diff --git a/storage/metric/memory.go b/storage/metric/memory.go
index a43f53be5c..625cf67d1d 100644
--- a/storage/metric/memory.go
+++ b/storage/metric/memory.go
@@ -41,38 +41,66 @@ func (v singletonValue) get() clientmodel.SampleValue {
 	return clientmodel.SampleValue(v)
 }
 
-type stream struct {
+type stream interface {
+	add(...*SamplePair)
+
+	clone() Values
+	expunge(age time.Time) Values
+
+	size() int
+	clear()
+
+	metric() clientmodel.Metric
+
+	getValueAtTime(t time.Time) Values
+	getBoundaryValues(in Interval) Values
+	getRangeValues(in Interval) Values
+}
+
+type arrayStream struct {
 	sync.RWMutex
 
-	metric clientmodel.Metric
+	m      clientmodel.Metric
 	values Values
 }
 
-func (s *stream) add(timestamp time.Time, value clientmodel.SampleValue) {
+func (s *arrayStream) metric() clientmodel.Metric {
+	return s.m
+}
+
+func (s *arrayStream) add(v ...*SamplePair) {
 	s.Lock()
 	defer s.Unlock()
 
-	// BUG(all): https://github.com/prometheus/prometheus/pull/265/files#r4336435.
-
-	s.values = append(s.values, &SamplePair{
-		Timestamp: timestamp.Round(time.Second).UTC(),
-		Value:     value,
-	})
+	s.values = append(s.values, v...)
 }
 
-func (s *stream) clone() Values {
+func (s *arrayStream) clone() Values {
 	s.RLock()
 	defer s.RUnlock()
 
-	// BUG(all): Examine COW technique.
-
 	clone := make(Values, len(s.values))
 	copy(clone, s.values)
 
 	return clone
 }
 
-func (s *stream) getValueAtTime(t time.Time) Values {
+func (s *arrayStream) expunge(t time.Time) Values {
+	s.Lock()
+	defer s.Unlock()
+
+	finder := func(i int) bool {
+		return s.values[i].Timestamp.After(t)
+	}
+
+	i := sort.Search(len(s.values), finder)
+	expunged := s.values[:i]
+	s.values = s.values[i:]
+
+	return expunged
+}
+
+func (s *arrayStream) getValueAtTime(t time.Time) Values {
 	s.RLock()
 	defer s.RUnlock()
 
@@ -102,7 +130,7 @@ func (s *stream) getValueAtTime(t time.Time) Values {
 	}
 }
 
-func (s *stream) getBoundaryValues(in Interval) Values {
+func (s *arrayStream) getBoundaryValues(in Interval) Values {
 	s.RLock()
 	defer s.RUnlock()
 
@@ -125,7 +153,7 @@ func (s *stream) getBoundaryValues(in Interval) Values {
 	}
 }
 
-func (s *stream) getRangeValues(in Interval) Values {
+func (s *arrayStream) getRangeValues(in Interval) Values {
 	s.RLock()
 	defer s.RUnlock()
 
@@ -143,13 +171,17 @@ func (s *stream) getRangeValues(in Interval) Values {
 	return result
 }
 
-func (s *stream) empty() bool {
-	return len(s.values) == 0
+func (s *arrayStream) size() int {
+	return len(s.values)
 }
 
-func newStream(metric clientmodel.Metric) *stream {
-	return &stream{
-		metric: metric,
+func (s *arrayStream) clear() {
+	s.values = Values{}
+}
+
+func newArrayStream(metric clientmodel.Metric) *arrayStream {
+	return &arrayStream{
+		m:      metric,
 		values: make(Values, 0, initialSeriesArenaSize),
 	}
 }
@@ -158,7 +190,7 @@ type memorySeriesStorage struct {
 	sync.RWMutex
 
