On 7/4/19 8:09 AM, Denis Plotnikov wrote:
> 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

s/by/at/

> 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
> 

Nice numbers.

> +++ 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

Missing a change to the header description at bytes 104-107 calling out
'1' as meaning zstd (it only calls out '0' or absent as meaning zlib).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org