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Re: [Xen-devel] Discussion: Add API to retrieve migration progress

On Tue, 2013-11-12 at 11:05 +0000, George Dunlap wrote:
> On Fri, Nov 8, 2013 at 11:13 AM, Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 08/11/13 08:05, Chunyan Liu wrote:
> >
> > Hi, list,
> >
> > One customer requests that we should show migration progress bar in 'xl
> > migrate' or 'virsh migrate', like '-h/--hash' option in 'rpm' command, so
> > that they could see clearly what happened in migration period. To deal with
> > that, we need to have a method to retrieve migration progress. And we hope
> > such stuff could be finally merged to upstream. How do you think?
> >
> >
> > There is no sensible way to determine timing here.
> >
> > A non-live migrate can be approximated based solely %age of ram transmitted.
> > However, with a live migrate, the actions of the live guest affect how long
> > the following iteration takes, and the longer an iteration takes, the more
> > likely it is that further iterations will be needed later.  Over the
> > timescales required to live migrate a sensibly sized guest, changed in
> > workload in dom0 can make a meaningful difference in time taken to send an
> > iteration, meaning the live guest can dirty more ram and cause a larger next
> > iteration.
> I don't think people necessarily want a 100% accurate prediction; they
> just want an idea how far along things are going (and a reassurance
> that things are actually moving).  I think if the algorithm assumes 2
> passes and then a clean-up phase, and just hard-codes in assumptions
> about what percentage each pass will take (e.g., 80% for first pass,
> 15% for 2nd pass, 5% for final clean-up), the results will be useful,
> and about as accurate as anyone can expect.

Note that the existing code has some algorithm, or at least it is
calling xc_report_progress_step with some numbers. Whatever it is is
probably "good enough".


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