Re: [PATCH for-5.1 1/2] qcow2: Implement v2 zero writes with discard if possible

[Max Reitz]

On 20.07.20 15:18, Kevin Wolf wrote:
> qcow2 version 2 images don't support the zero flag for clusters, so for
> write_zeroes requests, we return -ENOTSUP and get explicit zero buffer
> writes. If the image doesn't have a backing file, we can do better: Just
> discard the respective clusters.
> 
> This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has
> to assume that the existing target image may contain any data, so it has
> to write zeroes. Without this patch, this results in a fully allocated
> target image, even if the source image was empty.
> 
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/qcow2-cluster.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

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

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