Data Organization and I/O in a Parallel Ocean Circulation Model

We describe an efficient and scalable parallel I/O strategy for writing out gigabytes of data generated hourly in the ocean model simulations on massively parallel distributed-memory architectures. Working with Modular Ocean Model, using netCDF file system, and implemented on Cray T3E, the strategy speeds up I/O by a factor of 50 in the sequential case. In parallel case, on 32 processors up to 512 processors, our implementation writes out most model dynamic fields of 969 MB to a single netCDF file in 65 seconds, independent of the number of processors. The remap-and-write parallel strategy resolves the memory limitation problem and requires minimal collective I/O capability of the file system. Several critical optimizations on memory management and file access are carried out, ensuring scalability and speeding up numerical simulation due to the improved memory organizations.