The GPFS Filesystem
GPFS is a high-performance filesystem, allowing Calcul Québec users to carry out input/output (I/O) operations in parallel at a very high rate. GPFS is the filesystem used on Briarée, Guillimin and Hadès.
Generally speaking, you should reduce the opening and closing of files to a minimum. It's therefore preferable to combine all the data into a single file rather than writing a large number of small files. It's also much better to open the file a single time at the beginning of the program's execution and close it at the very end, instead of opening and closing it each time that you need to add some new data to it during program execution.
It is absolutely essential in order to obtain good I/O performance with GPFS to read and write your files in large chunks, if necessary using a matrix in your program that will serve as a memory cache. If you don't organize your I/O to function like this you risk disrupting all the other users of the filesystem if it crashes under the load of the huge number of small requests generated by poorly designed I/O in your program.