How can MPIO do anything extra if it knows the file maybe opened by
"something" else too? Say, MPIO knows another
independent application may be accessing the same file. What can
MPIO do about this extra information? It can't lock the file, it
can't sync the file for all steps. Basically, nothing can be guranteed.
For the other case, if MPIO knows the file is opened multiple times
within this MPI application, it *maybe* able to do some coordinations.
But it is not easy to recognize two opens are accessing the same file.
Having the same filenames does not necessarily mean the same file.
One could open one file, "datafile", then Change-directory,
open another file, "datafile", again. Definitely two files. On the
other side, Unix symbolic pointers will lead to the same physical with
different filenames. For Unix, the software would have to dig down to
major/minor device numbers and i-node numbers. I don't think we can
require all MPIO to support that.