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Re: [Qemu-block] [Qemu-devel] [PATCH v1 1/3] qcow2: introduce compressio
Re: [Qemu-block] [Qemu-devel] [PATCH v1 1/3] qcow2: introduce compression type feature
Tue, 09 Jul 2019 13:27:55 +0200
Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux)
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
> 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:
> + If the compression type is default the bit
> + 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
> + A single compression type is applied to all compressed
> + 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
> + 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
> be stored. Each extension has a structure like the following:
Re: [Qemu-block] [Qemu-devel] [PATCH v1 1/3] qcow2: introduce compression type feature,
Markus Armbruster <=
[Qemu-block] [PATCH v1 2/3] qcow2: rework the cluster compression routine, Denis Plotnikov, 2019/07/03
[Qemu-block] [PATCH v1 3/3] qcow2: add zstd cluster compression, Denis Plotnikov, 2019/07/03