Re: [Qemu-devel] [PATCH] block: add 'backing' option to drive_add

[Fam Zheng]

On Mon, 06/17 17:12, Kevin Wolf wrote:
> Am 17.06.2013 um 16:46 hat Paolo Bonzini geschrieben:
> > Il 17/06/2013 16:26, Kevin Wolf ha scritto:
> > > Am 17.06.2013 um 16:01 hat Paolo Bonzini geschrieben:
> > >> Il 17/06/2013 15:52, Kevin Wolf ha scritto:
> > >>> It's not a new thought that we need to change the block layer so that a
> > >>> BlockDriverState can't be "empty", but that one BlockDriverState always
> > >>> refers to one image. If you change media, you attach a different
> > >>> BlockDriverState to the device. Once you have this, you can start
> > >>> refcounting BlockDriverStates, so that the backing file remains usable
> > >>> while the guest device already uses a different image.
> > >>>
> > >>> Not that it's it easy to get there...
> > >>
> > >> I'm not sure that is safe to do.
> > >>
> > >> Consider the case where the guest switches from A to B during backup,
> > >> and then from B to A.  You get two BDS for the same file, which pretty
> > >> much means havoc.
> > > 
> > > Well, yes, it means that the management tool needs to know what it's
> > > doing. It shouldn't create a second BDS for A, but reattach the still
> > > existing one.
> > 
> > How?  That would require the management tool to know the full chain of
> > BDSes that were opened in the past.
> 
> They better know on which files they are operating. It's not like the
> management could be unaware of running backup jobs or things like that.
> 

Is there any case that QEMU needs to have two BDS pointing to the same
file?  If not, can we try to detect such case  on opening and try to
reuse the bs?

(Oops, CC'ing the right qemu-devel :p)

-- 
Fam