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http://msdn2.microsoft.com/en-us/library/aa363858.aspx
FILE_FLAG_NO_BUFFERING The system opens a file with no system caching. This flag does not affect hard disk caching. When combined with FILE_FLAG_OVERLAPPED, the flag gives maximum asynchronous performance, because the I/O does not rely on the synchronous operations of the memory manager. However, some I/O operations take more time, because data is not being held in the cache. Also, the file metadata may still be cached. To flush the metadata to disk, use the FlushFileBuffers function.