Re: ImageInfo oddities regarding compression

[Daniel P . Berrangé]

On Fri, Nov 27, 2020 at 11:06:36AM +0100, Markus Armbruster wrote:
> ImageInfo has an optional member @compressed:
> 
>     ##
>     # @ImageInfo:
>     [...]
>     # @compressed: true if the image is compressed (Since 1.7)
> 
> Doc bug: neglects to specify the default.  I guess it's false.
> 
> The only user appears to be vmdk_get_extent_info().  Goes back to
> v1.7.0's commits
> 
>     f4c129a38a vmdk: Implment bdrv_get_specific_info
>     cbe82d7fb3 qapi: Add optional field 'compressed' to ImageInfo
> 
> ImageInfo also has an optional member @format-specific.
> 
> Doc bug: neglects to specify when it's present.  I assume it's always
> present when member @format has a value that has a non-empty variant in
> @format-specific's type ImageInfoSpecific.
> 
> Format qcow2's variant is ImageInfoSpecificQCow2.  It has a mandatory
> member @compression-type.
> 
>    ##
>    # @Qcow2CompressionType:
>    #
>    # Compression type used in qcow2 image file
>    #
>    # @zlib: zlib compression, see <http://zlib.net/>
>    # @zstd: zstd compression, see <http://github.com/facebook/zstd>
>    #
>    # Since: 5.1
>    ##
>    { 'enum': 'Qcow2CompressionType',
>      'data': [ 'zlib', { 'name': 'zstd', 'if': 'defined(CONFIG_ZSTD)' } ] }
> 
> Apparently, you can't have a qcow2 image without compression.  Correct?
> 
> Can you imagine a use case for "without compression"?

Yes & no. An image always has a compression type, either implicitly
zlib as the historical default, or explicitly as a type recorded in
the header.  This doesn't mean compression is used.

There may be zero or more clusters actually using compression.
Typically compression will never be used, but there's still an
associated compression type regardless.


Regards,
Daniel
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