Re: [Qemu-devel] [PATCH v3] backup: allow target without .bdrv_get_info

[Jeff Cody <jcody@redhat.com>]

On Wed, Feb 15, 2017 at 05:58:13PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Currently backup to nbd target is broken, as nbd doesn't have
> .bdrv_get_info realization.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> v3: fix compilation (I feel like an idiot)
>     adjust wording (Fam)
>     
> v2: add WARNING
> 
> ===
> 
> Since commit
> 
> commit 4c9bca7e39a6e07ad02c1dcde3478363344ec60b
> Author: John Snow <jsnow@redhat.com>
> Date:   Thu Feb 25 15:58:30 2016 -0500
> 
>     block/backup: avoid copying less than full target clusters
> 
> backup to nbd target is broken, we have "Couldn't determine the cluster size of
> the target image".
> 
> Proposed NBD protocol extension - NBD_OPT_INFO should finally solve this problem.
> But until it is not realized, we need allow backup to nbd target due to backward
> compatibility.
> 
> Furthermore, is it entirely ok to disallow backup if bds lacks .bdrv_get_info?
> Which behavior should be default: to fail backup or to use default cluster size?
> 
> 
>  block/backup.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/block/backup.c b/block/backup.c
> index ea38733..d800a24 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -638,7 +638,16 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
>       * backup cluster size is smaller than the target cluster size. Even for
>       * targets with a backing file, try to avoid COW if possible. */
>      ret = bdrv_get_info(target, &bdi);
> -    if (ret < 0 && !target->backing) {
> +    if (ret == -ENOTSUP) {
> +        /* Cluster size is not defined */
> +        fprintf(stderr,
> +                "WARNING: Target block device doesn't provide information "
> +                "about block size and it doesn't have backing file. Default "
> +                "block size of %u bytes is used. If actual block size of "
> +                "target exceeds this default, backup may be unusable",
> +                BACKUP_CLUSTER_SIZE_DEFAULT);

You should use error_report, like in your v2 patch.  Just make sure to
include qemu/error-report.h.

> +        job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
> +    } else if (ret < 0 && !target->backing) {
>          error_setg_errno(errp, -ret,
>              "Couldn't determine the cluster size of the target image, "
>              "which has no backing file");
> -- 
> 1.8.3.1
>