tsdb: Replay m-map chunk only when required

M-map chunks replayed on startup are discarded if there
was no WAL and no snapshot loaded, because there is no
series created in the Head that it can map to. So only
load m-map chunks from disk if there is either a snapshot
loaded or there is WAL on disk.

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

tsdb/head.go

@@ -590,6 +590,7 @@ func (h *Head) Init(minValidTime int64) error {
 	snapIdx, snapOffset := -1, 0
 	refSeries := make(map[chunks.HeadSeriesRef]*memSeries)
 
+	snapshotLoaded := false
 	if h.opts.EnableMemorySnapshotOnShutdown {
 		level.Info(h.logger).Log("msg", "Chunk snapshot is enabled, replaying from the snapshot")
 		// If there are any WAL files, there should be at least one WAL file with an index that is current or newer
@@ -619,6 +620,7 @@ func (h *Head) Init(minValidTime int64) error {
 			var err error
 			snapIdx, snapOffset, refSeries, err = h.loadChunkSnapshot()
 			if err == nil {
+				snapshotLoaded = true
 				level.Info(h.logger).Log("msg", "Chunk snapshot loading time", "duration", time.Since(start).String())
 			}
 			if err != nil {
@@ -636,26 +638,36 @@ func (h *Head) Init(minValidTime int64) error {
 	}
 
 	mmapChunkReplayStart := time.Now()
-	mmappedChunks, oooMmappedChunks, lastMmapRef, err := h.loadMmappedChunks(refSeries)
-	if err != nil {
-		// TODO(codesome): clear out all m-map chunks here for refSeries.
-		level.Error(h.logger).Log("msg", "Loading on-disk chunks failed", "err", err)
-		if _, ok := errors.Cause(err).(*chunks.CorruptionErr); ok {
-			h.metrics.mmapChunkCorruptionTotal.Inc()
-		}
+	var (
+		mmappedChunks    map[chunks.HeadSeriesRef][]*mmappedChunk
+		oooMmappedChunks map[chunks.HeadSeriesRef][]*mmappedChunk
+		lastMmapRef      chunks.ChunkDiskMapperRef
+		err              error
+	)
+	if snapshotLoaded || h.wal != nil {
+		// If snapshot was not loaded and if there is no WAL, then m-map chunks will be discarded
+		// anyway. So we only load m-map chunks when it won't be discarded.
+		mmappedChunks, oooMmappedChunks, lastMmapRef, err = h.loadMmappedChunks(refSeries)
+		if err != nil {
+			// TODO(codesome): clear out all m-map chunks here for refSeries.
+			level.Error(h.logger).Log("msg", "Loading on-disk chunks failed", "err", err)
+			if _, ok := errors.Cause(err).(*chunks.CorruptionErr); ok {
+				h.metrics.mmapChunkCorruptionTotal.Inc()
+			}
 
-		// Discard snapshot data since we need to replay the WAL for the missed m-map chunks data.
-		snapIdx, snapOffset = -1, 0
+			// Discard snapshot data since we need to replay the WAL for the missed m-map chunks data.
+			snapIdx, snapOffset = -1, 0
 
-		// If this fails, data will be recovered from WAL.
-		// Hence we wont lose any data (given WAL is not corrupt).
-		mmappedChunks, oooMmappedChunks, lastMmapRef, err = h.removeCorruptedMmappedChunks(err)
-		if err != nil {
-			return err
+			// If this fails, data will be recovered from WAL.
+			// Hence we wont lose any data (given WAL is not corrupt).
+			mmappedChunks, oooMmappedChunks, lastMmapRef, err = h.removeCorruptedMmappedChunks(err)
+			if err != nil {
+				return err
+			}
 		}
+		level.Info(h.logger).Log("msg", "On-disk memory mappable chunks replay completed", "duration", time.Since(mmapChunkReplayStart).String())
 	}
 
-	level.Info(h.logger).Log("msg", "On-disk memory mappable chunks replay completed", "duration", time.Since(mmapChunkReplayStart).String())
 	if h.wal == nil {
 		level.Info(h.logger).Log("msg", "WAL not found")
 		return nil