Re: [PATCH v10 2/2] docs: qcow2: introduce compression type feature

[Eric Blake <eblake@redhat.com>]

On 1/20/20 11:13 AM, Vladimir Sementsov-Ogievskiy wrote:
> The patch add new additional field to qcow2 header: compression_type,

s/add/adds a/
s/to/to the/

> which specifies compression type. If field is absent or zero, default
> compression type is set: ZLIB, which corresponds to current behavior.
> 
> New compression type (ZSTD) is to be added in further commit.

It would be nice to have that patch as part of the same series, but it 
has already been posted to the list separately, so I'm okay with this 
series as just doc word-smithing while we get that patch series in soon.

> 
> Suggested-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   docs/interop/qcow2.txt | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
> index 355925c35e..4569f0dba3 100644
> --- a/docs/interop/qcow2.txt
> +++ b/docs/interop/qcow2.txt
> @@ -109,6 +109,11 @@ the next fields through header_length.
>                                   An External Data File Name header extension may
>                                   be present if this bit is set.
>   
> +                    Bit 3:      Compression type bit.  If this bit is set,
> +                                a non-default compression is used for compressed
> +                                clusters. The compression_type field must be
> +                                present and not zero.

Why must the compression_type field be non-zero?  If this bit is set, an 
older qemu cannot use the image, regardless of the contents of the 
compression_type field, so for maximum back-compat, a sane app will 
never set this bit when compression_type is zero.  But there is nothing 
technically wrong with setting this bit even when compression_type is 0, 
and newer qemu would still manage to use the image correctly with zlib 
compression if it were not for this arbitrary restriction.

> +
>                       Bits 3-63:  Reserved (set to 0)
>   
>            80 -  87:  compatible_features
> @@ -189,7 +194,16 @@ present*, if not altered by a specific incompatible bit.
>   *. A field is considered not present when header_length is less or equal to the
>   field's offset. Also, all additional fields are not present for version 2.
>   
> -        < ... No additional fields in the header currently ... >
> +              104:  compression_type
> +                    Defines the compression method used for compressed clusters.
> +                    All compressed clusters in an image use the same compression
> +                    type.
> +                    If the incompatible bit "Compression type" is set: the field

Ragged edge formatting looks awkward.  Either this is one paragraph 
("type.  If") or there should be a blank line to make it obvious there 
are two paragraphs.

> +                    must be present and non-zero (which means non-zlib
> +                    compression type). Otherwise, this field must not be present
> +                    or must be zero (which means zlib).
> +                    Available compression type values:
> +                        0: zlib <https://www.zlib.net/>

I'm still not sure I agree with the mandate that the field must be 
non-zero when the incompatible feature bit is set.

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