Re: [Qemu-devel] Qemu-img convert with -B

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Apr 27, 2011 at 4:05 AM, Brad Campbell
<lists2009@fnarfbargle.com> wrote:
> I see there is a bug raised about the behaviour of qemu-img when used to convert using an output backing file. It allocates every sector whether or not it already exists in the output backing file.

Please post the link to the bug report.

> Can someone verify these assumptions for me please?
> - I can bdrv_open() a file that has a chain of backing files, and the
> following is true :
>        - bdrv_read() returns the most recently allocated sector contents (or
> 0)

Correct.

>        - bdrv_is_allocated() will return false only if that sector is not
> allocated in _any_ of the files in the chain

Incorrect.  It returns true if the sector is allocated in the top-most
file, false otherwise.  In other words bdrv_is_allocated() is flat, it
does not traverse a chain of backing files.

Stefan