diff --git a/wal.go b/wal.go
index 1dadc8f2c..68c48838c 100644
--- a/wal.go
+++ b/wal.go
@@ -52,13 +52,16 @@ const (
 	WALEntryDeletes WALEntryType = 4
 )
 
-// SamplesCB is the callback after reading samples.
+// SamplesCB is the callback after reading samples. The passed slice
+// is only valid until the call returns.
 type SamplesCB func([]RefSample) error
 
-// SeriesCB is the callback after reading series.
+// SeriesCB is the callback after reading series. The passed slice
+// is only valid until the call returns.
 type SeriesCB func([]RefSeries) error
 
-// DeletesCB is the callback after reading deletes.
+// DeletesCB is the callback after reading deletes. The passed slice
+// is only valid until the call returns.
 type DeletesCB func([]Stone) error
 
 // WAL is a write ahead log that can log new series labels and samples.
@@ -594,6 +597,9 @@ func (w *SegmentWAL) sync() error {
 	if err := w.flush(); err != nil {
 		return err
 	}
+	if w.head() == nil {
+		return nil
+	}
 	return fileutil.Fdatasync(w.head().File)
 }
 
@@ -769,6 +775,10 @@ type walReader struct {
 	curBuf     []byte
 	lastOffset int64 // offset after last successfully read entry
 
+	seriesBuf    []RefSeries
+	sampleBuf    []RefSample
+	tombstoneBuf []Stone
+
 	err error
 }
 
@@ -996,7 +1006,8 @@ func (r *walReader) entry(cr io.Reader) (WALEntryType, byte, []byte, error) {
 }
 
 func (r *walReader) decodeSeries(flag byte, b []byte) ([]RefSeries, error) {
-	series := []RefSeries{}
+	r.seriesBuf = r.seriesBuf[:0]
+
 	dec := decbuf{b: b}
 
 	for len(dec.b) > 0 && dec.err() == nil {
@@ -1010,7 +1021,7 @@ func (r *walReader) decodeSeries(flag byte, b []byte) ([]RefSeries, error) {
 		}
 		sort.Sort(lset)
 
-		series = append(series, RefSeries{
+		r.seriesBuf = append(r.seriesBuf, RefSeries{
 			Ref:    ref,
 			Labels: lset,
 		})
@@ -1019,16 +1030,16 @@ func (r *walReader) decodeSeries(flag byte, b []byte) ([]RefSeries, error) {
 		return nil, dec.err()
 	}
 	if len(dec.b) > 0 {
-		return series, errors.Errorf("unexpected %d bytes left in entry", len(dec.b))
+		return r.seriesBuf, errors.Errorf("unexpected %d bytes left in entry", len(dec.b))
 	}
-	return series, nil
+	return r.seriesBuf, nil
 }
 
 func (r *walReader) decodeSamples(flag byte, b []byte) ([]RefSample, error) {
 	if len(b) == 0 {
 		return nil, nil
 	}
-	samples := []RefSample{}
+	r.sampleBuf = r.sampleBuf[:0]
 	dec := decbuf{b: b}
 
 	var (
@@ -1041,7 +1052,7 @@ func (r *walReader) decodeSamples(flag byte, b []byte) ([]RefSample, error) {
 		dtime := dec.varint64()
 		val := dec.be64()
 
-		samples = append(samples, RefSample{
+		r.sampleBuf = append(r.sampleBuf, RefSample{
 			Ref: uint64(int64(baseRef) + dref),
 			T:   baseTime + dtime,
 			V:   math.Float64frombits(val),
@@ -1049,20 +1060,20 @@ func (r *walReader) decodeSamples(flag byte, b []byte) ([]RefSample, error) {
 	}
 
 	if dec.err() != nil {
-		return nil, errors.Wrapf(dec.err(), "decode error after %d samples", len(samples))
+		return nil, errors.Wrapf(dec.err(), "decode error after %d samples", len(r.sampleBuf))
 	}
 	if len(dec.b) > 0 {
-		return samples, errors.Errorf("unexpected %d bytes left in entry", len(dec.b))
+		return r.sampleBuf, errors.Errorf("unexpected %d bytes left in entry", len(dec.b))
 	}
-	return samples, nil
+	return r.sampleBuf, nil
 }
 
 func (r *walReader) decodeDeletes(flag byte, b []byte) ([]Stone, error) {
 	dec := &decbuf{b: b}
-	var stones []Stone
+	r.tombstoneBuf = r.tombstoneBuf[:0]
 
 	for dec.len() > 0 && dec.err() == nil {
-		stones = append(stones, Stone{
+		r.tombstoneBuf = append(r.tombstoneBuf, Stone{
 			ref: dec.be64(),
 			intervals: Intervals{
 				{Mint: dec.varint64(), Maxt: dec.varint64()},
@@ -1073,7 +1084,7 @@ func (r *walReader) decodeDeletes(flag byte, b []byte) ([]Stone, error) {
 		return nil, dec.err()
 	}
 	if len(dec.b) > 0 {
-		return stones, errors.Errorf("unexpected %d bytes left in entry", len(dec.b))
+		return r.tombstoneBuf, errors.Errorf("unexpected %d bytes left in entry", len(dec.b))
 	}
-	return stones, nil
+	return r.tombstoneBuf, nil
 }
diff --git a/wal_test.go b/wal_test.go
index 17a71da75..45279b1b5 100644
--- a/wal_test.go
+++ b/wal_test.go
@@ -197,7 +197,7 @@ func TestSegmentWAL_Log_Restore(t *testing.T) {
 	const (
 		numMetrics = 50
 		iterations = 5
-		stepSize   = 1
+		stepSize   = 5
 	)
 	// Generate testing data. It does not make semantical sense but
 	// for the purpose of this test.
@@ -236,7 +236,7 @@ func TestSegmentWAL_Log_Restore(t *testing.T) {
 			if len(series) > 0 {
 				clsets := make([]RefSeries, len(series))
 				copy(clsets, series)
-				resultSeries = append(resultSeries, series)
+				resultSeries = append(resultSeries, clsets)
 			}
 
 			return nil
@@ -253,7 +253,9 @@ func TestSegmentWAL_Log_Restore(t *testing.T) {
 
 		delf := func(stones []Stone) error {
 			if len(stones) > 0 {
-				resultDeletes = append(resultDeletes, stones)
+				cst := make([]Stone, len(stones))
+				copy(cst, stones)
+				resultDeletes = append(resultDeletes, cst)
 			}
 
 			return nil