The docs basically explain a way to create a persistent volume in the sandbox, and use a host-mount volume to bind that persistent vol elsewhere in the container (so the volumes section lists two volumes). This is a workaround for persistent vols requiring a relative containerPath. While the example, as is, works great because it uses the Docker containerizer – it falls flat as soon as you change the container type from DOCKER to MESOS.
Root cause is a Mesos impl detail: https://issues.apache.org/jira/browse/MESOS-7770
Fabian Baier experienced this problem on both DCOS 1.9.x (our production cluster) and testing (pre-1.10).
Short term, I think we should probably update our docs w/ a "known issue" warning.
Current persistent vol docs: https://github.com/mesosphere/marathon/blob/master/docs/docs/persistent-volumes.md