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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