Am 26.09.2016 um 11:51 hat Pavel Dovgalyuk geschrieben: > > From: Kevin Wolf [mailto:kw...@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.dovga...@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.
Hm, this can happy today indeed. Originally, we only called bdrv_goto_snapshot() for all _top level_ BDSes, and this is still what you normally get. However, if you explicitly create a BDS (e.g. with its own -drive option), it is considered a top level BDS without actually being top level for the guest, and therefore the snapshotting function is called for it. Of course, this is highly inefficient because the goto_snapshot request is passed by the filter driver and then called another time for the lower node, effectively loading the snapshot a second time. On the other hand if you use a single -drive option to create both the qcow2 BDS and the blkreplay filter, we do need to pass down the goto_snapshot request because it won't be called for the qcow2 layer otherwise. I'm not completely sure yet what the right behaviour would be here. Kevin
