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.
