately, figuring out how to migrate snapshot information at the libvirt API level requires some engineering work - the current RPC protocol for migration is not set up to migrate an arbitrary amount of snapshots in a single call. On the other hand, if you are allowed to make more than one API call, the solution is already available; maybe we should patch virsh to learn how to make the series of API calls, to automate what I will describe below. > > Then I try to trace the code(Libvirt 0.9.8 to 1.0.3), I find out the code > in src/qemu/qemu_migration.c @ Line 1395 - 1440 (Libvirt 1.0.3) It's still unimplemented at the libvirt level, even in libvirt.git. > > It will check the VM which has snapshots or not. > > I just curious about this limitation, why the VM can't live migration with > snapshots? Doing it all in one RPC call would be a potential denial-of-service (RPCs are bounded in length to avoid consuming server resources, and taking lots of snapshots on the source could easily be made to exceed bounds). If someone can design a way to set up a series of RPC handshakes, then we could do it at the libvirt level in a single API call, but I'm not sure it is worth it. > > What happen if I skip this check? > > Does it has any suggestion way or virsh command with options to do live > migration with snapshots? The existing solution at the management tool layer is to migrate the snapshot information first, and then to migrate the domain. For each snapshot in 'virsh snapshot-list --name $dom', you will want to 'virsh snapshot-dumpxml $dom $name > file' on the source, then 'virsh snapshot-create --redefine $dom file' on the destination. Next, determine 'virsh snapshot-current --name $dom' on the source, and use 'virsh snapshot-current $dom $name' on the destination to set it as current (if there is a current snapshot). After the destination has all the snapshots, you then loop over 'virsh snapshot-delete --metadata $dom $name' on the source, at which point, live migration will now work. Patches to teach virsh how to do all this work in a single 'virsh migrate' are welcome. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org