调大oplog⼤大⼩小后，引起从库宕机，报错日志

[replication-2] could not find member to sync from
[rsBackgroundSync] too stale to catch up -- entering maintenancemode
[rsBackgroundSync] Our newest OpTime : { ts: Timestamp 1503977172000|27, t: 1 }
[rsBackgroundSync] Earliest OpTime available is { ts: Timestamp1503998451000|1, t: -1 }