上一篇logstash-output-jdbc使用中提到“运行bin/logstash -f test.conf时可能提示注册插件失败”，通过分析详细的错误日志，发现其赫然写着“connection is not available,request time out after 10000ms”，原来是mysql连接的问题，插件注册失败只是表象。
由于没有在logstash所在的机器A安装mysql，而是通过mysql connector 的jar包（logstash conf文件里指定了jar报路径），所以没有办法测试A到数据库机器B的连接是否成功，尝试过增加mysql connection_timeout的时间到300s，发现错误还是request time out after 10000ms，说明jar里有自己的时间设置。
百度良久，最后在mysql的my.ini文件（默认C:PragramDataMySQLMySQLServer 5.7目录下）中增加skip-name-resolve，重启mysql服务后，问题解决。原因是
MySQL处理用户连接时进行如下过程来验证用户的合法性（参考http://www.cnblogs.com/timeship/archive/2013/02/28/2937730.html）：
When a new client connects to mysqld, mysqld spawns a new thread to handle the request. This thread first checks whether the host name is in the host name cache. If not, the thread attempts to resolve the host name: The thread takes the IP address and resolves it to a host name (using gethostbyaddr()). It then takes that host name and resolves it back to the IP address (using gethostbyname()) and compares to ensure it is the original IP address.