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

[Peter Lieven <pl@kamp.de>]

Am 27.06.2017 um 15:16 schrieb Daniel P. Berrange:
> 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

It would also need a compress.level, but okay. This will make things
more complicated as one might not know what good values for
the algoritm are.

Wouldn't it be better to have sth like compress.level=1..9 and let
the implementation decide which parameters it choose for the algorithm?

Do you have a pointer where I can find out how to imlement that
in QAPI? Maybe the patches that introduces the encryption settings?

Thanks,
Peter