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

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Oct 09, 2020 at 10:10:18AM +0200, Markus Armbruster wrote:
> 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.

In the NBD protocol it's called a "metadata context".  This is
abbreviated to "...META_CONTEXT" in the names of constants,
eg. NBD_OPT_SET_META_CONTEXT.  For the reasons you gave I don't think
it's a very good name, but it's what we have.

https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md#metadata-querying

What it actually is, is a string, like "base:allocation" or
"qemu:dirty-bitmap:<NAME>" which selects which metadata you want to
query in NBD_CMD_BLOCK_STATUS commands.

Rich.

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