Re: [Qemu-devel] [PATCH v2] backup: allow target without .bdrv_get_info

[Fam Zheng <famz@redhat.com>]

On Wed, 02/15 14:42, Vladimir Sementsov-Ogievskiy wrote:
> 15.02.2017 14:33, Fam Zheng wrote:
> > On Wed, 02/15 14:09, 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>
> > > ---
> > > 
> > > 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.
> > If you respin, including above in the commit message is even better, IMO.
> > 
> > > 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 | 10 +++++++++-
> > >   1 file changed, 9 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/block/backup.c b/block/backup.c
> > > index ea38733..a464b63 100644
> > > --- a/block/backup.c
> > > +++ b/block/backup.c
> > > @@ -638,7 +638,15 @@ 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 */
> > > +        error_report("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, destination image "
> > > +                     "may become unusable");
> > "Image unusable" might be too strong here because it sounds like a serious
> > damage to the image, while actually what we mean is "backup unusable".
> 
> Finally, "... default, backup may be unusable"?

Yes, looks good.

Fam