NB-NB大牛-请进-下载大小疑点

NB----NB大牛------请进-----下载大小问题
这几天一直折腾下载的一个东西，一直有下载不了大文件的念头闪烁，今天终于深测了下。发现文件大概到了600多M的时候，客户端就没反应了，一直响应着。服务器端日志显示下载完成
下面贴上源码：  
  System.IO.Stream iStream = null;

  // Buffer to read 10K bytes in chunk:
  byte[] buffer = new Byte[10000];

  // Length of the file:
  int length;

  // Total bytes to read:
  long dataToRead;

  // Identify the file to download including its path.
  string filepath = fileName + ".zip";

  // Identify the file name.
  string filename = System.IO.Path.GetFileName(filepath);
  Logger.Log.Info("文件名："+filename);
  try
  {
  // Open the file.
  iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open,
  System.IO.FileAccess.Read, System.IO.FileShare.Read);

  // Total bytes to read:
  dataToRead = iStream.Length;
  Logger.Log.Info("大小：" + dataToRead);
  Response.ContentType = "application/octet-stream";
  Response.AddHeader("Content-Disposition", "attachment; filename="+Server.UrlEncode(filename));

  // Read the bytes.
  while (dataToRead > 0)
  {
  // Verify that the client is connected.
  if (Response.IsClientConnected)
  {
  // Read the data in buffer.
  length = iStream.Read(buffer, 0, 10000);
  Logger.Log.Info("大小length：" + length);
  // Write the data to the current output stream.
  Response.OutputStream.Write(buffer, 0, length);

  // Flush the data to the HTML output.
  Response.Flush();

  buffer = new Byte[10000];
  dataToRead = dataToRead - length;
  }
  else
  {
  //prevent infinite loop if user disconnects
  dataToRead = -1;
  }
  }
  Logger.Log.Info("下载成功！");
  }
  catch (Exception ex)
  {
  // Trap the error, if any.
  Logger.Log.Info("异常：" + ex.ToString());
  //Response.Write("Error : " + ex.Message);
  }
  finally
  {
  if (iStream != null)
  {
  //Close the file.
  iStream.Close();
  }
  }

日志：
INFO ,12-03-21 13:31:20,Page_Load,页面加载中........
INFO ,12-03-21 13:31:20,Page_Load,执行文件打包操作
INFO ,12-03-21 13:31:21,ForeignFileToZIP,执行SP_RPT_GETFILEDOWNLOAD获取文件的信息900
INFO ,12-03-21 13:31:21,ForeignFileToZIP,执行SP_RPT_GetFilesToExcel获取说明文档信息900
INFO ,12-03-21 13:31:21,ForeignFileToZIP,创建文件夹D:\download\外来文件_31868e20-3a43-419d-b6f4-39425736f2d4\外来文件