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

[Peter Xu <peterx@redhat.com>]

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?

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,

-- 
Peter Xu