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

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Mon, Mar 10, 2014 at 03:36:06PM +0100, Olaf Hering wrote:
> 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.

Ok, this is already supported by the libvirt virDomainMigrateSetMaxDowntime
API. Strangely you can't set it immediately when invoking 'virsh migrate',
only able to set it once running via 'virsh migrate-setmaxdowntime'.

It makes sense to support it as an arg to 'virsh migrate' itself too,
though I suggest you call it '--maxdowntime' for consistent naming
with the API & existing command.

> --timeout N: if a VM is busy and its workload causes many new dirty
> pages the migrate command would take forever. This option is supposed to
> stop the migration attempt if the number of new dirty pages is too high.
> It would change the semantics of "virsh migrate --timeout n", which
> currently forces a suspend (according to the help text).

The '--timeout' arg isn't anything that's part of the libvirt API,
it is implemented exclusively in virsh client code. That said I
still don't think we can change its semantics in the way you
describe. '--timeout' is a rather poor choice of name for what it
does currently, but we're stuck with it.  So for your proposed
semantics, I think we'll have to introduce a separate '--abort N'
argument to virsh.

Regards,
Daniel
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