updated code much more reasonable now

gistfile1.txt

    class Program
    {
        const int KILO = 1024;
        const long MEGA = 1024 * KILO;
        const long GIGA = 1024 * MEGA;

        static void Main(string[] args)
        {
            var size = 7 * KILO;
            var count = 1000;            
            if(args.Length == 2)
            {
                count = int.Parse(args[0]);
                size = int.Parse(args[1]);
            }
            int flushat = 10;
            var filename = "shitbird";
            var watch = new Stopwatch();
            watch.Start();
            //UseUnBuffered(count, filename, size, flushat);
            SimpleUnbuffered(count, filename, (uint)size, flushat);
            PrintWatch("unbuffered", watch, count, size);
            File.Delete(filename);
            watch.Restart();
            SimpleFlush(count, filename, size, flushat);
            //UseFlush(count, filename, size, flushat);
            PrintWatch("flushing", watch, count, size);
            File.Delete(filename);
        }

        private static void SimpleUnbuffered(int count, string filename, uint size, int flushat)
        {
            var holding = new byte[size*flushat];
            var buffer = new byte[size];
            InitBuffer(buffer);
            int flags = WinApi.FILE_FLAG_NO_BUFFERING | WinApi.FILE_FLAG_SEQUENTIAL_SCAN;     // default to simmple no buffering
            using (var stream = new FileStream(filename, FileMode.OpenOrCreate))
            {
                stream.SetLength(size * count);
            }
            var handle = WinApi.CreateFile(filename, (int)FileAccess.ReadWrite, FileShare.None, IntPtr.Zero, FileMode.OpenOrCreate,
                                      flags, IntPtr.Zero);
            
            if (handle.IsInvalid)
            {
                throw new Win32Exception();
            }
            for (int i = 0; i < count; i++)
            {
                if (i % flushat == 0 && i != 0)
                {
                    int foo = 0;
                    int towrite = buffer.Length;
                    while (towrite > 0)
                    {
                        var ret = WinApi.WriteFile(handle, buffer, (uint) towrite, ref foo, IntPtr.Zero);
                        if (!ret)
                            throw new Win32Exception();
                        towrite -= foo;
                    }
                } else
                {
                    Buffer.BlockCopy(buffer,0, holding,(int) ((i%flushat) * size), buffer.Length);
                }
            }
            WinApi.CloseHandle(handle.DangerousGetHandle());
            handle.Close();
        }

        private static void SimpleFlush(int count, string filename, int size, int flushat)
        {
            var buffer = new byte[size];
            InitBuffer(buffer);

            using (var stream = GetFileBufferedStream(filename, size))
            {
                stream.SetLength(size * count);
                for (int i = 0; i < count; i++)
                {
                    stream.Write(buffer, 0, buffer.Length);
                    if (i % flushat == 0)
                    {
                        stream.Flush(false);
                        FlushFileBuffers(stream.SafeFileHandle);
                    }
                }
            }
        }
        [DllImport("kernel32.dll", SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool FlushFileBuffers(SafeFileHandle hFile);

        private static void InitBuffer(byte[] buffer)
        {
            for (int i = 0; i < buffer.Length; i++)
            {
                buffer[i] = (byte)(i % 255);
            }
        }
        private static void PrintWatch(string which, Stopwatch watch, int count, int size)
        {
            Console.WriteLine(which + " took: " + watch.Elapsed.TotalMilliseconds + " to write " + count * size);
        }

        private static FileStream GetFileBufferedStream(string filename, int bufferSize)
        {
            return new FileStream(filename, FileMode.Create, FileSystemRights.WriteData, FileShare.ReadWrite, bufferSize, FileOptions.SequentialScan);
        }