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

[Stefan Hajnoczi]

On Wed, Apr 27, 2011 at 4:05 AM, Brad Campbell
<address@hidden> 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