镜像操作请参考：http://blog.csdn.net/dba_huangzj/article/details/35995083
应用程序数据库连接字符串(带见证服务器即自动故障转移)：
DBHelper DBHelparKQ = new DBHelper("Data Source=主体;Failover Partner=镜像;Initial Catalog=数据库;User ID=帐号;Password=密码");
_conn.ConnectionString ="Data Source=192.168.1.2;Failover Partner=192.168.1.3;Initial Catalog=DB_Test;User ID=sa;Password=Aa123456";
主体与镜像手动切换指令
use master; alter database DB_Test set partner failover;