Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero

[Avi Kivity]

On 03/12/2012 01:04 PM, Kevin Wolf wrote:
> >>
> >> qcow2 can't handle clusters that are referenced twice from the same L1
> >> table. This would require a reverse lookup to adjust the QCOW_O_COPIED
> >> flags in the L2 tables containing the other references.
> > 
> > Don't follow, sorry.  What adjustment are you talking about?
> > 
> > If it's a 1->0 transition, is it mandatory to adjust the flag?  That is,
> > it it legal to have a refcount of exactly one, but have the flag clear?
>
> According to the spec it's illegal and qemu-img check will complain,
> too. In practice, I'm not entirely sure if it will cause real corruption
> or just unnecessary COWs. I believe it may be the latter.

We could retro-doc it but I'm not a huge fan of this practice.

> But it's not worth the trouble anyway when we can have a real zero flag
> in qcow3.

ok.  Thanks for the explanations.

-- 
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