4GigB variable size limit

[Katie Antypas <kantypas@xxxxxxx>]

Hi Everyone,

I'm jumping into the discussion late here, but coming from a perspective 
of trying to find and develop an IO strategy which will work at the 
petascale level, the 4 GigB variable size limitation is a major 
barrier.  Already a 1000^3 grid variable can not fit into a single 
netcdf variable.  Users at NERSC and other supercomputing centers 
regularly run problems of this size or greater and IO demands are only 
going to get bigger.  We don't believe chopping up data structures into 
pieces is a good long term solution or strategy.  There isn't a natural 
way to break up the data and chunking eliminates the elegance, ease and 
purpose of a parallel IO library.  Besides the direct code changes, 
analytics and visualization tools become more complicated as datafiles 
from the same simulation but of different sizes would not have the same 
number variables.  Restarting a simulation from a checkpoint file on a 
different number of processors would also become more convoluted.

The view from NERSC is that if Parallel-NetCDF is to be viable option 
for users running large parallel simulations, this is a limitation that 
must be lifted...

Katie Antypas
NERSC User Services Group
Lawrence Berkeley National Lab

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