[netcdfgroup] netCDF4 file bit-for-bit reproducibility?

[Charlie Zender]

Hi and happy new year to the netCDF group,

Will netCDF4 make bit-for-bit (BFB) reproducible files?
In other words will running a determinstic model twice on the same
machine (without changing compiler, netCDF library, etc.) produce
identical output files?
I thought that netCDF4 would, like netCDF3, use deterministic
algorithms without any date-stamps within the file, and thus produce
BFB files. Apparently I am wrong.

To determine whether files were BFB I checked their SHA1 sums.
netCDF3 produces BFB files and netCDF4 (snapshot) does not (below).
BFB output files make models easier to debug, so it would be helpful
if netCDF4 continued the netCDF3 BFB "tradition". Is this possible?
Am I missing something? Is HDF5 the culprit?

Thanks,
Charlie

netCDF3 is BFB:
zender at virga:~$ ncks -O -h ~/nco/data/in.nc ~/foo_31.nc
zender at virga:~$ ncks -O -h ~/nco/data/in.nc ~/foo_32.nc
zender at virga:~$ sha1sum ~/foo_31.nc
769f05e2bf0f079f3a47883199ff11ed55578208  /home/zender/foo_31.nc
zender at virga:~$ sha1sum ~/foo_32.nc
769f05e2bf0f079f3a47883199ff11ed55578208  /home/zender/foo_32.nc

netCDF4 is not BFB:
zender at virga:~$ ncks -O -h -4 ~/nco/data/in.nc ~/foo_41.nc
zender at virga:~$ ncks -O -h -4 ~/nco/data/in.nc ~/foo_42.nc
zender at virga:~$ sha1sum ~/foo_41.nc
6eb94cf1530b2f90c7b1375cdc79df52b8afb411  /home/zender/foo_41.nc
zender at virga:~$ sha1sum ~/foo_42.nc
a1e9492f30eddadcb50258fca8f226bbf5c65bcb  /home/zender/foo_42.nc

/usr/local/netcdf4/bin/ncgen -k hdf5 -b -o ~/foo_41.nc ~/nco/data/in.cdl
/usr/local/netcdf4/bin/ncgen -k hdf5 -b -o ~/foo_42.nc ~/nco/data/in.cdl
zender at virga:~/nco/doc$ sha1sum ~/foo_41.nc
bb8d6c754940a018802815a07796ea0c5c080f2b  /home/zender/foo_41.nc
zender at virga:~/nco/doc$ sha1sum ~/foo_42.nc
6bb284edac01feef7eaa30ba36c1bdd7146d025c  /home/zender/foo_42.nc

-- 
Charlie Zender, Department of Earth System Science, UC Irvine
Sab. at CNRS/LGGE-Grenoble until 20080715 :) 011+33+476+824236
Laboratoire de Glaciologie et Géophysique de l'Environnement
54 rue Molière BP 96, 38402 Saint Martin d'Hères Cedex, France