Peter Xu <pet...@redhat.com> writes: > On Wed, Jun 14, 2023 at 12:47:41PM -0300, Fabiano Rosas wrote: >> Peter Xu <pet...@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-faro...@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+4qsdb...@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.
