How do I specify that I do not need my large datasets to be backed up?
If you create a folder named "nobackup", inside any folder, then all data stored inside this folder will not be backed-up. Simply move (mv or rsync) your data into a folder with the proper name.
Also note that all projects have a separate nobackup folder under the /proj/xyz/ hierarchy (and also under the /proj/xyz/private/ hierarchy) with a separate quota limit than the ordinary backed up project folder. You can read more about this in our disk storage guide.