Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Kevin Wolf]

Am 10.09.2010 17:53, schrieb Anthony Liguori:
> On 09/10/2010 10:18 AM, Kevin Wolf wrote:
>> Am 10.09.2010 17:02, schrieb Anthony Liguori:
>>    
>>> What makes us future proof is having a good feature support.  qcow2
>>> doesn't have this.  We have a good way at making purely informational
>>> changes and also making changes that break the format.  Those features
>>> are independent so they can be backported in a compatible way too.
>>>      
>> I might have agreed that it's useful to be able to backport them
>> independently if we had had lots of such features added in the past. But
>> we haven't.
>>    
> 
> I think part of why we haven't had them is that the mechanisms aren't 
> very flexible.
> 
> A good example of where feature support would be very nice is for 
> changing the way snapshots metadata is recorded in qcow2.
> 
> It would be nice to be able to represent snapshots with a uuid.  If you 
> added new metadata that had uuid based snapshots that were hierarchical 
> and added a feature bit, it would have some nice properties.
> 
> Since most images don't have snapshots, the common case would be a qcow2 
> that was fully backwards compatible.  You would also get a graceful 
> failure for using a new image with an old QEMU.

Well, snapshots have an ID today (which is different from their name).
Nobody stops you from putting a UUID there. Fully backwards compatible,
no feature flag needed. I think Miguel was planning to actually do this.

Kevin