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

[Kevin Wolf]

Am 27.06.2019 um 17:42 hat Alberto Garcia geschrieben:
> On Thu 27 Jun 2019 05:38:56 PM CEST, Alberto Garcia 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. And you'd have to fall back to explicit writes of a
zeroed buffer rather than using efficient operations more often.

Kevin