PATH disk is a Mesos concept to partition a disk into parts for different tasks to use. But this concept does not fit well with CSI.
For example, it is impossible to create two PATH disks on a single external EBS volume to be used on two agents at the same time. In other words, all PATH disks carved out from the same EBS volume are somehow correlated, but this correlation is hard to be expressed in Mesos.
Another example is that we cannot rely on the CSI plugins to enforce the quota of a PATH volume on a CSI volume, since this concept is transparent to CSI. As a result, Mesos might need to issue some filesystem-specific operations directly to the volume's filesystem for quota support. This seems doable for now, but we don't know if by-passing the CSI plugin to interact with the volume filesystems would cause any problem in the future.
So we might want to consider dropping PATH support for CSI volumes for now. If we find it beneficial to support PATH and won't cause any problem in the future, we can always add this feature back.