Re: [PATCH v2] qapi: Allow getting flat output from 'query-named-block-nodes'

[Eric Blake]

On 1/20/20 2:50 AM, Peter Krempa wrote:
> When a management application manages node names there's no reason to
> recurse into backing images in the output of query-named-block-nodes.
>
> Add a parameter to the command which will return just the top level
> structs.
>
> Signed-off-by: Peter Krempa <address@hidden>
> ---
>
> Diff to v1:
>   - rewrote setting of 'return_flat' in qmp_query_named_block_nodes
>   - tried to clarify the QMP schema docs for the new field
>
> This patch does not aim to fix the rather suboptimal original
> documentation of the command as that is going to end up in a bunch of
> bikeshedding.
>
> While I know that there are plans for a new command that should fix
> this, the plans were already there for quite some time without much
> happening. This is a quick fix to a real problem, because if you have
> (maybe unpractically) deep backing chains, the returned JSON is getting
> huge. (140 nesting levels exceeds 10MiB of JSON)

Yep, O(n^2) output growth based on a depth of N is not ideal.

> +++ b/qapi/block-core.json
> @@ -1752,6 +1752,9 @@
>   #
>   # Get the named block driver list
>   #
> +# @flat: Omit the nested data about backing image ("backing-image" key) if 
> true.
> +#        Default is false (Since 5.0)
> +#
>   # Returns: the list of BlockDeviceInfo
>   #
>   # Since: 2.0
> @@ -1805,7 +1808,9 @@
>   #                    } } ] }
>   #
>   ##
> -{ 'command': 'query-named-block-nodes', 'returns': [ 'BlockDeviceInfo' ] }
> +{ 'command': 'query-named-block-nodes',
> +  'returns': [ 'BlockDeviceInfo' ],
> +  'data': { '*flat': 'bool' } }

Reviewed-by: Eric Blake <address@hidden>

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