code: public static int getNumWriteBehindBuffers(int numBuffers) {
  		int numIOBufs = (int) (Math.log(numBuffers) / Math.log(4) - 1.5);
  		return numIOBufs > 6 ? 6 : numIOBufs;
  	}
docs: Determines the number of buffers to be used for asynchronous write behind. It is currently
  computed as the logarithm of the number of buffers to the base 4, rounded up, minus 2.
  The upper limit for the number of write behind buffers is however set to six.
  
  @param numBuffers The number of available buffers.
  @return The number