Re: [PATCH v10 28/34] qcow2: Add subcluster support to qcow2_co_pwrite_zeroes()

[Max Reitz]

On 03.07.20 17:58, Alberto Garcia wrote:
> This works now at the subcluster level and pwrite_zeroes_alignment is
> updated accordingly.
> 
> qcow2_cluster_zeroize() is turned into qcow2_subcluster_zeroize() with
> the following changes:
> 
>    - The request can now be subcluster-aligned.
> 
>    - The cluster-aligned body of the request is still zeroized using
>      zero_in_l2_slice() as before.
> 
>    - The subcluster-aligned head and tail of the request are zeroized
>      with the new zero_l2_subclusters() function.
> 
> There is just one thing to take into account for a possible future
> improvement: compressed clusters cannot be partially zeroized so
> zero_l2_subclusters() on the head or the tail can return -ENOTSUP.
> This makes the caller repeat the *complete* request and write actual
> zeroes to disk. This is sub-optimal because
> 
>    1) if the head area was compressed we would still be able to use
>       the fast path for the body and possibly the tail.
> 
>    2) if the tail area was compressed we are writing zeroes to the
>       head and the body areas, which are already zeroized.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2.h         |  4 +--
>  block/qcow2-cluster.c | 81 +++++++++++++++++++++++++++++++++++++++----
>  block/qcow2.c         | 33 +++++++++---------
>  3 files changed, 94 insertions(+), 24 deletions(-)

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

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