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. 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? -- Peter Xu
