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

On Mon, Mar 10, 2014 at 03:47:49PM +0100, Jiri Denemark wrote:
> 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.

I think it is valuable to have it as a parameter to 'migrate' - since
'migrate' blocks, you'd have to open up a second shell to set the
downtime which is kind of tedious.

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