Re: [Qemu-block] [PATCH] [RFC] qcow2: add compression type feature

[Max Reitz]

On 16.05.19 09:50, Denis Plotnikov wrote:
> 
> 
> On 29.04.2019 1:32, Max Reitz wrote:
>> On 05.02.19 10:08, Denis Plotnikov wrote:
>>> The patch adds some preparation parts for incompatible compression type
>>> feature into QCOW2 header that indicates that *all* compressed clusters
>>> must be (de)compressed using a certain compression type.
>>>
>>> It is implied that the compression type is set on the image creation and
>>> can be changed only later by image convertion, thus the only compression
>>> algorithm is used for the image.
>>>
>>> The plan is to add support for ZSTD and then may be something more effective
>>> in the future.
>>>
>>> ZSDT compression algorithm consumes 3-5 times less CPU power with a
>>> comparable comression ratio with zlib. It would be wise to use it for
>>> data compression f.e. for backups.
>>>
>>> The default compression is ZLIB.
>>>
>>> Signed-off-by: Denis Plotnikov <address@hidden>
>>> ---
>>>   block/qcow2.c | 25 +++++++++++++++++++++++++
>>>   block/qcow2.h | 26 ++++++++++++++++++++++----
>>>   2 files changed, 47 insertions(+), 4 deletions(-)
>>
>> Are there plans to pursue this further?
> Yes, I'm going to come up with the first version in a few weeks

OK.

>> [...]
>>
>>> diff --git a/block/qcow2.h b/block/qcow2.h
>>> index 32cce9eee2..fdde5bbefd 100644
>>> --- a/block/qcow2.h
>>> +++ b/block/qcow2.h
>>> @@ -112,6 +112,10 @@
>>>   #define QCOW2_OPT_REFCOUNT_CACHE_SIZE "refcount-cache-size"
>>>   #define QCOW2_OPT_CACHE_CLEAN_INTERVAL "cache-clean-interval"
>>>   
>>> +/* Compression types */
>>> +#define QCOW2_COMPRESSION_TYPE_ZLIB    0
>>> +#define QCOW2_COMPRESSION_TYPE_ZSTD    1
>>
>> We probably want QAPI types anyway (qemu-img info should report the
>> compression type), so I think we could use them instead.
> I'm not sure that I understood the idea. Could you please explain what 
> is meant here? We don't need those constants and should use the 
> constants defined somewhere else (where)?

qemu-img info can report format-specific information.  I think the
compression type should be listed there as well, once there is more than
one.

Format-specific information is a QAPI type, namely
ImageInfoSpecificQCow2.  Therefore, everything it contains needs to be a
QAPI object, and this includes potential compression information.  We
thus need a QAPI enum like Qcow2CompressionType, and that would
automatically give us these values.

Max

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