Re: [PATCH V2] migration: file URI

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jun 14, 2023 at 12:47:41PM -0300, Fabiano Rosas wrote:
>> 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.
>
> IIUC the best timing is when migration starts, not earlier, not later.
>

Sorry, I meant "when" as in "which migration instances".

> If that's always the case, it's better qemu guarantee that?  Or am I wrong
> that libvirt wants to not do it in some cases?

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.