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.
1、检查一下connect URL的设置；
2、如果读的Load Balance和你设置的参数的预期不一样，可以考虑配合设置Tag，进一步配置从什么member读。
供参考。
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