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

[Kevin Wolf]

Am 18.06.2013 um 09:00 hat Fam Zheng geschrieben:
> On Tue, 06/18 08:32, Kevin Wolf wrote:
> > Am 18.06.2013 um 05:58 hat Fam Zheng geschrieben:
> > > 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?
> > 
> > No, I think there's no case where this would make sense.
> > 
> > > If not, can we try to detect such case  on opening and try to
> > > reuse the bs?
> > 
> > We can't do it reliably, think about symlinks or even hard links, or
> > things like /dev/fdset/..., let alone remote protocols that refer to the
> > same image file etc.
> > 
> > We can check the obvious cases and error out for them, but that's about
> > what we can do. I don't think we should try to fix things automagically
> > when we can't do it right.
> 
> It's impossible to know a remote protocol points to the same image with
> local file path, that's not in QEMU's scope, but we have a good chance
> to detect (strcmp with existing bs->filename) and error out Paolo's
> A-B-A problem, don't we?

Yes, catching 50% of the misuses is better than catching none.

My point was that we shouldn't "try to reuse the bs" when we detect that
the file is already open, because that makes it a feature that users are
supposed to use and that doesn't work consistently across backends and
will therefore cause endless pain.

If we detect it (in order to protect the user from his own mistakes), we
must treat it as a misuse and return an error.

Kevin