Re: [Qemu-devel] [PATCH v2 3/3] qcow2: add zstd cluster compression

[Markus Armbruster <armbru@redhat.com>]

Denis Plotnikov <dplotnikov@virtuozzo.com> writes:

> zstd significantly reduces cluster compression time.
> It provides better compression performance maintaining
> the same level of compression ratio in comparison with
> zlib, which, by the moment, has been the only compression
> method available.
>
> The performance test results:
> Test compresses and decompresses qemu qcow2 image with just
> installed rhel-7.6 guest.
> Image cluster size: 64K. Image on disk size: 2.2G
>
> The test was conducted with brd disk to reduce the influence
> of disk subsystem to the test results.
> The results is given in seconds.
>
> compress cmd:
>   time ./qemu-img convert -O qcow2 -c -o compression_type=[zlib|zstd]
>                   src.img [zlib|zstd]_compressed.img
> decompress cmd
>   time ./qemu-img convert -O qcow2
>                   [zlib|zstd]_compressed.img uncompressed.img
>
>            compression               decompression
>          zlib       zstd           zlib         zstd
> ------------------------------------------------------------
> real     65.5       16.3 (-75 %)    1.9          1.6 (-16 %)
> user     65.0       15.8            5.3          2.5
> sys       3.3        0.2            2.0          2.0
>
> Both ZLIB and ZSTD gave the same compression ratio: 1.57
> compressed image size in both cases: 1.4G
>
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
[...]
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index 7cf068f814..4344e858cb 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -538,6 +538,9 @@ Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
>                      Another compressed cluster may map to the tail of the final
>                      sector used by this compressed cluster.
>  
> +                    The layout of the compressed data depends on the compression
> +                    type used for the image (see compressed cluster layout).
> +
>  If a cluster is unallocated, read requests shall read the data from the backing
>  file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
>  no backing file or the backing file is smaller than the image, they shall read
> @@ -790,3 +793,19 @@ In the image file the 'enabled' state is reflected by the 'auto' flag. If this
>  flag is set, the software must consider the bitmap as 'enabled' and start
>  tracking virtual disk changes to this bitmap from the first write to the
>  virtual disk. If this flag is not set then the bitmap is disabled.
> +
> +=== Compressed cluster layout ===
> +
> +The compressed cluster data may have a different layout depending on the
> +compression type used for the image, and store specific data for the particular
> +compression type.
> +
> +Compressed data layout for the available compression types:
> +(x = data_space_length - 1)
> +
> +    zlib:
> +        Byte  0 -  x:     the compressed data content
> +                          all the space provided used for compressed data
> +    zstd:
> +        Byte  0 -  3:     the length of compressed data
> +              4 -  x:     the compressed data content

Adding <http://zlib.net/> and <http://github.com/facebook/zstd> here as
well wouldn't hurt, would it?

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 835dd3c37f..2021e03a84 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -4215,11 +4215,12 @@
>  # Compression type used in qcow2 image file
>  #
>  # @zlib:  zlib compression, see <http://zlib.net/>
> +# @zstd:  zstd compression, see <http://github.com/facebook/zstd>
>  #
>  # Since: 4.1
>  ##
>  { 'enum': 'Qcow2CompressionType',
> -  'data': [ 'zlib' ] }
> +  'data': [ 'zlib', { 'name': 'zstd', 'if': 'defined(CONFIG_ZSTD)' } ] }
>  
>  ##
>  # @BlockdevCreateOptionsQcow2:

QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>