Re: [Qemu-devel] [PATCH V5 01/10] specs/qcow2: add compress format extension

[Eric Blake]

On 07/25/2017 03:29 PM, Peter Lieven wrote:

>>> +                   deflate: Standard zlib deflate compression without
>>> +                            compression header
>> Why did you name it "deflate" instead of "zlib" again?
> 
> zlib provides raw deflate encoding and gzip encoding both with and without 
> header afaik.
> We use raw deflate encoding without header in QEMU. And the name of the 
> algorithm is deflate.
> I found it more appropriate than zlib.

Okay, I can live with that.

> 
>>
>>> +
>>> +              14:  compress_level (uint8_t)
>>> +
>>> +                   0 = default compress level (valid for all formats, 
>>> default)
>>> +
>>> +                   Additional valid compression levels for deflate 
>>> compression:
>>> +
>>> +                   All values between 1 and 9. 1 gives best speed, 9 gives 
>>> best
>>> +                   compression. The default compression level is defined 
>>> by zlib
>>> +                   and currently defaults to 6.
>>> +
>>> +              15:  compress_window_size (uint8_t)
>>> +
>>> +                   Window or dictionary size used by the compression 
>>> format.
>>> +                   Currently only used by the deflate compression 
>>> algorithm.
>> What must this be set to for other algorithms?  I guess we get to that
>> in later patches.
> 
> Its only used by the deflate algorithm. At this point we only have that 
> algorithm.
> lzo has no such encoding. If we add e.g. lzma it has a dictionary size which 
> is
> a power of two (typcial value would be 2^20).

So for lzo, we should document that the field must be 0 (when we get to
adding it).

> 
> Can you also have a look at the remainder of the series?

It's on my list for 2.11 reviews, although 2.10 freeze stuff is taking
priority, so it may be a few days yet.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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