Re: [libvirt] limit downtime during life migration from xl/virsh

On Mon, Mar 10, 2014 at 15:36:06 +0100, Olaf Hering wrote: > > During live migration of VMs from one host to another the VM is > suspended for an unpredictable amount of time. The actual downtime > depends on how many new pages will be dirty and the band width to the > destination host. Since VM memory size grows faster than transfer rates > the currently available tuneables will cause troubles for workloads > within the VM which can not handle large timejumps. > > I have already written code to tweak the inner loop doing the actual > migration work in libxc. But the patchset exposes the details of the > loop to the cmdline, as such it is not portable nor is it a friendly UI > for the hostadmin. > > > Here is my proposal for a new option for virsh and 2 new options for xl: > > [xl | virsh --live] --max-suspend-time N --timeout N VM host > > --max-suspend-time N: as the name suggests, the VM downtime must not be > longer than specified. The code doing the migration has to estimate the > transfer speed. If the VM is about to be suspended, it has to check if > the remaining dirty pages can be transfered within the required > timeframe. If not, the migration is aborted, the VM continues to run on > the src host, the new VM on the dst host is destroyed and an error is > returned. Libvirt already has virDomainMigrateSetMaxDowntime API with this semantics. However, using virsh, one can set it with virsh migrate-setmaxdowntime command while migration is happening. Not sure if exposing it as yet another parameter of already quite complicated migrate command would buy us much.