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

[Vladimir Sementsov-Ogievskiy]

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 <address@hidden>
> > ---
> >
> > v2: add WARNING
> >
> > ===
> >
> > Since commit
> >
> > commit 4c9bca7e39a6e07ad02c1dcde3478363344ec60b
> > Author: John Snow <address@hidden>
> > 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"?


> > +        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
> Fam


--
Best regards,
Vladimir