Re: [Qemu-devel] [PATCH v5 1/3] qcow2: introduce compression type feature

[Eric Blake <eblake@redhat.com>]

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On 9/4/19 10:29 AM, Denis Plotnikov wrote:
> 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 <dplotnikov@virtuozzo.com>
> ---

> +++ b/docs/interop/qcow2.txt

> @@ -165,6 +170,18 @@ 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 clusters.
> +                    A single compression type is applied to all compressed image
> +                    clusters.
> +                    The compression type is set on image creation only.

This sentence adds nothing for me; you could delete it, and the spec
would still be just as useful (and less questionable about whether I can
'qemu-img amend' an image to use a different compression type in-place
by rewriting all existing compressed clusters).

With or without that sentence,

Reviewed-by: Eric Blake <eblake@redhat.com>

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


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