Re: [PATCH v3 2/6] nbd: Add 'qemu-nbd -A' to expose allocation depth

[Markus Armbruster]

Eric Blake <eblake@redhat.com> writes:

> Allow the server to expose an additional metacontext to be requested
> by savvy clients.  qemu-nbd adds a new option -A to expose the
> qemu:allocation-depth metacontext through NBD_CMD_BLOCK_STATUS; this
> can also be set via QMP when using block-export-add.
>
> qemu as client can be hacked into viewing this new context by using
> the now-misnamed x-dirty-bitmap option when creating an NBD blockdev
> (even though our x- naming means we could rename it, I did not think
> it worth breaking back-compat of tools that have been using it while
> waiting for a better solution).  It is worth noting the decoding of
> how such context information will appear in 'qemu-img map
> --output=json':
>
> NBD_STATE_DEPTH_UNALLOC => "zero":false, "data":true
> NBD_STATE_DEPTH_LOCAL   => "zero":false, "data":false
> NBD_STATE_DEPTH_BACKING => "zero":true,  "data":true
>
> libnbd as client is probably a nicer way to get at the information
> without having to decipher such hacks in qemu as client. ;)
>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> Message-Id: <20200930121105.667049-6-eblake@redhat.com>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> [eblake: comment tweak suggested by Vladimir]
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
[...]
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 3758ea991269..249bd434f4eb 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3882,9 +3882,12 @@
>  #
>  # @tls-creds: TLS credentials ID
>  #
> -# @x-dirty-bitmap: A "qemu:dirty-bitmap:NAME" string to query in place of
> +# @x-dirty-bitmap: A metacontext name such as "qemu:dirty-bitmap:NAME" or

What is a "metacontext"?

Aside: nbd.h spells it "meta context".  The two spellings suggest
different meanings to me.  Using meta- as a prefix modifies the prefixed
word, as in metadata (data that provides information about other data).
Separated by space, meta becomes a noun.  None of its common meanings
seem to apply here.  Recommend to pick an appropriate one of
"metacontext", "meta-context", "meta context", and stick to it.

> +#                  "qemu:allocation-depth" to query in place of the
>  #                  traditional "base:allocation" block status (see
> -#                  NBD_OPT_LIST_META_CONTEXT in the NBD protocol) (since 3.0)
> +#                  NBD_OPT_LIST_META_CONTEXT in the NBD protocol; and
> +#                  yes, naming this option x-context would have made
> +#                  more sense) (since 3.0)
>  #
>  # @reconnect-delay: On an unexpected disconnect, the nbd client tries to
>  #                   connect again until succeeding or encountering a serious
> diff --git a/qapi/block-export.json b/qapi/block-export.json
> index 65804834d905..524cd3a94400 100644
> --- a/qapi/block-export.json
> +++ b/qapi/block-export.json
> @@ -78,11 +78,15 @@
>  #          NBD client can use NBD_OPT_SET_META_CONTEXT with
>  #          "qemu:dirty-bitmap:NAME" to inspect the bitmap. (since 4.0)
>  #
> +# @alloc: Also export the allocation map for @device, so the NBD client
> +#         can use NBD_OPT_SET_META_CONTEXT with "qemu:allocation-depth"
> +#         to inspect allocation details. (since 5.2)
> +#
>  # Since: 5.0
>  ##
>  { 'struct': 'BlockExportOptionsNbd',
>    'data': { '*name': 'str', '*description': 'str',
> -            '*bitmap': 'str' } }
> +            '*bitmap': 'str', '*alloc': 'bool' } }
>
>  ##
>  # @NbdServerAddOptions:
[...]