Lisandro Dalcin <[email protected]> writes:

...

I'm not sure of this suggested change, in that a "bad for MPI-IO" workload (like each rank randomly seeking around a big file) might not be better with rank 0 acting as a service rank.

Please note my main question is unrelated to MPI-IO. It is about the original POSIX-based implementation of binary viewers. For mode READ, all processes open the file (with the open() system call), but in the current implementation, only process zero ever reads the file (unless the user gets the file descriptor and start issuing low-level PetscBinaryRead() calls). So I do not see the point of opening the file on all processes (and then stress metadata servers on parallel filesystem), if we are not going to ever read from rank != 0. Let's just fix things to open the file at rank==0 only.

Okay, this much I agree on. If a rank doesn't actually read, it shouldn't open the file to read.