Set the min time of Head properly after truncation (#8212)

* Set the min time of Head properly after truncation

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

* Fix lint

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

* Enhance compaction plan logic for completely deleted small block

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

* Fix review comments

Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

tsdb/compact.go

@@ -214,6 +214,11 @@ func (c *LeveledCompactor) plan(dms []dirMeta) ([]string, error) {
 	for i := len(dms) - 1; i >= 0; i-- {
 		meta := dms[i].meta
 		if meta.MaxTime-meta.MinTime < c.ranges[len(c.ranges)/2] {
+			// If the block is entirely deleted, then we don't care about the block being big enough.
+			// TODO: This is assuming single tombstone is for distinct series, which might be no true.
+			if meta.Stats.NumTombstones > 0 && meta.Stats.NumTombstones >= meta.Stats.NumSeries {
+				return []string{dms[i].dir}, nil
+			}
 			break
 		}
 		if float64(meta.Stats.NumTombstones)/float64(meta.Stats.NumSeries+1) > 0.05 {

tsdb/head.go

@@ -820,9 +820,22 @@ func (h *Head) truncateMemory(mint int64) (err error) {
 	h.metrics.headTruncateTotal.Inc()
 	start := time.Now()
 
-	h.gc()
+	actualMint := h.gc()
 	level.Info(h.logger).Log("msg", "Head GC completed", "duration", time.Since(start))
 	h.metrics.gcDuration.Observe(time.Since(start).Seconds())
+	if actualMint > h.minTime.Load() {
+		// The actual mint of the Head is higher than the one asked to truncate.
+		appendableMinValidTime := h.appendableMinValidTime()
+		if actualMint < appendableMinValidTime {
+			h.minTime.Store(actualMint)
+			h.minValidTime.Store(actualMint)
+		} else {
+			// The actual min time is in the appendable window.
+			// So we set the mint to the appendableMinValidTime.
+			h.minTime.Store(appendableMinValidTime)
+			h.minValidTime.Store(appendableMinValidTime)
+		}
+	}
 
 	// Truncate the chunk m-mapper.
 	if err := h.chunkDiskMapper.Truncate(mint); err != nil {
@@ -1054,10 +1067,8 @@ func (h *Head) appender() *headAppender {
 	cleanupAppendIDsBelow := h.iso.lowWatermark()
 
 	return &headAppender{
-		head: h,
-		// Set the minimum valid time to whichever is greater the head min valid time or the compaction window.
-		// This ensures that no samples will be added within the compaction window to avoid races.
-		minValidTime:          max(h.minValidTime.Load(), h.MaxTime()-h.chunkRange.Load()/2),
+		head:                  h,
+		minValidTime:          h.appendableMinValidTime(),
 		mint:                  math.MaxInt64,
 		maxt:                  math.MinInt64,
 		samples:               h.getAppendBuffer(),
@@ -1067,6 +1078,12 @@ func (h *Head) appender() *headAppender {
 	}
 }
 
+func (h *Head) appendableMinValidTime() int64 {
+	// Setting the minimum valid time to whichever is greater, the head min valid time or the compaction window,
+	// ensures that no samples will be added within the compaction window to avoid races.
+	return max(h.minValidTime.Load(), h.MaxTime()-h.chunkRange.Load()/2)
+}
+
 func max(a, b int64) int64 {
 	if a > b {
 		return a
@@ -1335,13 +1352,14 @@ func (h *Head) Delete(mint, maxt int64, ms ...*labels.Matcher) error {
 }
 
 // gc removes data before the minimum timestamp from the head.
-func (h *Head) gc() {
+// It returns the actual min times of the chunks present in the Head.
+func (h *Head) gc() int64 {
 	// Only data strictly lower than this timestamp must be deleted.
 	mint := h.MinTime()
 
