Re: [Qemu-devel] [6907] Introducing qcow2 extensions (Uri Lublin)

[Anthony Liguori]

Avi Kivity wrote:
> Anthony Liguori wrote:
> > > We should introduce a notion of compatible vs. incompatible extensions.
> > >
> > > A compatible extension my be ignored by the qcow2 code if it does 
> > > not understand the magic number.  An incompatible extension causes 
> > > an abort.  This allows both more flexibility in how we can change 
> > > the file format.  I believe ext* does the same thing.
> >
> > I was assuming that all extensions would be compatible.  I don't like 
> > the idea of having qcow2 files floating around that require specific 
> > version of QEMU.
> >
> > For that, we should just bump to qcow3 (that's what versioning is 
> > for, right? :-).
>
> The problem is that you would need to bump the version each time you 
> made an incompatible change.  We'd end up with qcow4351 very quickly.
>
> One option is to declare qcow3 as qcow2 + all incompatible extensions 
> just before 0.11.  Another is to require explicit user action to 
> enable an incompatible option.  If you did that, you could expect only 
> to upgrade qemu, not degrade, but that's a reasonable assumption in 
> many scenarios.

Well what sort of incompatible extensions are we talking about?  Are 
there things being kicked around?

Regards,

Anthony Liguori