Re: [Qemu-devel] [PATCH] qcow3 - arbitrary metadata

[Anthony Liguori <anthony@codemonkey.ws>]

Nathaniel McCallum wrote:
> On Mon, Jul 28, 2008 at 7:04 PM, Laurent Vivier <laurent@lvivier.info 
> <mailto:laurent@lvivier.info>> wrote:
>
>
>     Le 28 juil. 08 à 21:56, Nathaniel McCallum a écrit :
>
>         A project I'm working on requires the ability to store
>         arbitrary metadata in the VM disk image.  Thus, here is a
>         patch that implements that as qcow3.  It basically replaces
>         the header.backing_store_{offset|size} with
>         header.metadata_{offset|size}.  Metadata is then defined as
>         NULL-byte separated 'key:value' pairs.  The attached qcow3
>         then stores the backing file as
>         'Backing-File:/home/me/backing_file.img' in the metadata
>         section.  I've included two patches.  One is the full patch
>         against the latest SVN (qcow3.patch).  The second patch is
>         just the diff between qcow2.c and qcow3.c so that you can
>         easily see the changes.
>
>         I've also wondered if it might be possible to backport these
>         changes into qcow2 instead of qcow3.  However, this would
>         break older versions of qemu that claim to support qcow2.
>
>         Nathaniel
>         <qcow3.patch><qcow2-qcow3.patch>
>
>
>     I think this has already been tried exactly one year ago modifying
>     snapshot...
>
>     http://www.mail-archive.com/kvm-devel@lists.sourceforge.net/msg05103.html
>
>     but now Fabrice is working on a configuration file, perhaps you
>     can store your metadata in it.
>
>
> Well, the point is to not have a separate configuration file.
>
> I've dug up a few more bits of info.  First, the offset option will 
> never work without changing at least qcow2's on disk format.

Never is such a strong word.  What I was thinking is that you could do 
the offset thing based on a URI of some sort.  For instance:

qemu -drive file=offset:foo.img:offset=32,if=ide

The syntax of -drive erks me here but you get the gist.  Then backing 
files would work fine.

>   This is because when you start qemu in snapshot mode, it creates a 
> new qcow2 image and uses it as the main image.  However, the offset 
> was meant to apply to the underlying image and there is no field in 
> qcow2 to specify the offset of the backing_file.
>
> Second, when creating a new qcow2 file, the l1_table_offset is 4096.  
> There does not appear to be anything stored between the end of the 
> header and the 4096.  So, in theory, you could put anything you want 
> there.  No?

There's no guarantee that that space won't be used in future versions of 
QCOW2 so you can't just cram arbitrary stuff in there.

Regards,

Anthony Liguori

> Nathaniel
>