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

[Fam Zheng]

On Tue, 06/18 08:37, Markus Armbruster wrote:
> Fam Zheng <address@hidden> writes:
> 
> > 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?
> 
> Maybe, I don't know.
> 
> >        If not, can we try to detect such case  on opening
> 
> Gee, what a nice swamp you found there!
> 
> For local files, you can compare (dev-major, dev-minor, inode).
> 
> Beyond that, you tend not to get comparisons, but best guesses.
> 
> >                                                           and try to
> > reuse the bs?
> 
> I doubt reusing the BDS is correct in the general case.
> 
Maybe I meant basically the same as Kevin, but just that QEMU finds out
A has existing BDS, and reattach it.
> >> > > 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.
I assume it should be less wrong than having two BDS for the same file.

-- 
Fam