It would be nice if the data persistence layer was pluggable instead of being a hard-code boolean switch between the two implementations:

CollectableService.java

private BasePersister createPersister(ServiceParameters params, RrdRepository repository) {
if (Boolean.getBoolean("org.opennms.rrd.storeByGroup")) {
return new GroupPersister(params, repository);
} else {
return new OneToOnePersister(params, repository);
}

Obviously this is a global and not a per-resource configuration.