1.默认情况下，集群中primary支持读写，secondary是不支持的
需要rs.slaveOk();
测试语句：
cfg={ _id:"testrs", members:[ {_id:0,host:'xxx.xxx.xxx.xxx:19130',priority:2}, {_id:1,host:''xxx.xxx.xxx.xxx:19130',priority:1},
{_id:2,host:''xxx.xxx.xxx.xxx:19130',arbiterOnly:true}] };
rs.initiate(cfg)
db.test.insert({id:1,name:'francs'} );
mongo --port xxx --host xxx