wal: do not call fsync when it is not necessary

wal/wal.go

@@ -502,6 +502,8 @@ func (w *WAL) Save(st raftpb.HardState, ents []raftpb.Entry) error {
 		return nil
 	}
 
+	mustSync := mustSync(st, w.state, len(ents))
+
 	// TODO(xiangli): no more reference operator
 	for i := range ents {
 		if err := w.saveEntry(&ents[i]); err != nil {
@@ -517,7 +519,10 @@ func (w *WAL) Save(st raftpb.HardState, ents []raftpb.Entry) error {
 		return err
 	}
 	if fstat.Size() < segmentSizeBytes {
-		return w.sync()
+		if mustSync {
+			return w.sync()
+		}
+		return nil
 	}
 	// TODO: add a test for this code path when refactoring the tests
 	return w.cut()
@@ -543,3 +548,15 @@ func (w *WAL) SaveSnapshot(e walpb.Snapshot) error {
 func (w *WAL) saveCrc(prevCrc uint32) error {
 	return w.encoder.encode(&walpb.Record{Type: crcType, Crc: prevCrc})
 }
+
+func mustSync(st, prevst raftpb.HardState, entsnum int) bool {
+	// Persistent state on all servers:
+	// (Updated on stable storage before responding to RPCs)
+	// currentTerm
+	// votedFor
+	// log entries[]
+	if entsnum != 0 || st.Vote != prevst.Vote || st.Term != prevst.Term {
+		return true
+	}
+	return false
+}