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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
References: <20160926080757.6992.74311.stgit@PASHA-ISP>
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Date: Mon, 26 Sep 2016 12:51:11 +0300
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Subject: Re: [Qemu-devel] [PATCH v5 7/9] block: don't make snapshots for
 filters
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To: 'Kevin Wolf' <kwolf@redhat.com>, 'Pavel Dovgalyuk' <pavel.dovgaluk@ispras.ru>
Cc: qemu-devel@nongnu.org, peter.maydell@linaro.org, quintela@redhat.com, jasowang@redhat.com, mst@redhat.com, pbonzini@redhat.com

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> Am 26.09.2016 um 10:08 hat Pavel Dovgalyuk geschrieben:
> > This patch disables snapshotting for block driver filters.
> > It is needed, because snapshots should be created
> > in underlying disk images, not in filters itself.
> >
> > Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
> 
> But that's exactly what the existing code implements? If a driver
> doesn't provide .bdrv_snapshot_goto, the request is redirected to
> bs->file.
> 
> >  block/snapshot.c |    3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/block/snapshot.c b/block/snapshot.c
> > index bf5c2ca..8998b8b 100644
> > --- a/block/snapshot.c
> > +++ b/block/snapshot.c
> > @@ -184,6 +184,9 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
> >      if (!drv) {
> >          return -ENOMEDIUM;
> >      }
> > +    if (drv->is_filter) {
> > +        return 0;
> > +    }
> 
> This, on the other hand, doesn't redirect the request, but silently
> ignores it. That is, loading the snapshot will apparently succeed, but
> it wouldn't actually load anything and the disk would stay in its
> current state.

In my use case bdrv_all_goto_snapshot iterates all block drivers, including
filters and disk images. Therefore skipping goto for images is ok.

Maybe I should move this check to bdrv_can_snapshot?

> 
> >      if (drv->bdrv_snapshot_goto) {
> >          return drv->bdrv_snapshot_goto(bs, snapshot_id);
> >      }


Pavel Dovgalyuk