Re: [libvirt PATCH 0/6] Introduce Local Migration Support in Libvirt

On 2/3/20 5:43 AM, Daniel P. Berrangé wrote: > I'm (re-)sending this patch series on behalf of Shaju Abraham > <shaju.abraham(a)nutanix.com&gt; who has tried to send this several times > already. > > Red Hat's email infrastructure is broken, accepting the mails and then > failing to deliver them to mailman, or any other Red Hat address. > Unfortunately it means that while we can send comments back to Shaju > on this thread, subscribers will then probably fail to see any responses > Shaju tries to give :-( To say this is bad is an understatement. I have > yet another ticket open tracking & escalating this awful problem but > can't give any ETA on a fix :-( > > Anyway, with that out of the way, here's Shaju's original cover letter > below.... > > 1) What is this patch series about? > > Local live migration of a VM is about Live migrating a VM instance with in the > same node. Traditional libvirt live migration involves migrating the VM from a > source node to a remote node. The local migrations are forbidden in Libvirt for > a myriad of reasons. This patch series is to enable local migration in Libvirt. > > 2) Why Local Migration is important? > > The ability to Live migrate a VM locally paves the way for hypervisor upgrades > without shutting down the VM. For example to upgrade qemu after a security > upgrade, we can locally migrate the VM to the new qemu instance. By utilising > capabilities like "bypass-shared-memory" in qemu, the hypervisor upgrades are > faster. > > 3) Why is local migration difficult in Libvirt? > > Libvirt always assumes that the name/UUID pair is unique with in a node. During > local migration there will be two different VMs with the same UUID/name pair > which will confuse the management stack. There are other path variables like > monitor path, config paths etc which assumes that the name/UUID pair is unique. > So during migration the same monitor will be used by both the source and the > target. We cannot assign a temporary UUID to the target VM, since UUID is a part > of the machine ABI which is immutable. > To decouple the dependecy on UUID/name, a new field (the domain id) is included > in all the PATHs that Libvirt uses. This will ensure that all instances of the > VM gets a unique PATH. Since localhost migration is difficult, and there will likely be some growing pains until the feature is fully baked, perhaps it is best to have a knob for enabling/disabling it. The namespace feature suffered similar growing pains and having the ability to disable it in qemu.conf proved quite handy at times.