Advanced MPI-IO page is unclear

[harveydevereux]

I have made use of this page to get a good performance boost (2-3x) on my I/O heavy MPI workflow. I want to document why it has improved in the publication.

But I'm not actually sure which option is doing what, it says,

Performance is improved by either switching to a different File Collective algorithm in ompio 
or switching to using ROMIO for output. This is easily done by placing one of the following 
export lines in you submission script

and then lists two mutually exclusive lines

export OMPI_MCA_io=^ompio
export OMPI_MCA_fcoll=two_phase

Which one sets ROMIO and which one sets to "a different File Collective algorithm in ompio" (and what is that)? Based upon this ^ompio make use of ROMIO.

Can this be made explicit, say

 # force use of ROMIO
export OMPI_MCA_io=^ompio

# ...?
export OMPI_MCA_fcoll=two_phase

Thanks