An approach to allocating storage that has track managed storage and cylinder managed storage. The approach involves receiving a request to allocate a data set for new data and determining if the size of the new data exceeds a breakpoint value. The size of the new data may be determined by allocating a buffer data set that is the size of the breakpoint value, and writing the new data to the buffer data set. If the new data only partially fills the buffer data set, then the size of the new data is smaller than the breakpoint value. If the new data overfills the buffer data set, then the size of the new data set is larger than the breakpoint value. New data that is larger than the breakpoint value is automatically stored in cylinder managed storage, while new data that is smaller than the breakpoint value is automatically stored in track managed storage.