MPAS I/O performance degradation during Parallel NetCDF header reallocation

[amstokely]

If MPAS encounters an I/O scenario that requires Parallel NetCDF to reallocate the file header (for example, when ncmpi_enddef is called after the header has grown beyond its preallocated padding while in define mode), a significant I/O performance degradation may be observed, which can appear as an I/O hang. This behavior was the root cause of issue 384 in MPAS-Workflow. In that case, the perceived hang occurred when overwriting the config_block_decomp_file_prefix global attribute in the analysis NetCDF file with a longer string. Using Forge DDT, the slowdown was traced to the call to ncmpi_enddef following this attribute update.