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 <dplotni...@virtuozzo.com>
> @@ -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
