Re: [Qemu-devel] [PATCH v4 1/3] qcow2: introduce compression type feature

[Kevin Wolf]

Am 28.08.2019 um 14:56 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 2x faster than ZLIB providing a comparable compression ratio
> and therefore provides a performance advantage in backup scenarios.
> 
> The default compression is ZLIB. Images created with ZLIB compression type
> are backward compatible with older qemu versions.
> 
> Signed-off-by: Denis Plotnikov <address@hidden>

> @@ -359,6 +364,13 @@ typedef struct BDRVQcow2State {
>  
>      bool metadata_preallocation_checked;
>      bool metadata_preallocation;
> +    /*
> +     * Compression type used for the image. Default: 0 - ZLIB
> +     * The image compression type is set on image creation.
> +     * The only way to change the compression type is to convert the image
> +     * with the desired compression type set
> +     */
> +    uint32_t compression_type;

Should this use the enum Qcow2CompressionType instead of a plain int?

If you don't need to respin, I can make this change while applying.

Kevin