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

[Kevin Wolf <kwolf@redhat.com>]

Am 10.09.2010 19:10, schrieb Anthony Liguori:
> On 09/10/2010 11:05 AM, Kevin Wolf wrote:
>> 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.
>>    
> 
> The problem is that management tools have to make a decision about what 
> to do with ID's that aren't UUIDs which means that in our management 
> interface, we can't just expose UUIDs but instead we have to expose 
> strings that may sometimes be UUIDs.
> 
> I don't think it buys us a lot to get the backwards compatibility.

No matter how you store the UUID and no matter how many feature flags
you're going to throw on it, you'll always have old images and new
images. Management tools will have to cope with that (or break backwards
compatibility, of course - they still have that choice).

Kevin