On Mon, Mar 04, 2024 at 05:15:05PM -0300, Fabiano Rosas wrote: > Peter Xu <pet...@redhat.com> writes: > > > On Mon, Mar 04, 2024 at 08:53:24PM +0800, Peter Xu wrote: > >> On Mon, Mar 04, 2024 at 12:42:25PM +0000, Daniel P. Berrangé wrote: > >> > On Mon, Mar 04, 2024 at 08:35:36PM +0800, Peter Xu wrote: > >> > > Fabiano, > >> > > > >> > > On Thu, Feb 29, 2024 at 12:29:54PM -0300, Fabiano Rosas wrote: > >> > > > => guest: 128 GB RAM - 120 GB dirty - 1 vcpu in tight loop dirtying > >> > > > memory > >> > > > >> > > I'm curious normally how much time does it take to do the final > >> > > fdatasync() > >> > > for you when you did this test. > > I measured and it takes ~4s for the live migration and ~2s for the > non-live. I didn't notice this before because the VM goes into > postmigrate, so it's paused anyway. > > >> > > > >> > > I finally got a relatively large system today and gave it a quick shot > >> > > over > >> > > 128G (100G busy dirty) mapped-ram snapshot with 8 multifd channels. > >> > > The > >> > > migration save/load does all fine, so I don't think there's anything > >> > > wrong > >> > > with the patchset, however when save completes (I'll need to stop the > >> > > workload as my disk isn't fast enough I guess..) I'll always hit a > >> > > super > >> > > long hang of QEMU on fdatasync() on XFS during which the main thread > >> > > is in > >> > > UNINTERRUPTIBLE state. > >> > > >> > That isn't very surprising. If you don't have O_DIRECT enabled, then > >> > all that disk I/O from the migrate is going to be in RAM, and thus the > >> > fdatasync() is likely to trigger writing out alot of data. > >> > > >> > Blocking the main QEMU thread though is pretty unhelpful. That suggests > >> > the data sync needs to be moved to a non-main thread. > >> > >> Perhaps migration thread itself can also be a candidate, then. > >> > >> > > >> > With O_DIRECT meanwhile there should be essentially no hit from > >> > fdatasync. > >> > >> The update of COMPLETED status can be a good place of a marker point to > >> show such flush done if from the gut feeling of a user POV. If that makes > >> sense, maybe we can do that sync before setting COMPLETED. > > At the migration completion I believe the multifd threads will have > already cleaned up and dropped the reference to the channel, it might be > too late then. > > In the multifd threads, we'll be wasting (like we are today) the extra > syscalls after the first sync succeeds. > > >> > >> No matter which thread does that sync, it's still a pity that it'll go into > >> UNINTERRUPTIBLE during fdatasync(), then whoever wants to e.g. attach a gdb > >> onto it to have a look will also hang. > > > > Or... would it be nicer we get rid of the fdatasync() but leave that for > > upper layers? QEMU used to support file: migration already, it never > > manage cache behavior; it does smell like something shouldn't be done in > > QEMU when thinking about it, at least mapped-ram is nothing special to me > > from this regard. > > > > User should be able to control that either manually (sync), or Libvirt can > > do that after QEMU quits; after all Libvirt holds the fd itself? It should > > allow us to get rid of above UNINTERRUPTIBLE / un-debuggable period of QEMU > > went away. Another side benefit: rather than holding all of QEMU resources > > (especially, guest RAM) when waiting for a super slow disk flush, Libvirt / > > upper layer can do that separately after releasing all the QEMU resources > > first. > > I like the idea of QEMU having a self-contained > implementation. Specially since we'll add O_DIRECT support, which is > already quite heavy-handed if we're talking about managing cache > behavior. > > However, it's not trivial to find the right place to add the sync. > Wherever we put it there will be some implications, such as ensuring the > sync works even after migration failure, avoiding concurrent cleanup, > etc. > > In any case, I don't think it's correct to have the sync at > qio_channel_close(), now that we've seen it might block for a long > time. We could at the very least have a qio_channel_flush()[1] which the > QIOChannelFile implements with fdatasync(). Then the clients can choose > when to sync.
Yes, I agree with de-coupling it. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
