code: public void purchaseReservedVolume(String volumeId, int reservationLength, String reservationTimeUnit) {
          this.purchaseReservedVolume(new PurchaseReservedVolumeRequest()
                  .withVolumeId(volumeId)
                  .withBilling(new Billing().withReservation(new Reservation()
                          .withReservationLength(reservationLength)
                          .withReservationTimeUnit(reservationTimeUnit))));
      }
docs: PurchaseReserved the instance with fixed duration.
  
  You can not purchaseReserved the instance which is resizing.
  This is an asynchronous interface,
  you can get the latest status by invoke {@link #getVolume(String)}
  
  @param volumeId The id of volume which will be renew.
  @param reservationLength The fixed duration to renew,available is [1,2,3,4,5,6,7,8,9,12,24,36]
  @param reservationTimeUnit The timeUnit to renew the instance,only support "month" now.