Re: [Qemu-block] [RFC] Proposed qcow2 extension: subcluster allocation

[Alberto Garcia]

On Thu 13 Apr 2017 11:02:10 AM CEST, Kevin Wolf <address@hidden> wrote:
> I think you still need it if you don't want to look at the refcount
> blocks for every write. When you take an internal snapshot, you just
> increase the refcount of the L2 tables at first and keep the contents
> the same, including the subcluster information. On the first write to
> the cluster, like with normal images you need to copy the whole
> cluster, and whether this is the case is determined with the COPIED
> flag. (The copy can actually keep unallocated/zeroed subclusters that
> way, but data has to be copied for the whole cluster.)
>
> Of course, in all cases the COPIED flag is just an optimisation
> because it's always possible to look at the refcount blocks, but I
> don't think there is any difference between subclustered and
> traditional images in this respect.

Exactly :)

Berto