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diffuse handling of image creation from another path

see attachement!



On Tue, Dec 03, 2013 at 03:22:28PM -0000, Florian Miksch wrote:
Hi,
This does not look like a bug.  Please see the explanation below...

Inlining Florian's reproducer script so we can discuss it via email:

> #!/bin/bash
> # This script does create a backing image and overlay to
> # reproduce the reported bug.
> #...
>  
> mkdir a
> # Create an image from another path e.g. in the directory 'a'
> qemu-img create -f qcow2 a/blob.img 10G
>  
> # Create an overlay image from another path in the same directory
> qemu-img create -f qcow2 -b a/blob.img a/ovl.img
>  
> # Get Info in the new directory
> cd a
> qemu-img info ovl.img
> # Output:
> #       image: ovl.img
> #       file format: qcow2
> #       virtual size: 10G (10737418240 bytes)
> #       disk size: 196K
> #       cluster_size: 65536
> #       backing file: a/blob.img
>  
> # Get the Info from another directory
> cd ..
> qemu-img info a/ovl.img
> # Output:
> #       image: a/ovl.img
> #       file format: qcow2
> #       virtual size: 10G (10737418240 bytes)
> #       disk size: 196K
> #       cluster_size: 65536
> #       backing file: a/blob.img (actual path: a/a/blob.img)
>  
> # Bug:
> #       Compare 'image'
> #       Compare 'backing file'
> #       Look at 'actual path'

The behavior you are showing here is explained as follows:

Backing file paths can be relative or absolute and are stored inside
the image file.  In this case you are providing a relative backing
file path.

Relative backing file paths are interpreted against the image filename.
In other words:

  join_path(dirname('a/ovl.img'), 'a/blob.img') -> 'a/a/blob.img'

>  
> # 'qemu-img info' takes the recommended path as name of the image
> # and the actual path is then: a/a/blob.img    
>  
> qemu-img commit a/ovl.img
> # Now commit fails  
> # --> ERROR "No such file or directory

The problem was:
> qemu-img create -f qcow2 -b a/blob.img a/ovl.img

If you want to use a relative backing file path, remember that it is
relative to the image file:

cd a && qemu-img create -f qcow2 -b blob.img ovl.img

Then 'qemu-img commit a/ovl.img' will work as expected.

Stefan