On 28.06.20 13:02, Alberto Garcia wrote:
> When writing to a qcow2 file there are two functions that take a
> virtual offset and return a host offset, possibly allocating new
> clusters if necessary:
> 
>    - handle_copied() looks for normal data clusters that are already
>      allocated and have a reference count of 1. In those clusters we
>      can simply write the data and there is no need to perform any
>      copy-on-write.
> 
>    - handle_alloc() looks for clusters that do need copy-on-write,
>      either because they haven't been allocated yet, because their
>      reference count is != 1 or because they are ZERO_ALLOC clusters.
> 
> The ZERO_ALLOC case is a bit special because those are clusters that
> are already allocated and they could perfectly be dealt with in
> handle_copied() (as long as copy-on-write is performed when required).
> 
> In fact, there is extra code specifically for them in handle_alloc()
> that tries to reuse the existing allocation if possible and frees them
> otherwise.
> 
> This patch changes the handling of ZERO_ALLOC clusters so the
> semantics of these two functions are now like this:
> 
>    - handle_copied() looks for clusters that are already allocated and
>      which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
>      reference count of 1).
> 
>    - handle_alloc() looks for clusters for which we need a new
>      allocation (all other cases).
> 
> One important difference after this change is that clusters found
> in handle_copied() may now require copy-on-write, but this will be
> necessary anyway once we add support for subclusters.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Reviewed-by: Eric Blake <eblake@redhat.com>
> ---
>  block/qcow2-cluster.c | 256 +++++++++++++++++++++++-------------------
>  1 file changed, 141 insertions(+), 115 deletions(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>