Re: [Qemu-devel] [PATCH v1 1/3] qcow2: introduce compression type feature

[Markus Armbruster]

I'd like to recommend a few commas.

Denis Plotnikov <address@hidden> writes:

> 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 <address@hidden>
[...]
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index af5711e533..5b8a8d15fe 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -109,7 +109,12 @@ in the description of a field.
>                                  An External Data File Name header extension 
> may
>                                  be present if this bit is set.
>  
> -                    Bits 3-63:  Reserved (set to 0)
> +                    Bit 3:      Compression type bit. The bit must be set if
> +                                the compression type differs from default: 
> zlib.
> +                                If the compression type is default the bit 
> must
> +                                be unset.
> +
> +                    Bits 4-63:  Reserved (set to 0)
>  
>           80 -  87:  compatible_features
>                      Bitmask of compatible features. An implementation can
> @@ -165,6 +170,21 @@ in the description of a field.
>                      Length of the header structure in bytes. For version 2
>                      images, the length is always assumed to be 72 bytes.
>  
> +        104 - 107:  compression_type
> +                    Defines the compression method used for compressed 
> clusters.
> +                    A single compression type is applied to all compressed 
> image
> +                    clusters.
> +                    The compression type is set on image creation only.
> +                    The default compression type is zlib.
> +                    When the deafult compression type is used the compression

Comma after used, please.

> +                    type bit (incompatible feature bit 3) must be unset.
> +                    When any other compression type is used the compression

Likewise.

> +                    type bit must be set.
> +                    Qemu versions older than 4.1 can use images created with
> +                    the default compression type without any additional
> +                    preparations and cannot use images created with any other

Comma after preparations, please.

> +                    compression type.
> +
>  Directly after the image header, optional sections called header extensions 
> can
>  be stored. Each extension has a structure like the following:
>  
[...]