OVMS: Oracle 10g OpenVMS - Fragmented RMAN backup files

Does anyone know if there is a way to preallocate rman backup saveset files?  We are trying to decrease the elapsed time our backups are running and have noticed that backup files that rman creates are badly fragmented.   The more fragmented the files, the slower it runs.  Depending on the number of channels we've tried, it can almost totally consume the 4 CPUs we have, just keeping track of the file header information.

I've restricted the MAXPIECESIZE to 15G and that has helped it run faster, but I can't find any other parameters to set that would allow us to preallocate the files?  Maybe I'm missing something.