Re: [Qemu-devel] [PATCH v0 1/3] qcow2: introduce compression type feature

[Kevin Wolf]

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 <address@hidden>

> @@ -771,3 +779,30 @@ 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.
> +
> +
> +== Compression type extension ==
> +
> +The compression type extension is an optional header extension. It stores the
> +compression type used for disk clusters (de)compression.
> +A single compression type is applied to all compressed disk clusters,
> +with no way to change compression types per cluster. Two clusters of the 
> image
> +couldn't be compressed with different compression types.
> +
> +The compression type is set on image creation. The only way to change
> +the compression type is to convert the image explicitly.
> +
> +The compression type extension is present if and only if the incompatible
> +compression type bit is set. When the bit is not set the compression type
> +header must be absent.
> +
> +When the compression type bit is not set and the compression type header
> +extension is absent, ZLIB compression is used for compressed clusters.
> +This defines default image compression type: ZLIB.
> +Qemu < 4.1 can use images created with compression type ZLIB without any
> +additional preparations and cannot use images created with compression
> +types != ZLIB.
> +
> +Available compression types:
> +    0: ZLIB
> +    1: ZSTD

This section shouldn't be added at the end of the document, but after
the last section that describes a header extension (I think this is
'Full disk encryption header pointer').

Also, let me mention that I'm not reviewing the documentation wording in
detail because Markus has already given a lot of good feedback that I
don't want to duplicate.

Kevin