Re: [Qemu-devel] [RFC] Re-evaluating subcluster allocation for qcow2 images

[Alberto Garcia]

On Fri 28 Jun 2019 11:20:57 AM CEST, Kevin Wolf wrote:
>> >> I would consider 64k cluster/8k subcluster as too extreme for me.
>> 
>> I forgot to add: this 64k/8k ratio is only with my current prototype.
>> 
>> In practice if we go with the 128-bit L2 entries we would have 64
>> subclusters per cluster, or 32 if we want to have a separate
>> QCOW_OFLAG_ZERO for each subcluster (would we need this?).
>
> Yes, I think we'd want to have a separate zero flag for each
> subcluster.  Otherwise, when writing to a zero cluster, you'd have to
> COW the whole supercluster again.

Yes if the original cluster had the QCOW_OFLAG_ZERO bit, not if it was
simply unallocated.

Berto