Yes, Rex-Ray is EMC specific. Take a look at Robin Systems (https://robinsystems.com/) for stateful containers. They have examples of running hadoop, cassandra, mongodb, etc all on commodity hardware.
Its not like good container storage solutions don't exist for databases and other stateful applications. The problem is in expecting it to be free and open source. Building orchestration or simple file or object storage is easy, but building high performance, resilient, scale out storage that can run on cheap commodity boxes is a difficult task. Once you get over the "free" requirement, there are some good options like ScaleIO and Robin Systems. https://robinsystems.com/containerization-platform-enterpris...