Re: [Qemu-devel] [PATCH 1/3] block: Additional info string in ImageInfo and BDI

[Eric Blake <eblake@redhat.com>]

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On 09/05/2013 06:05 AM, Max Reitz wrote:
> Add a string for additional information to ImageInfo and
> BlockDriverInfo. Also, use this string to emit the compatibility level
> and lazy_refcount value (on compat=1.1) for qcow2.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---

> +++ b/qapi-schema.json
> @@ -238,6 +238,9 @@
>  #
>  # @backing-image: #optional info of the backing image (since 1.6)
>  #
> +# @info-string: #optional string supplying additional format-specific
> +# information (since 1.7)
> +#
>  # Since: 1.3
>  #
>  ##
> @@ -248,7 +251,7 @@
>             '*cluster-size': 'int', '*encrypted': 'bool',
>             '*backing-filename': 'str', '*full-backing-filename': 'str',
>             '*backing-filename-format': 'str', '*snapshots': ['SnapshotInfo'],
> -           '*backing-image': 'ImageInfo' } }
> +           '*backing-image': 'ImageInfo', '*info-string': 'str' } }

This may work for HMP, but is LOUSY for use by QMP clients.  If you are
passing back more than a single piece of information, you are now
requiring the QMP client to do a parse of a free-form string to learn
those pieces of information.  I'd much rather see a full JSON schema
where EVERY piece of information passed back gets its own optional
field, or even where the additional information is a union type
discriminated by the image, so that we have full structure of the
information being returned rather than just an ad-hoc blobbed string.

Please rework this so that QMP clients like libvirt can easily probe
what compat mode a qcow2 image uses, without having to parse a free-form
string.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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