code: public InputStream openStream(ProgressMonitor p) throws IOException {
      if(onNFS() ) return PersistNFS .openStream(_key  );
      if(onHDFS()) return PersistHdfs.openStream(_key,p);
      if(onS3()  ) return PersistS3  .openStream(_key,p);
      if(onTachyon()) return PersistTachyon.openStream(_key,p);
      if( isFrame() ) throw new IllegalArgumentException("Tried to pass a Frame to openStream (maybe tried to parse a (already-parsed) Frame?)");
      assert _type==TypeMap.PRIM_B : "Expected byte[] type but got "+TypeMap.className(_type);
      return new ByteArrayInputStream(memOrLoad());
    }
docs: Creates a Stream for reading bytes