Re: [Qemu-devel] [PATCH 4/4] block/qcow2: add zlib-fast compression algorithm

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Jun 27, 2017 at 02:34:10PM +0200, Peter Lieven wrote:
> this adds support for optimized zlib settings which almost
> tripples the compression speed while maintaining about
> the same compressed size.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  block/qcow2-cluster.c |  3 ++-
>  block/qcow2.c         | 11 +++++++++--
>  block/qcow2.h         |  1 +
>  qemu-img.texi         |  1 +
>  4 files changed, 13 insertions(+), 3 deletions(-)
> diff --git a/qemu-img.texi b/qemu-img.texi
> index 043c1ba..83a5db2 100644
> --- a/qemu-img.texi
> +++ b/qemu-img.texi
> @@ -627,6 +627,7 @@ The following options are available if support for the respective libraries
>  has been enabled at compile time:
>  
>     zlib            Uses standard zlib compression (default)
> +   zlib-fast       Uses zlib compression with optimized compression parameters

This looks like a poor design from a future proofing POV. I would much
rather see us introduce a more flexible modelling of compression at the
QAPI layer which lets us have tunables for each algorith, in the same
way that the qcow2 built-in encryption now has ability to set tunables
for each algorithm.

eg your "zlib-fast" impl which just uses zlib with a window size of -15
could be expressed as

   qemu-img create -o compress.format=zlib,compress.window=-15



>     lzo             Uses LZO1X compression


Regards,
Daniel
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