Re: [Qemu-devel] [PATCH for 2.1 0/2] Fix commit of oversized layer

[Fam Zheng]

On Thu, 07/10 11:25, Kevin Wolf wrote:
> Am 10.07.2014 um 10:42 hat Fam Zheng geschrieben:
> > On Fri, 06/27 11:44, Kevin Wolf wrote:
> > > In general, it feels like it would be the right thing to do, especially
> > > considering the goal of operation categories in the final state, but on
> > > the other hand it means that RESIZE would have to be excluded from
> > > bs->backing_blocker, too, allowing standalone resize commands on backing
> > > files. Not sure that this would be a good idea...
> > 
> > Is it really dangerous if we relax the backing_blocker on resize? In 
> > general, I
> > expect the only critical category of operation is chain manipulation,
> > particularly bdrv_swap.
> 
> I'm not completely sure about backing_blockers on resize, but I don't
> think there's currently a way to make it safe, except if the image could
> be safely removed from the backing chain altogether.
> 
> In any case, as long as block jobs don't set blockers on all images
> they touch but rather just on the top-level one, allowing to resize
> arbitrary nodes is dangerous because the block jobs can't cope with
> BDSes changing their size in the middle of the operation.

What's the worst case here? Something like block job never complete?  User
might get to some error state in this case but that's expected.

Because guest can't cope with devices changing their size in the middle of some
of its operations, as well.

Fam