Re: [Qemu-devel] [PATCH v4] backup: allow target without .bdrv_get_info

[Kevin Wolf <kwolf@redhat.com>]

Am 28.02.2017 um 20:33 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 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>
> ---
> 
> v4: use error_report()
>     add article
> 
> 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 | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/block/backup.c b/block/backup.c
> index ea38733849..ea160e9e82 100644
> --- a/block/backup.c
> +++ b/block/backup.c
> @@ -24,6 +24,7 @@
>  #include "qemu/cutils.h"
>  #include "sysemu/block-backend.h"
>  #include "qemu/bitmap.h"
> +#include "qemu/error-report.h"
>  
>  #define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
>  #define SLICE_TIME 100000000ULL /* ns */
> @@ -638,7 +639,16 @@ 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) {

I think this should be if (ret == -ENOTSUP && !target->backing) because
the warning explicitly says "doesn't have a backing file", and the case
with a backing file is already handled below (without a warning).

I can fix this while applying if you agree.

> +        /* Cluster size is not defined */
> +        error_report("WARNING: The target block device doesn't provide "
> +                     "information about the block size and it doesn't have a "
> +                     "backing file. The default block size of %u bytes is "
> +                     "used. If the actual block size of the target exceeds "
> +                     "this default, the backup may be unusable",
> +                     BACKUP_CLUSTER_SIZE_DEFAULT);
> +        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");

Kevin