A big advantage of file mapping is that it doesn't influence system cache. If your application does excessive I/O by means of ReadFile, your system cache will grow, consuming more and more physical memory. If your OS is 32 bit and you have much more than 1GB memory, than you're lucky, since on 32 bit Windows the size of system cache is limited by 1GB. Otherwise system cache will consume all available physical memory and the memory manager will soon start purging pages of other processes to disk, intensifying disk operations instead of actually lessen them. The effect is especially noticeable on 64 bit Windows, where the cache size is limited only by available physical memory. File mapping on the other hand doesn't lead to overgrowing of system cache and at the same time doesn't degrade the performance.
