Am 28.05.2019 um 16:37 hat Denis Plotnikov geschrieben:
> The patch adds some preparation parts for incompatible compression type
> feature to 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 conversion, thus compression type
> defines the only compression algorithm used for the image.
>
> The goal of the feature is to add support of other compression algorithms
> to qcow2. For example, ZSTD which is more effective on compression than ZLIB.
> It works roughly x2 faster than ZLIB providing a comparable compression ratio
> and therefore provide a performance advantage in backup scenarios.
>
> The default compression is ZLIB. Images created with ZLIB compression type
> is backward compatible with older qemu versions.
>
> Signed-off-by: Denis Plotnikov <dplotni...@virtuozzo.com>
> @@ -3119,6 +3123,10 @@
> # an image, the data file name is loaded from the
> image
> # file. (since 4.0)
> #
> +# @compression-type: compression method to use for image clusters
> compression
> +# The comression method is set on image creation and
> can
> +# be changed via image converting only. (since 4.1)
> +#
> # Since: 2.9
> ##
> { 'struct': 'BlockdevOptionsQcow2',
> @@ -3134,7 +3142,8 @@
> '*refcount-cache-size': 'int',
> '*cache-clean-interval': 'int',
> '*encrypt': 'BlockdevQcow2Encryption',
> - '*data-file': 'BlockdevRef' } }
> + '*data-file': 'BlockdevRef',
> + '*compression-type': 'Qcow2CompressionType' } }
qcow2_open() doesn't actually parse this option (and it couldn't do
anything useful with it because the image is fixed to a single
compression type), so this shouldn't be added.
> ##
> # @SshHostKeyCheckMode:
> @@ -4206,6 +4215,19 @@
> 'data': [ 'v2', 'v3' ] }
>
>
> +##
> +# @Qcow2CompressionType:
> +#
> +# Compression type used in qcow2 image file
> +#
> +# @zlib : gzip compressor
> +# @zstd : zstd compression
> +#
> +# Since: 4.1
> +##
> +{ 'enum': 'Qcow2CompressionType',
> + 'data': [ 'zlib', 'zstd' ] }
I think it would be cleaner to start with only 'zlib' here, like your C
code that doesn't implement anything for non-zlib compression types yet.
'zstd' can be added to the enum when it's actually implemented. This
will also make schema introspection provide the right information with a
build that includes this patch, but not the zstd compression type
patch.
Kevin
