I'm using the experimental external_volumes feature with a Docker Volume driver. We've modified our driver to allow a 'null' Options map to do on-the-fly provisioning but there seems to be something quirky how the external volumes code validates extra options. I've been reading the docs here: https://mesosphere.github.io/marathon/docs/external-volumes.html and from what I can tell, this should work:
Note: sizeInGiB and description are "nimble" driver specific options.
Yet, this is what I get back:
This is marathon 1.3.9 on CentOS using the official repos.