Re: [PATCH V2] migration: file URI

[Steven Sistare]

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.

- Steve