 	wmCache                 *WatermarkCache
-	fingerprintToSeries     map[clientmodel.Fingerprint]*stream
+	fingerprintToSeries     map[clientmodel.Fingerprint]stream
 	labelPairToFingerprints map[LabelPair]clientmodel.Fingerprints
 	labelNameToFingerprints map[clientmodel.LabelName]clientmodel.Fingerprints
 }
@@ -184,7 +216,10 @@ func (s *memorySeriesStorage) AppendSample(sample *clientmodel.Sample) error {
 	fingerprint := &clientmodel.Fingerprint{}
 	fingerprint.LoadFromMetric(sample.Metric)
 	series := s.getOrCreateSeries(sample.Metric, fingerprint)
-	series.add(sample.Timestamp, sample.Value)
+	series.add(&SamplePair{
+		Value:     sample.Value,
+		Timestamp: sample.Timestamp,
+	})
 
 	if s.wmCache != nil {
 		s.wmCache.Set(fingerprint, &watermarks{High: sample.Timestamp})
@@ -202,11 +237,11 @@ func (s *memorySeriesStorage) CreateEmptySeries(metric clientmodel.Metric) {
 	s.getOrCreateSeries(metric, fingerprint)
 }
 
-func (s *memorySeriesStorage) getOrCreateSeries(metric clientmodel.Metric, fingerprint *clientmodel.Fingerprint) *stream {
+func (s *memorySeriesStorage) getOrCreateSeries(metric clientmodel.Metric, fingerprint *clientmodel.Fingerprint) stream {
 	series, ok := s.fingerprintToSeries[*fingerprint]
 
 	if !ok {
-		series = newStream(metric)
+		series = newArrayStream(metric)
 		s.fingerprintToSeries[*fingerprint] = series
 
 		for k, v := range metric {
@@ -231,20 +266,12 @@ func (s *memorySeriesStorage) Flush(flushOlderThan time.Time, queue chan<- clien
 
 	s.RLock()
 	for fingerprint, stream := range s.fingerprintToSeries {
-		finder := func(i int) bool {
-			return stream.values[i].Timestamp.After(flushOlderThan)
-		}
-
-		stream.Lock()
-
-		i := sort.Search(len(stream.values), finder)
-		toArchive := stream.values[:i]
-		toKeep := stream.values[i:]
+		toArchive := stream.expunge(flushOlderThan)
 		queued := make(clientmodel.Samples, 0, len(toArchive))
-
+		// NOTE: This duplication will go away soon.
 		for _, value := range toArchive {
 			queued = append(queued, &clientmodel.Sample{
-				Metric:    stream.metric,
+				Metric:    stream.metric(),
 				Timestamp: value.Timestamp,
 				Value:     value.Value,
 			})
@@ -255,18 +282,15 @@ func (s *memorySeriesStorage) Flush(flushOlderThan time.Time, queue chan<- clien
 		// https://github.com/prometheus/prometheus/issues/275
 		queue <- queued
 
-		stream.values = toKeep
-
-		if len(toKeep) == 0 {
+		if stream.size() == 0 {
 			emptySeries = append(emptySeries, fingerprint)
 		}
-		stream.Unlock()
 	}
 	s.RUnlock()
 
 	s.Lock()
 	for _, fingerprint := range emptySeries {
-		if s.fingerprintToSeries[fingerprint].empty() {
+		if s.fingerprintToSeries[fingerprint].size() == 0 {
 			s.dropSeries(&fingerprint)
 		}
 	}
@@ -279,7 +303,7 @@ func (s *memorySeriesStorage) dropSeries(fingerprint *clientmodel.Fingerprint) {
 	if !ok {
 		return
 	}
-	for k, v := range series.metric {
+	for k, v := range series.metric() {
 		labelPair := LabelPair{
 			Name:  k,
 			Value: v,
@@ -299,13 +323,11 @@ func (s *memorySeriesStorage) appendSamplesWithoutIndexing(fingerprint *clientmo
 	series, ok := s.fingerprintToSeries[*fingerprint]
 
 	if !ok {
-		series = newStream(clientmodel.Metric{})
+		series = newArrayStream(clientmodel.Metric{})
 		s.fingerprintToSeries[*fingerprint] = series
 	}
 
