From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41557) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bot2b-0006YL-Md for qemu-devel@nongnu.org; Tue, 27 Sep 2016 10:07:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bot2W-0005dh-Ma for qemu-devel@nongnu.org; Tue, 27 Sep 2016 10:07:24 -0400 From: "Pavel Dovgalyuk" References: <20160926080757.6992.74311.stgit@PASHA-ISP> <20160926080838.6992.95614.stgit@PASHA-ISP> <20160926092346.GE6093@noname.str.redhat.com> <002001d217db$845fc030$8d1f4090$@ru> <20160926131721.GH6093@noname.str.redhat.com> In-Reply-To: <20160926131721.GH6093@noname.str.redhat.com> Date: Tue, 27 Sep 2016 17:06:55 +0300 Message-ID: <001101d218c8$67aa1010$36fe3030$@ru> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Language: ru Subject: Re: [Qemu-devel] [PATCH v5 7/9] block: don't make snapshots for filters List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: 'Kevin Wolf' Cc: 'Pavel Dovgalyuk' , qemu-devel@nongnu.org, peter.maydell@linaro.org, quintela@redhat.com, jasowang@redhat.com, mst@redhat.com, pbonzini@redhat.com, mreitz@redhat.com, qemu-block@nongnu.org > From: Kevin Wolf [mailto:kwolf@redhat.com] > Am 26.09.2016 um 11:51 hat Pavel Dovgalyuk geschrieben: > > > 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 > > > > > > 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. How this can be specified in command line? I believed that separate -drive option is required. > > I'm not completely sure yet what the right behaviour would be here. Pavel Dovgalyuk