Re: [PATCH v2] qcow2: Fix preallocation on images with unaligned sizes

[Alberto Garcia]

ping

On Wed 17 Jun 2020 04:00:36 PM CEST, Alberto Garcia wrote:
> When resizing an image with qcow2_co_truncate() using the falloc or
> full preallocation modes the code assumes that both the old and new
> sizes are cluster-aligned.
>
> There are two problems with this:
>
>   1) The calculation of how many clusters are involved does not always
>      get the right result.
>
>      Example: creating a 60KB image and resizing it (with
>      preallocation=full) to 80KB won't allocate the second cluster.
>
>   2) No copy-on-write is performed, so in the previous example if
>      there is a backing file then the first 60KB of the first cluster
>      won't be filled with data from the backing file.
>
> This patch fixes both issues.
>
> Signed-off-by: Alberto Garcia <berto@igalia.com>