Re: [Qemu-devel] [PATCH 1/4] block/qcow2: add compression_algorithm create option

[Denis V. Lunev]

On 06/27/2017 04:27 PM, Peter Lieven wrote:
> Am 27.06.2017 um 15:20 schrieb Daniel P. Berrange:
>> On Tue, Jun 27, 2017 at 02:34:07PM +0200, Peter Lieven wrote:
>>> this patch adds a new compression_algorithm option when creating
>>> qcow2 images.
>>> The current default for the compresison algorithm is zlib and zlib
>>> will be
>>> used when this option is omitted (like before).
>>>
>>> If the option is specified e.g. with:
>>>
>>>   qemu-img create -f qcow2 -o compression_algorithm=zlib image.qcow2 1G
>> IMHO we should introduce a nested struct "compress" struct to hold
>> the format
>> name, and any other format specific arguments, in a way that maps
>> nicely to
>> any future QAPI representmatch of create options. eg
>>
>> { 'enum': 'BlockdevQcow2CompressFormat',
>>    'data': [ 'zlib', 'lzo' ] }
>>
>> { 'union': 'BlockdevQcow2Compress',
>>    'base': { 'format': 'BlockdevQcow2CompressFormat' },
>>    'discriminator': 'format',
>>    'data': { 'zlib': 'BlockdevQcow2CompressZLib',
>>              'lzo': 'BlockdevQcow2CompressLZO'} }
>>
>> so it would map to
>>
>>   qemu-img create -f qcow2 -o compress.format=zlib image.qcow2 1G
>>
>> and let us have other compress.XXXX options specific to each format
>
> Or would it be possible to start with just a compress.level (int)
> parameter.
> In fact that would be sufficient for almost all formats (or better use
> algorithms?).
> The windowBits can be default to -15 in the future. It seems the old
> choice of -12
> was just not optimal. We just have to use it for backwards
> compatiblity if the compress
> options are not specified.
>
> Peter
>
We can put generic parameters on top (in generic header) but
put algorithm-dependent container inside. This could be
viable for the future to avoid incompatible format changes.

Den