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

[Kevin Wolf]

Am 28.06.2019 um 11:53 hat Alberto Garcia geschrieben:
> 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.

Right, but that leaving clusters simply unallocated doesn't quite cut it
is something we already noticed before writing the spec for v3. Only
really necessary when you have a backing file, of course, but that's one
of the more interesting cases for subclusters anyway.

Kevin