-	for _, sample := range samples {
-		series.add(sample.Timestamp, sample.Value)
-	}
+	series.add(samples...)
 }
 
 func (s *memorySeriesStorage) GetFingerprintsForLabelSet(l clientmodel.LabelSet) (fingerprints clientmodel.Fingerprints, err error) {
@@ -367,7 +389,7 @@ func (s *memorySeriesStorage) GetMetricForFingerprint(f *clientmodel.Fingerprint
 	}
 
 	metric := clientmodel.Metric{}
-	for label, value := range series.metric {
+	for label, value := range series.metric() {
 		metric[label] = value
 	}
 
@@ -436,7 +458,7 @@ func (s *memorySeriesStorage) Close() {
 	s.Lock()
 	defer s.Unlock()
 
-	s.fingerprintToSeries = map[clientmodel.Fingerprint]*stream{}
+	s.fingerprintToSeries = map[clientmodel.Fingerprint]stream{}
 	s.labelPairToFingerprints = map[LabelPair]clientmodel.Fingerprints{}
 	s.labelNameToFingerprints = map[clientmodel.LabelName]clientmodel.Fingerprints{}
 }
@@ -447,7 +469,7 @@ func (s *memorySeriesStorage) GetAllValuesForLabel(labelName clientmodel.LabelNa
 
 	valueSet := map[clientmodel.LabelValue]bool{}
 	for _, series := range s.fingerprintToSeries {
-		if value, ok := series.metric[labelName]; ok {
+		if value, ok := series.metric()[labelName]; ok {
 			if !valueSet[value] {
 				values = append(values, value)
 				valueSet[value] = true
@@ -460,7 +482,7 @@ func (s *memorySeriesStorage) GetAllValuesForLabel(labelName clientmodel.LabelNa
 
 func NewMemorySeriesStorage(o MemorySeriesOptions) *memorySeriesStorage {
 	return &memorySeriesStorage{
-		fingerprintToSeries:     make(map[clientmodel.Fingerprint]*stream),
+		fingerprintToSeries:     make(map[clientmodel.Fingerprint]stream),
 		labelPairToFingerprints: make(map[LabelPair]clientmodel.Fingerprints),
 		labelNameToFingerprints: make(map[clientmodel.LabelName]clientmodel.Fingerprints),
 		wmCache:                 o.WatermarkCache,
diff --git a/storage/metric/memory_test.go b/storage/metric/memory_test.go
index 2ddfa19425..a86a7424e2 100644
--- a/storage/metric/memory_test.go
+++ b/storage/metric/memory_test.go
@@ -24,12 +24,13 @@ import (
 
 func BenchmarkStreamAdd(b *testing.B) {
 	b.StopTimer()
-	s := newStream(clientmodel.Metric{})
-	times := make([]time.Time, 0, b.N)
-	samples := make([]clientmodel.SampleValue, 0, b.N)
+	s := newArrayStream(clientmodel.Metric{})
+	samples := make(Values, b.N)
 	for i := 0; i < b.N; i++ {
-		times = append(times, time.Date(i, 0, 0, 0, 0, 0, 0, time.UTC))
-		samples = append(samples, clientmodel.SampleValue(i))
+		samples = append(samples, &SamplePair{
+			Timestamp: time.Date(i, 0, 0, 0, 0, 0, 0, time.UTC),
+			Value:     clientmodel.SampleValue(i),
+		})
 	}
 
 	b.StartTimer()
@@ -37,9 +38,7 @@ func BenchmarkStreamAdd(b *testing.B) {
 	var pre runtime.MemStats
 	runtime.ReadMemStats(&pre)
 
-	for i := 0; i < b.N; i++ {
-		s.add(times[i], samples[i])
-	}
+	s.add(samples...)
 
 	var post runtime.MemStats
 	runtime.ReadMemStats(&post)