Cache disk fingerprint->metric lookups in memory.

storage/metric/memory.go

@@ -175,14 +175,28 @@ func (s *memorySeriesStorage) AppendSample(sample model.Sample) error {
 	s.Lock()
 	defer s.Unlock()
 
-	metric := sample.Metric
-	fingerprint := model.NewFingerprintFromMetric(metric)
-	series, ok := s.fingerprintToSeries[*fingerprint]
+	fingerprint := model.NewFingerprintFromMetric(sample.Metric)
+	series := s.getOrCreateSeries(sample.Metric, fingerprint)
+	series.add(sample.Timestamp, sample.Value)
 
 	if s.wmCache != nil {
 		s.wmCache.Set(fingerprint, &Watermarks{High: sample.Timestamp})
 	}
 
+	return nil
+}
+
+func (s *memorySeriesStorage) CreateEmptySeries(metric model.Metric) {
+	s.Lock()
+	defer s.Unlock()
+
+	fingerprint := model.NewFingerprintFromMetric(metric)
+	s.getOrCreateSeries(metric, fingerprint)
+}
+
+func (s *memorySeriesStorage) getOrCreateSeries(metric model.Metric, fingerprint *model.Fingerprint) *stream {
+	series, ok := s.fingerprintToSeries[*fingerprint]
+
 	if !ok {
 		series = newStream(metric)
 		s.fingerprintToSeries[*fingerprint] = series
@@ -201,10 +215,7 @@ func (s *memorySeriesStorage) AppendSample(sample model.Sample) error {
 			s.labelNameToFingerprints[k] = labelNameValues
 		}
 	}
-
-	series.add(sample.Timestamp, sample.Value)
-
-	return nil
+	return series
 }
 
 // Append raw samples, bypassing indexing. Only used to add data to views,

storage/metric/tiered.go

@@ -666,6 +666,7 @@ func (t *TieredStorage) GetMetricForFingerprint(f *model.Fingerprint) (model.Met
 	}
 	if m == nil {
 		m, err = t.DiskStorage.GetMetricForFingerprint(f)
+		t.memoryArena.CreateEmptySeries(m)
 	}
 	return m, err
 }