Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (s *Session) Fsync(async bool) error {
	return s.Run(bson.D{{"fsync", 1}, {"async", async}}, nil)
}
func (s *Session) Fsync(async bool) error {
	return s.Run(bson.D{{"fsync", 1}, {"async", async}}, nil)
}
// Fsync flushes in-memory writes to disk on the server the session
// is established with. If async is true, the call returns immediately,
// otherwise it returns after the flush has been made.