we deploy one replicaset with three machines running mongod(one primary, two secondary). These machines have 10Gbps ethernet bandwidth for mongodb ,and SSD card for mongodb datafile. data items for db to handle are totally 41000000 per week. When replication runs a long time, the two secondary mongod reports "changing sync target because current sync target's most recent OpTime is Jan 4 19:47:18:230 which is more than 30 seconds behind member 172.28.11.142:27017 whose most recent OpTime is 1451908070"