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 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. > > 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. Thanks, -- Peter Xu
