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From: Robert Latham (robl_at_[hidden])
Date: 2007-01-09 15:56:41

On Mon, Jan 08, 2007 at 02:32:14PM -0700, Tom Lund wrote:
> Rainer,
> Thank you for taking time to reply to my querry. Do I understand
> correctly that external32 data representation for i/o is not
> implemented? I am puzzled since the MPI-2 standard clearly indicates
> the existence of external32 and has lots of words regarding how nice
> this feature is for file interoperability. So do both Open MPI and
> MPIch2 not adhere to the standard in this regard? If this is really the
> case, how difficult is it to define a custom data representation that is
> 32-bit big endian on all platforms? Do you know of any documentation
> that explains how to do this?
> Thanks again.

Hi Tom

You do understand correctly. I do not know of an MPI-IO
implementation that supports external32.

When you say "custom data representation" do you mean an MPI-IO
user-defined data representation?

An alternate approach would be to use a higher level library like
parallel-netcdf or HDF5 (configured for parallel i/o). Those
libraries already define a file format and implement all the necessary
data conversion routines, and they have a wealth of ancilary tools and
programs to work with their respective file formats. Additionally,
those higher-level libraries will offer you more features than MPI-IO
such as the ability to define atributes on variables and datafiles.
Even better, there is the potential that these libraries might offer
some clever optimizations for your workload, saving you the effort.
Further, you can use those higher-level libraries on top of any MPI-IO
implementation, not just OpenMPI or MPICH2.

This is a little bit of a diversion from your original question, but
to sum it up, I'd say one potential answer to the lack of external32
is to use a higher level library and sidestep the issue of MPI-IO data
representations altogether.


Rob Latham
Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
Argonne National Lab, IL USA                 B29D F333 664A 4280 315B