Re: [Qemu-devel] Re: block: format vs. protocol, and how they stack

[Daniel P. Berrange]

On Tue, Jun 22, 2010 at 05:40:02PM +0100, Jamie Lokier wrote:
> Kevin Wolf wrote:
> > > The "protocol" parlance breaks down when we move away from the simple
> > > stuff.  For instance, qcow2 needs two children: the block driver
> > > providing the delta bits (in qcow2 format), and the block driver
> > > providing the base bits (whose configuration happens to be stored in the
> > > delta bits).  
> > 
> > Backing files are different. When talking about opening images (which is
> > what we do here) the main difference is that they can be opened only
> > after the image itself has been opened. I don't think we should include
> > them in this discussion.
> 
> Imho, being unable to override the qcow2 backing file from the command
> line / monitor is very annoying, if you've moved files from another
> machine or just renamed them for tidiness.  It's especially bad if the
> supplied qcow2 file has an absolute path in it, quite bad if it has
> subdirectories or ".." components, annoying if you've been given
> several qcow2 files all of which have the name "backing-file" stored
> in them which are different images because they were originally on
> different machines, and awful if it has the name of a block device in it.

FYI, in the scenario where you've moved backing files around, you can 
use qemu-img to update the location

  qemu-img rebase -u -b /path/to/newbackingfile.img demo.img

Regards,
Daniel
-- 
|: Red Hat, Engineering, London    -o-   http://people.redhat.com/berrange/ :|
|: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :|
|: http://autobuild.org        -o-         http://search.cpan.org/~danberr/ :|
|: GnuPG: 7D3B9505  -o-   F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|