Re: [PATCH 1/2] block/rbd: support selected key-value-pairs via QAPI

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jun 16, 2025 at 11:25:54AM +0200, Ilya Dryomov wrote:
> On Thu, May 15, 2025 at 1:29 PM Fiona Ebner <f.ebner@proxmox.com> wrote:
> >
> > Currently, most Ceph configuration options are not exposed via QAPI.
> > While it is possible to specify a dedicated Ceph configuration file,
> > specialized options are often only required for a selection of images
> > on the RBD storage, not all of them. To avoid the need to generate a
> > dedicated Ceph configuration file for each image (or for each required
> > combination of options), support a selection of key-value pairs via
> > QAPI.
> >
> > Initially, this is just 'rbd_cache_policy'. For example, this is
> > useful with small images used as a pflash for EFI variables. Setting
> > the 'rbd_cache_policy' to 'writeback' yields a substantial improvement
> > there [0].
> >
> > The function qemu_rbd_extract_key_value_pairs() was copied/adapted
> > from the existing qemu_rbd_extract_encryption_create_options().
> >
> > [0]: https://bugzilla.proxmox.com/show_bug.cgi?id=3329#c9
> >
> > Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

snip

> >  ##
> >  # @BlockdevOptionsRbd:
> >  #
> > @@ -4327,6 +4360,9 @@
> >  #     authentication.  This maps to Ceph configuration option "key".
> >  #     (Since 3.0)
> >  #
> > +# @key-value-pairs: Key-value pairs for additional Ceph configuraton.
> > +#     (Since 10.1)
> > +#
> >  # @server: Monitor host address and port.  This maps to the "mon_host"
> >  #     Ceph option.
> >  #
> > @@ -4342,6 +4378,7 @@
> >              '*user': 'str',
> >              '*auth-client-required': ['RbdAuthMode'],
> >              '*key-secret': 'str',
> > +            '*key-value-pairs' : 'RbdKeyValuePairs',
> 
> To side-step all of the above, have you considered implementing
> a straightforward passthrough to Ceph instead?  Something like
> 
>   '*key-value-pairs': ['RbdKeyValuePair']
> 
> where RbdKeyValuePair is just a pair arbitrary strings (and
> key-value-pairs is thus an optional list of those).  rados_conf_set()
> would be called just the same but the user would be able to override
> any Ceph option they wish, not just a few that we thought of here.

Passing through arbitrary key/value pairs as strings is essentially
abdicating our design responsibility in QAPI. enums would no longer
be introspectable. Integers / booleans would require abnormal formatting
by clients. API stability / deprecation promises can no longer be made.
and more besides.

Given that limitation, if we did go the string pairs route, I would
expect it to be marked as "unstable" in the QAPI schema, so apps have
a suitable warning NOT to rely on this.

With regards,
Daniel
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