Re: [Qemu-devel] [PATCH 0/4] export internal snapshot by qemu-nbd

[Wenchao Xia]

于 2013-7-22 10:10, Fam Zheng 写道:
> On Fri, 07/19 17:03, Wenchao Xia wrote:
> > 于 2013-7-18 13:43, Stefan Hajnoczi 写道:
> > > On Wed, Jul 17, 2013 at 10:03:51PM +0800, Wenchao Xia wrote:
> > > > This series allow user to read internal snapshot's contents without qemu-img
> > > > convert. Another purpose is that, when qemu is online and have taken an
> > > > internal snapshot, let user invoke qemu-nbd to do any thing on it except write.
> > >
> > > I agree with Eric and Kevin that we cannot access image files while QEMU
> > > has them open.
> > >
> > > A bit more detail about using the run-time NBD server to do this safely:
> > >
> > > Internal snapshots are not first-class block layer objects today.  They
> > > are not BlockDriverStates, instead you must access their data through
> > > bdrv_snapshot_goto() or bdrv_snapshot_load_tmp().
> >    Also cc to Fam Zheng an Kevin:
> >    I think BlockDriverStates modification, is based on a more basic
> > question: does qcow2's data structure on disk, allow multiple snapshot
> > reader, while one active writer, without lock? I think the modification
> > showed above, already said "yes" to this question. If yes, just
> > create a new BlockDriverStates, we should say yes in spec.
> >    I wonder how vmdk allow direct snapshot access by VixDiskLib 1.1,
> > without network expense on host, it should be the format said support
> > of it.
> Is the VMDK snapshot internal? Which subformat of VMDK do you mean here?
>
> Thanks.
  Rechecked VMDK's doc, guess it is using backing chain. Still, It helps
if internal snapshot can do similar things.

--
Best Regards

Wenchao Xia