replicaset有兩個secondary，Eventual mode，但在使用labix/mgo.v2連接mongo時，發現所有的read都只集中在一個secondary上了，僅有很少的量會訪問另外一個secondary 。

// Read preference modes are specific to mgo: Eventual Mode = 0 // Same as Nearest, but may change servers between reads. Monotonic Mode = 1 // Same as SecondaryPreferred before first write. Same as Primary after first write. Strong Mode = 2 // Same as Primary.