"To mitigate the impact of mongodump on the performance of the replica set, use mongodump to capture backups from a secondary member of a replica set. Alternatively, you can shut down a secondary and use mongodump with the data files directly. If you shut down a secondary to capture data with mongodump ensure that the operation can complete before its oplog becomes too stale to continue replicating."

How I remember in 3.0 mongodump stops doing backups directly from files. Only via mongod.