At least for the topic of why romio fails with HDF5, I believe this is the fix we need (has to do with how romio processes the MPI datatypes in its flatten routine). I made a different fix a long time ago in SMPI for that, then somewhat more recently it was re-broke it and I had to re-fix it. So the below takes a little more aggressive approach, not totally redesigning the flatten function, but taking over how the array size counter is handled.
https://github.com/open-mpi/ompi/pull/3975
Mark Allen
https://github.com/open-mpi/ompi/pull/3975
Mark Allen
