Only checkpoint chunkdescs and series that need persisting. (#2340)

This decreases checkpoint size by not checkpointing things
that don't actually need checkpointing.

This is fully compatible with the v2 checkpoint format,
as it makes series appear as though the only chunksdescs
in memory are those that need persisting.

storage/local/persistence.go

@@ -668,8 +668,10 @@ func (p *persistence) checkpointSeriesMapAndHeads(fingerprintToSeries *seriesMap
 			fpLocker.Lock(m.fp)
 			defer fpLocker.Unlock(m.fp)
 
-			if len(m.series.chunkDescs) == 0 {
-				// This series was completely purged or archived in the meantime. Ignore.
+			chunksToPersist := len(m.series.chunkDescs) - m.series.persistWatermark
+			if len(m.series.chunkDescs) == 0 || chunksToPersist == 0 {
+				// This series was completely purged or archived in the meantime or has
+				// no chunks that need persisting. Ignore.
 				return
 			}
 			realNumberOfSeries++
@@ -688,7 +690,9 @@ func (p *persistence) checkpointSeriesMapAndHeads(fingerprintToSeries *seriesMap
 			if _, err = w.Write(buf); err != nil {
 				return
 			}
-			if _, err = codable.EncodeVarint(w, int64(m.series.persistWatermark)); err != nil {
+			// persistWatermark. We only checkpoint chunks that need persisting, so
+			// this is always 0.
+			if _, err = codable.EncodeVarint(w, int64(0)); err != nil {
 				return
 			}
 			if m.series.modTime.IsZero() {
@@ -700,37 +704,25 @@ func (p *persistence) checkpointSeriesMapAndHeads(fingerprintToSeries *seriesMap
 					return
 				}
 			}
-			if _, err = codable.EncodeVarint(w, int64(m.series.chunkDescsOffset)); err != nil {
+			// chunkDescsOffset.
+			if _, err = codable.EncodeVarint(w, int64(m.series.chunkDescsOffset+m.series.persistWatermark)); err != nil {
 				return
 			}
 			if _, err = codable.EncodeVarint(w, int64(m.series.savedFirstTime)); err != nil {
 				return
 			}
-			if _, err = codable.EncodeVarint(w, int64(len(m.series.chunkDescs))); err != nil {
+			// Number of chunkDescs.
+			if _, err = codable.EncodeVarint(w, int64(chunksToPersist)); err != nil {
 				return
 			}
-			for i, chunkDesc := range m.series.chunkDescs {
-				if i < m.series.persistWatermark {
-					if _, err = codable.EncodeVarint(w, int64(chunkDesc.FirstTime())); err != nil {
-						return
-					}
-					lt, err := chunkDesc.LastTime()
-					if err != nil {
-						return
-					}
-					if _, err = codable.EncodeVarint(w, int64(lt)); err != nil {
-						return
-					}
-				} else {
-					// This is a non-persisted chunk. Fully marshal it.
-					if err = w.WriteByte(byte(chunkDesc.C.Encoding())); err != nil {
-						return
-					}
-					if err = chunkDesc.C.Marshal(w); err != nil {
-						return
-					}
+			for _, chunkDesc := range m.series.chunkDescs[m.series.persistWatermark:] {
+				if err = w.WriteByte(byte(chunkDesc.C.Encoding())); err != nil {
+					return
+				}
+				if err = chunkDesc.C.Marshal(w); err != nil {
+					return
 				}
-				p.checkpointChunksWritten.Observe(float64(len(m.series.chunkDescs) - m.series.persistWatermark))
+				p.checkpointChunksWritten.Observe(float64(chunksToPersist))
 			}
 			// Series is checkpointed now, so declare it clean. In case the entire
 			// checkpoint fails later on, this is fine, as the storage's series

storage/local/persistence_test.go

@@ -493,8 +493,8 @@ func testCheckpointAndLoadSeriesMapAndHeads(t *testing.T, encoding chunk.Encodin
 	if err != nil {
 		t.Fatal(err)
 	}
-	if loadedSM.length() != 4 {
-		t.Errorf("want 4 series in map, got %d", loadedSM.length())
+	if loadedSM.length() != 3 {
+		t.Errorf("want 3 series in map, got %d", loadedSM.length())
 	}
 	if loadedS1, ok := loadedSM.get(m1.FastFingerprint()); ok {
 		if !reflect.DeepEqual(loadedS1.metric, m1) {
@@ -518,28 +518,6 @@ func testCheckpointAndLoadSeriesMapAndHeads(t *testing.T, encoding chunk.Encodin
 	} else {
 		t.Errorf("couldn't find %v in loaded map", m1)
 	}
-	if loadedS3, ok := loadedSM.get(m3.FastFingerprint()); ok {
-		if !reflect.DeepEqual(loadedS3.metric, m3) {
-			t.Errorf("want metric %v, got %v", m3, loadedS3.metric)
-		}
-		if loadedS3.head().C != nil {
-			t.Error("head chunk not evicted")
-		}
-		if loadedS3.chunkDescsOffset != 0 {
-			t.Errorf("want chunkDescsOffset 0, got %d", loadedS3.chunkDescsOffset)
-		}
-		if !loadedS3.headChunkClosed {
-			t.Error("headChunkClosed is false")
-		}
-		if loadedS3.head().ChunkFirstTime != 2 {
-			t.Errorf("want ChunkFirstTime in head chunk to be 2, got %d", loadedS3.head().ChunkFirstTime)
-		}
-		if loadedS3.head().ChunkLastTime != 2 {
-			t.Errorf("want ChunkLastTime in head chunk to be 2, got %d", loadedS3.head().ChunkLastTime)
-		}
-	} else {
-		t.Errorf("couldn't find %v in loaded map", m3)
-	}
 	if loadedS4, ok := loadedSM.get(m4.FastFingerprint()); ok {
 		if !reflect.DeepEqual(loadedS4.metric, m4) {
 			t.Errorf("want metric %v, got %v", m4, loadedS4.metric)
@@ -594,20 +572,17 @@ func testCheckpointAndLoadSeriesMapAndHeads(t *testing.T, encoding chunk.Encodin
 		if !reflect.DeepEqual(loadedS5.metric, m5) {
 			t.Errorf("want metric %v, got %v", m5, loadedS5.metric)
 		}
-		if got, want := len(loadedS5.chunkDescs), chunkCountS5; got != want {
+		if got, want := len(loadedS5.chunkDescs), chunkCountS5-3; got != want {
 			t.Errorf("got %d chunkDescs, want %d", got, want)
 		}
-		if got, want := loadedS5.persistWatermark, 3; got != want {
+		if got, want := loadedS5.persistWatermark, 0; got != want {
 			t.Errorf("got persistWatermark %d, want %d", got, want)
 		}
-		if !loadedS5.chunkDescs[2].IsEvicted() {
-			t.Error("3rd chunk not evicted")
-		}
-		if loadedS5.chunkDescs[3].IsEvicted() {
-			t.Error("4th chunk evicted")
+		if loadedS5.chunkDescs[0].IsEvicted() {
+			t.Error("1st chunk evicted")
 		}
-		if loadedS5.chunkDescsOffset != 0 {
-			t.Errorf("want chunkDescsOffset 0, got %d", loadedS5.chunkDescsOffset)
+		if loadedS5.chunkDescsOffset != 3 {
+			t.Errorf("want chunkDescsOffset 3, got %d", loadedS5.chunkDescsOffset)
 		}
 		if loadedS5.headChunkClosed {
 			t.Error("headChunkClosed is true")