Re: [Qemu-devel] [PATCH 00/21][RFC] postcopy live migration

[Takuya Yoshikawa]

(2012/01/01 18:52), Dor Laor wrote:
> > But we really need to think hard about whether this is the right thing
> > to take into the tree. I worry a lot about the fact that we don't test
> > pre-copy migration nearly enough and adding a second form just
> > introduces more things to test.
>
> It is an issue but it can't be a merge criteria, Isaku is not blame of pre copy 
> live migration lack of testing.
>
> I would say that 90% of issues of live migration problems are not related to 
> the pre|post stage but more of issues of device model save state. So post-copy 
> shouldn't add a significant regression here.

Though they may be only 10% the remaining issues tend to be hard to find.

> Probably it will be good to ask every migration patch writer to write an 
> additional unit test for migration.
>
> > It's also not clear to me why post-copy is better. If you were going to
> > sit down and explain to someone building a management tool when they
> > should use pre-copy and when they should use post-copy, what would you
> > tell them?
>
> Today, we have a default of max-downtime of 100ms.
> If either the guest work set size or the host networking throughput can't match 
> the downtime, migration won't end.
> The mgmt user options are:
> - increase the downtime more and more to an actual stop
> - fail migrate
>
> W/ post-copy there is another option.
> Performance measurements will teach us (probably prior to commit) when this 
> stage is valuable. Most likely, we better try first with pre-copy and if we 
> can't meet the downtime we can optionally use post-copy.

It is difficult to recommend mixing two methods which have different 
requirements
to users:

        post-copy cannot be canceled and, probably, needs some 
dedicated/reliable
        lines to make it sure that guests will not be broken during copy stage.

What we want to know, from user's point of view, is clear/simple criteria:

        what is needed for post-copy
        for what services we should select post-copy


        Takuya

> Here's a paper by Umesh (the migration thread writer):
> http://osnet.cs.binghamton.edu/publications/hines09postcopy_osr.pdf
>
> Regards,
> Dor