为什么Mongo 2.6.5会不断记录SyncSourceFeedback

Question

我正在使用Mongo 2.6.5运行3节点副本集，其中db1是主要的，而db2和db3是次要的。

db2，其中一个辅助人员，连续（每秒一次或两次）记录此行：

2015-12-15T21:06:09.852+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017
2015-12-15T21:06:10.363+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017
2015-12-15T21:06:10.877+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017
2015-12-15T21:06:11.404+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017
2015-12-15T21:06:11.917+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017
2015-12-15T21:06:12.445+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017
2015-12-15T21:06:12.956+0000 [SyncSourceFeedbackThread] replset setting syncSourceFeedback to db1.example.com:27017

rs.status()会返回此信息：

{
    "set" : "rs0",
    "date" : ISODate("2015-12-15T21:01:16Z"),
    "myState" : 1,
    "members" : [
        {
            "_id" : 1,
            "name" : "db1.example.com:27017",
            "health" : 1,
            "state" : 1,
            "stateStr" : "PRIMARY",
            "uptime" : 33612785,
            "optime" : Timestamp(1450213276, 2),
            "optimeDate" : ISODate("2015-12-15T21:01:16Z"),
            "electionTime" : Timestamp(1443584172, 1),
            "electionDate" : ISODate("2015-09-30T03:36:12Z"),
            "self" : true
        },
        {
            "_id" : 2,
            "name" : "db2.example.com:27017",
            "health" : 1,
            "state" : 2,
            "stateStr" : "SECONDARY",
            "uptime" : 6513888,
            "optime" : Timestamp(1450213275, 2),
            "optimeDate" : ISODate("2015-12-15T21:01:15Z"),
            "lastHeartbeat" : ISODate("2015-12-15T21:01:15Z"),
            "lastHeartbeatRecv" : ISODate("2015-12-15T21:01:16Z"),
            "pingMs" : 1,
            "syncingTo" : "db1.example.com:27017"
        },
        {
            "_id" : 4,
            "name" : "db3.example.com:27017",
            "health" : 1,
            "state" : 2,
            "stateStr" : "SECONDARY",
            "uptime" : 6629116,
            "optime" : Timestamp(1450213273, 4),
            "optimeDate" : ISODate("2015-12-15T21:01:13Z"),
            "lastHeartbeat" : ISODate("2015-12-15T21:01:14Z"),
            "lastHeartbeatRecv" : ISODate("2015-12-15T21:01:16Z"),
            "pingMs" : 0,
            "syncingTo" : "db1.example.com:27017"
        }
    ],
    "ok" : 1
}

问题是什么？