[netcdfgroup] Netcdf-4 and parallel I/O

• To: netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: [netcdfgroup] Netcdf-4 and parallel I/O
• From: "Hernan G. Arango" <arango@xxxxxxxxxxxxxxxxxx>
• Date: Mon, 26 Jan 2009 15:09:27 -0500

Hi All,

I implemented the parallel I/O in ROMS (http://www.myroms.org) using
the latest snapshot of the NetCDF-4/HDF5 libraries.

I have two questions:

(1) The documentation for "nf90_open_par" says that it is only
    possible to open a NetCDF4 format file for parallel processing.
    This makes sense if the file is open for writing.  However,
    I think that it should be able to open a classic format file
    (NetCDF 3.x) for just reading since the library is backward
    compatible.  I tested this in ROMS and in fact it is reading
    a classic format in parallel as far as I can see in the
    debugger. I am not going inside the libraries.  So the
    question is this actually happening?  Is the library reading
    a classic format in parallel?  If so, perhaps the documentation
    needs to be revised.

http://www.unidata.ucar.edu/software/netcdf/docs_snapshot/netcdf-f90.html#NF90_005fOPEN_005fPAR

(2) Why doesn't the library create identical files with the
    same executable? Are there any time makings or other execution
    information hidden in the created NetCDF-4 file.  I tried to run
    the same execute twice and a binary difference tells me
    that the files are different:

    > diff new.nc old.nc
    Binary files new.nc and old.nc differ

    However, if I dump the data into ASCII files, using "ncdump", the
    files are identical.

    I need to understand this issue because this is an important
    parallel partition test.  Regardless of the parallel partition
    or number of MPI nodes, we should get identical solutions.
    Otherwise, we have a parallel bug.

    If there is an internal marking, how should we deactivate it?

Thank you, H

-----------------------------------------------------------------------

Hernan G. Arango           Institute of Marine and Coastal Sciences
arango@xxxxxxxxxxxxxxxxxx  Rutgers University
off:  (732) 932-6555 x266  71 Dudley Road
FAX:  (732) 932-6520       New Brunswick, NJ 08901-8521, USA

http://marine.rutgers.edu/po/arango
http://marine.rutgers.edu/po/arango/rocco
http://marine.rutgers.edu/roms
http://www.myroms.org
http://www.ocean-modeling.org