 	// Drop old chunks and remember series IDs and hashes if they can be
 	// deleted entirely.
-	deleted, chunksRemoved := h.series.gc(mint)
+	deleted, chunksRemoved, actualMint := h.series.gc(mint)
 	seriesRemoved := len(deleted)
 
 	h.metrics.seriesRemoved.Add(float64(seriesRemoved))
@@ -1382,6 +1400,8 @@ func (h *Head) gc() {
 		panic(err)
 	}
 	h.symbols = symbols
+
+	return actualMint
 }
 
 // Tombstones returns a new reader over the head's tombstones
@@ -1813,11 +1833,12 @@ func newStripeSeries(stripeSize int, seriesCallback SeriesLifecycleCallback) *st
 
 // gc garbage collects old chunks that are strictly before mint and removes
 // series entirely that have no chunks left.
-func (s *stripeSeries) gc(mint int64) (map[uint64]struct{}, int) {
+func (s *stripeSeries) gc(mint int64) (map[uint64]struct{}, int, int64) {
 	var (
-		deleted            = map[uint64]struct{}{}
-		deletedForCallback = []labels.Labels{}
-		rmChunks           = 0
+		deleted                  = map[uint64]struct{}{}
+		deletedForCallback       = []labels.Labels{}
+		rmChunks                 = 0
+		actualMint         int64 = math.MaxInt64
 	)
 	// Run through all series and truncate old chunks. Mark those with no
 	// chunks left as deleted and store their ID.
@@ -1830,6 +1851,10 @@ func (s *stripeSeries) gc(mint int64) (map[uint64]struct{}, int) {
 				rmChunks += series.truncateChunksBefore(mint)
 
 				if len(series.mmappedChunks) > 0 || series.headChunk != nil || series.pendingCommit {
+					seriesMint := series.minTime()
+					if seriesMint < actualMint {
+						actualMint = seriesMint
+					}
 					series.Unlock()
 					continue
 				}
@@ -1864,7 +1889,11 @@ func (s *stripeSeries) gc(mint int64) (map[uint64]struct{}, int) {
 		deletedForCallback = deletedForCallback[:0]
 	}
 
-	return deleted, rmChunks
+	if actualMint == math.MaxInt64 {
+		actualMint = mint
+	}
+
+	return deleted, rmChunks, actualMint
 }
 
 func (s *stripeSeries) getByID(id uint64) *memSeries {

tsdb/head_test.go

@@ -1897,3 +1897,37 @@ func TestErrReuseAppender(t *testing.T) {
 	require.Error(t, app.Commit())
 	require.Error(t, app.Rollback())
 }
+
+func TestHeadMintAfterTruncation(t *testing.T) {
+	chunkRange := int64(2000)
+	head, _ := newTestHead(t, chunkRange, false)
+
+	app := head.Appender(context.Background())
+	_, err := app.Add(labels.Labels{{Name: "a", Value: "b"}}, 100, 100)
+	require.NoError(t, err)
+	_, err = app.Add(labels.Labels{{Name: "a", Value: "b"}}, 4000, 200)
+	require.NoError(t, err)
+	_, err = app.Add(labels.Labels{{Name: "a", Value: "b"}}, 8000, 300)
+	require.NoError(t, err)
+	require.NoError(t, app.Commit())
+
+	// Truncating outside the appendable window and actual mint being outside
+	// appendable window should leave mint at the actual mint.
+	require.NoError(t, head.Truncate(3500))
+	require.Equal(t, int64(4000), head.MinTime())
+	require.Equal(t, int64(4000), head.minValidTime.Load())
+
+	// After truncation outside the appendable windown if the actual min time
+	// is in the appendable window then we should leave mint at the start of appendable window.
+	require.NoError(t, head.Truncate(5000))
+	require.Equal(t, head.appendableMinValidTime(), head.MinTime())
+	require.Equal(t, head.appendableMinValidTime(), head.minValidTime.Load())
+
+	// If the truncation time is inside the appendable window, then the min time
+	// should be the truncation time.
+	require.NoError(t, head.Truncate(7500))
+	require.Equal(t, int64(7500), head.MinTime())
+	require.Equal(t, int64(7500), head.minValidTime.Load())
+
+	require.NoError(t, head.Close())
+}