Re: [Qemu-block] [PATCH] [RFC] qcow2: add compression type feature

[Eric Blake]

On 2/5/19 3:08 AM, Denis Plotnikov wrote:
> The patch adds some preparation parts for incompatible compression type
> feature into 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 convertion, thus the only compression

s/convertion/conversion/

> algorithm is used for the image.
> 
> The plan is to add support for ZSTD and then may be something more effective
> in the future.
> 
> ZSDT compression algorithm consumes 3-5 times less CPU power with a

s/ZSDT/ZSTD/

> comparable comression ratio with zlib. It would be wise to use it for

s/comression/compression/

> data compression f.e. for backups.
> 
> The default compression is ZLIB.
> 
> Signed-off-by: Denis Plotnikov <address@hidden>
> ---
>  block/qcow2.c | 25 +++++++++++++++++++++++++
>  block/qcow2.h | 26 ++++++++++++++++++++++----
>  2 files changed, 47 insertions(+), 4 deletions(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 8c91b92865..cb3d6cc1c0 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -73,6 +73,7 @@ typedef struct {
>  #define  QCOW2_EXT_MAGIC_FEATURE_TABLE 0x6803f857
>  #define  QCOW2_EXT_MAGIC_CRYPTO_HEADER 0x0537be77
>  #define  QCOW2_EXT_MAGIC_BITMAPS 0x23852875
> +#define  QCOW2_EXT_MAGIC_COMPRESSION_TYPE 0x434D5052

This appears to be adding a new header extension magic number, but
didn't actually modify the specification. I'd expect this patch to touch
docs/interop/qcow2.txt before it can be considered complete.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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