Re: [Qemu-devel] [PATCH V4 3/4] Add backing drive while performing backup.

[Fam Zheng]

On Wed, 07/17 13:04, Ian Main wrote:
> This patch adds the original source drive as a backing drive to our target
> image so that the target image will appear complete during backup.  This
> is especially useful for SYNC_MODE_NONE as it allows export via NBD to
> have a complete point-in-time snapshot available for export.
> 
> Signed-off-by: Ian Main <address@hidden>
> ---
>  block/backup.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/block/backup.c b/block/backup.c
> index 68abd23..e086e76 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -323,6 +323,10 @@ static void coroutine_fn backup_run(void *opaque)
>  
>      hbitmap_free(job->bitmap);
>  
> +    /* Set the target backing drive back to NULL before calling delete or
> +     * it will also delete the underlying drive. */
> +    target->backing_hd = NULL;
> +
>      bdrv_iostatus_disable(target);
>      bdrv_delete(target);
>  
> @@ -362,6 +366,15 @@ void backup_start(BlockDriverState *bs, BlockDriverState 
> *target,
>          return;
>      }
>  
> +    /* Manually set the backing hd to be the backup source drive so
> +     * that all reads done while we are backing up will be passed
> +     * on to the original source drive.  This allows reading from the
> +     * image while the backup is in progress, or in the case of
> +     * SYNC_MODE_NONE allows a complete image to be present for export.
> +     * Note that we do this for all modes including SYNC_MODE_TOP as
> +     * even then it allows on-the-fly reading. */
> +    target->backing_hd = bs;
> +

Also set target->backing_file and target->backing_format here? Paolo?

Thanks.

Fam