On Fri, Jan 23, 2026 at 11:10:54AM -0300, Fabiano Rosas wrote: > Peter Xu <[email protected]> writes: > > > COLO caches all device states in a buffer channel `fb'. Flush it right > > before pushing the buffer to make sure everything needed will be flushed, > > add a comment explaning it. > > > > Signed-off-by: Peter Xu <[email protected]> > > --- > > migration/colo.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/migration/colo.c b/migration/colo.c > > index f92803dd29..e2307f1907 100644 > > --- a/migration/colo.c > > +++ b/migration/colo.c > > @@ -472,8 +472,6 @@ static int > > colo_do_checkpoint_transaction(MigrationState *s, > > qemu_savevm_state_complete_precopy_iterable(s->to_dst_file, false); > > qemu_put_byte(s->to_dst_file, QEMU_VM_EOF); > > > > - qemu_fflush(fb); > > - > > /* > > * We need the size of the VMstate data in Secondary side, > > * With which we can decide how much data should be read. > > @@ -484,6 +482,8 @@ static int > > colo_do_checkpoint_transaction(MigrationState *s, > > goto out; > > } > > > > + /* Flush to make sure everything lands bioc->data */ > > + qemu_fflush(fb); > > qemu_put_buffer(s->to_dst_file, bioc->data, bioc->usage); > > ret = qemu_fflush(s->to_dst_file); > > Reviewed-by: Fabiano Rosas <[email protected]> > > Wait, aren't we wasting a memcpy here? Isn't this the same, but without > the copy of bioc->data to s->to_dst_file->buf: > > qemu_fflush(fb); // ensure bioc->data > is populated > qemu_fflush(s->to_dst_file); // send buffered data > from migration stream > qemu_put_buffer_async(s->to_dst_file, bioc->data, ...); // point > s->to_dst_file->iov to bioc->data > qemu_fflush(s->to_dst_file); // send > s->to_dst_file->iov contents
Good point! Same to postcopy.. I quickly measured a small VM in postcopy path, async put can shrink the time needed (plus the flush) from 91us to 38us. Not a huge difference, but I don't see why we don't do it either. I'll test more and add those when I repost. -- Peter Xu
