Re: [PATCH V2] migration: file URI

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Mon, Jun 12, 2023 at 03:39:34PM -0400, Steven Sistare wrote:
>> On 6/12/2023 2:44 PM, Peter Xu wrote:
>> > Hi, Steve,
>> > 
>> > On Wed, Jun 07, 2023 at 11:38:59AM -0700, Steve Sistare wrote:
>> >> Extend the migration URI to support file:<filename>.  This can be used for
>> >> any migration scenario that does not require a reverse path.  It can be used
>> >> as an alternative to 'exec:cat > file' in minimized containers that do not
>> >> contain /bin/sh, and it is easier to use than the fd:<fdname> URI.  It can
>> >> be used in HMP commands, and as a qemu command-line parameter.
>> > 
>> > I have similar question on the fixed-ram work,
>> 
>> Sorry, what is the "fixed-ram work"?
>
> https://lore.kernel.org/all/20230330180336.2791-1-farosas@suse.de
>
> It has similar requirement to migrate to a file, though slightly different
> use case.
>
>> 
>> > on whether we should assume
>> > the vm stopped before doing so.  Again, it leaves us space for
>> > optimizations on top without breaking anyone.
>> 
>> I do not assume the vm is stopped.  The migration code will stop the vm
>> in migration_iteration_finish.
>> 
>> > The other thing is considering a very busy guest, migration may not even
>> > converge for "file:" URI (the same to other URIs) but I think that doesn't
>> > make much sense to not converge for a "file:" URI.  The user might be very
>> > confused too.
>> 
>> The file URI is mainly intended for the case where guest ram is backed by shared memory 
>> and preserved in place, in which case writes are not tracked and convergence is not an
>> issue.  If not shared memory, the user should be advised to stop the machine first.
>> I should document these notes in qemu-options and/or migration.json.
>
> My question was whether we should treat "file:" differently from most of
> other URIs.  It makes the URI slightly tricky for sure, but it also does
> make sense to me because "file:" implies more than the rest URIs, where
> we're sure about the consequence of the migration (vm stops), in that case
> keeping vm live makes it less performant, and also weird.
>
> It doesn't need to be special in memory type being shared, e.g. what if
> there's a device that contains a lot of data to migrate in the future?
> Assuming "shared memory will always migrate very fast" may not hold true.
>
> Do you think it makes more sense to just always stop VM when migrating to
> file URI?  Then if someone tries to restart the VM or cancel the migration,
> we always do both (cancel migration, then start VM).

From our discussions in the other thread, I have implemented a
MIGRATION_CAPABILITY_SUSPEND to allow the management layer to decide
whether the guest should be stopped by QEMU before the migration.

I'm not opposed to coupling file URI with a stopped VM, although I
think, at least for fixed-ram, libvirt would prefer to be able to decide
when to stop.