Many systems (e.g. Google Cloud Compute instances) come pre-installed with OpenMPI v1.X. Since v2.X has been out for a while, is it always preferable to upgrade and use that instead? What are the main differences? Is there a difference in general performance and stability? I have been looking for this information everywhere but besides the detailed and incremental change lists I can't find a general overview.
Mostly adding MPI-3 features, deprecating support for older systems, and better support for MPI-IO (moved from ROMIO to OMPIO).
More info is available in these slides presented at the Supercomputing 2014 conference.