Mirroring of network servers has been considered to be effective for load balancing. However, the cost of setting up new mirror servers is enormously high. In this paper, we propose a dynamic file allocation model with a simple mirroring function for handling significant changes of network traffic in the Internet. According to the load fluctuation, we can dynamically reallocate files using this model. We show that our model accomplishes satisfactory performance and reduces cost by adding a simple mirroring function to all existent servers instead of setting up mirror servers afresh.