Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread

[Andrey Gruzdev]

On 22.03.2021 23:17, Peter Xu wrote:

> On Fri, Mar 19, 2021 at 05:52:47PM +0300, Andrey Gruzdev wrote:
> > Added missing qemu_fflush() on buffer file holding precopy device state.
> > Increased initial QIOChannelBuffer allocation to 512KB to avoid reallocs.
> > Typical configurations often require >200KB for device state and VMDESC.
> >
> > Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
> > ---
> >  migration/migration.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/migration/migration.c b/migration/migration.c
> > index ca8b97baa5..32b48fe9f5 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -3812,7 +3812,7 @@ static void *bg_migration_thread(void *opaque)
> >       * with vCPUs running and, finally, write stashed non-RAM part of
> >       * the vmstate from the buffer to the migration stream.
> >       */
> > -    s->bioc = qio_channel_buffer_new(128 * 1024);
> > +    s->bioc = qio_channel_buffer_new(512 * 1024);
> >      qio_channel_set_name(QIO_CHANNEL(s->bioc), "vmstate-buffer");
> >      fb = qemu_fopen_channel_output(QIO_CHANNEL(s->bioc));
> >      object_unref(OBJECT(s->bioc));
> > @@ -3866,6 +3866,8 @@ static void *bg_migration_thread(void *opaque)
> >      if (qemu_savevm_state_complete_precopy_non_iterable(fb, false, false)) {
> >          goto fail;
> >      }
> > +    qemu_fflush(fb);
> >       
> What will happen if the vmstates are bigger than 512KB?  Would the extra data
> be dropped?
>     
No, the buffer shall be reallocated and the original content shall be kept.
> In that case, I'm wondering whether we'll need a qemu_file_get_error() after
> the flush to detect it, and whether we need to retry with a bigger buffer size?
>
> Thanks,
>
>     
I think for QIOBufferChannel we don't need that, as I can see from the buffer channel
implementation.