On Thu, May 23, 2024 at 04:05:48PM -0300, Fabiano Rosas wrote: > We've recently added support for direct-io with multifd, which brings > performance benefits, but creates a non-uniform user interface by > coupling direct-io with the multifd capability. This means that users > cannot keep the direct-io flag enabled while disabling multifd. > > Libvirt in particular already has support for direct-io and parallel > migration separately from each other, so it would be a regression to > now require both options together. It's relatively simple for QEMU to > add support for direct-io migration without multifd, so let's do this > in order to keep both options decoupled. > > We cannot simply enable the O_DIRECT flag, however, because not all IO > performed by the migration thread satisfies the alignment requirements > of O_DIRECT. There are many small read & writes that add headers and > synchronization flags to the stream, which at the moment are required > to always be present. > > Fortunately, due to fixed-ram migration there is a discernible moment > where only RAM pages are written to the migration file. Enable > direct-io during that moment. > > Signed-off-by: Fabiano Rosas <faro...@suse.de>
Is anyone going to consume this? How's the performance? It doesn't look super fast to me if we need to enable/disable dio in each loop.. then it's a matter of whether we should bother, or would it be easier that we simply require multifd when direct-io=on. Thanks, -- Peter Xu
