Hi Katie:
It sounds to me like you're talking about the 4G total size limit on a variable. Allowing that limit to be 2^64 seems reasonable.
Allowing individual dimension lengths to be greater than 2^31 is a bigger deal,
since array indexes are limited to 32 bit signed ints (at least in Java). Im
not sure if you are requesting that. It sounds like unstructured meshes might
push that limit someday, but do you have another use case for that?
Katie Antypas wrote:
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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