Re: [PATCH V2] migration: file URI

[Peter Xu <peterx@redhat.com>]

On Tue, Jun 20, 2023 at 02:36:58PM -0400, Steven Sistare wrote:
> On 6/15/2023 10:50 AM, Fabiano Rosas wrote:
> > Peter Xu <peterx@redhat.com> writes:
> > 
> >> On Wed, Jun 14, 2023 at 02:59:54PM -0300, Fabiano Rosas wrote:
> >>> In this message Daniel mentions virDomainSnapshotXXX which would benefit
> >>> from using the same "file" migration, but being done live:
> >>>
> >>> https://lore.kernel.org/r/ZD7MRGQ+4QsDBtKR@redhat.com
> >>>
> >>> And from your response here:
> >>>  https://lore.kernel.org/r/ZEA759BSs75ldW6Y@x1n
> >>>
> >>> I had understood that having a new SUSPEND cap to decide whether to do
> >>> it live or non-live would be enough to cover all use-cases.
> >>
> >> Oh, I probably lost some of the contexts there, sorry about that - so it's
> >> about not being able to live snapshot on !LINUX worlds properly, am I
> >> right?
> >>
> > 
> > Right, so that gives us for now a reasonable use-case for keeping live
> > migration behavior possible with "file:".
> > 
> >> In the ideal world where we can always synchronously tracking guest pages
> >> (like what we do with userfaultfd wr-protections on modern Linux), the
> >> !SUSPEND case should always be covered by CAP_BACKGROUND_SNAPSHOT already
> >> in a more performant way.  IOW, !SUSPEND seems to be not useful to Linux,
> >> because whenever we want to set !SUSPEND we should just use BG_SNAPSHOT.
> >>
> > 
> > I agree.
> > 
> >> But I think indeed the live snapshot support is not good enough. Even on
> >> Linux, it lacks different memory type supports, multi-process support, and
> >> also no-go on very old kernels.  So I assume the fallback makes sense, and
> >> then we can't always rely on that.
> >>
> >> Then I agree we can keep "file:" the same as others like proposed here, but
> >> I'd like to double check with all of us so we're on the same page..
> > 
> > +1
> > 
> >> And maybe we should mention some discussions into commit message or
> >> comments where proper in the code, so we can track what has happened
> >> easier.
> >>
> > 
> > I'll add some words where appropriate in my series as well. A v2 is
> > already overdue with all the refactorings that have happened in the
> > migration code.
> 
> Peter, should one of us proceed to submit the file URI as a stand-alone patch, 
> since we both need it, and it has some value on its own? 
> 
> My version adds a watch on the incoming channel so we do not block monitor commands. 
> It also adds tracepoints like the other URI's.
> 
> Fabiano's version adds a nice unit test.  
> 
> Maybe we should submit a small series with both.

I fully agree.  I didn't check the details, but if we know the shared bits
it'll be great if we arrange it before-hand, and then it might also be the
best too for all sides.  Thanks for raising this.

-- 
Peter Xu