Re: [Qemu-devel] QCow v2

[Fabrice Bellard <fabrice@bellard.org>]

Hi,

I am not sure it is interesting to store the configuration of the VM in 
the disk image (there can be several disk images for a given VM). 
Moreover, extensions to the qcow header must be added at its end.

Regarding my current plans, the next qcow evolution will be the support 
for multiple snapshots. Each snapshot will be tagged by a label.

Fabrice.

Nathaniel McCallum wrote:
> Mark's notes on the qcow format got me to thinking how useful it would
> be to be able to store other information in the qcow image itself.  For
> instance you could store the configuration for the virtual machine in
> the image which could be extracted and then start the virtual machine.
> 
> So... I'm proposing that the qcow image be extended to support this
> scenario.  Something as simple as:
> 
>   typedef struct QCowHeader {
>       uint32_t magic;
>       uint32_t version;
> 
>       uint8_t  embedded_data_type;
>       uint64_t embedded_data_offset;
>       uint32_t embedded_data_size;
>       uint32_t mtime;
> 
>       uint64_t size;
> 
>       uint8_t  cluster_bits;
>       uint8_t  l2_bits;
>       uint32_t crypt_method;
> 
>       uint64_t l1_table_offset;
>   } QCowHeader;
> 
> Thus, embedded_data_type is a constant signifying the type of the data.
> embedded_data_type could be a string, etc.  Of course, there are a million
> other ways this could be implemented.  The idea being that we could store
> more than just backing store.
> 
> One possible application could be that you could store config info in
> the image and have a qemu-loader app that extracts the info and starts
> the virtual machine.  Another application could be just misc. metadata:
> copyright, contact info, VM version, etc...
> 
> I can provide a patch to block-qcow.c if there is interest.
> 
> Nathaniel
> 
> 
> 
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