replicaset有两个secondary，Eventual mode，但在使用labix/mgo.v2连接mongo时，发现所有的read都只集中在一个secondary上了，仅有很少的量会访问另外一个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.