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

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

(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
>