This is similar to MESOS-5081 and was left as a TODO in the first patch for the XFS isolator.
// TODO(jpeach) If there's no disk resource attached, we should set the
// minimum quota (1 block), since a zero quota would be unconstrained.
Document semantics of absent disk resources
Posix disk isolator allows unrestricted sandbox disk usage if the executor/task doesn't specify disk resource