Create a checkpoint only at the end of Compact call (#8067)

* Create a checkpoint only at the end of Compact call

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

* Fix review comments

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

* Fix Bartek's offline reviews

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

* Introduce TruncateInMemory and TruncateWAL

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

* Small enhancements and test fixing attempts

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

* Fix tests

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

* Add TestOneCheckpointPerCompactCall

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

* Don't truncate WAL on block compaction

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

* Simplified the algo.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

* Better protection around calling truncateWAL, truncate WAL on Head compaction error

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

Co-authored-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>

tsdb/block.go

@@ -172,7 +172,7 @@ type BlockMetaCompaction struct {
 	// ULIDs of all source head blocks that went into the block.
 	Sources []ulid.ULID `json:"sources,omitempty"`
 	// Indicates that during compaction it resulted in a block without any samples
-	// so it should be deleted on the next reload.
+	// so it should be deleted on the next reloadBlocks.
 	Deletable bool `json:"deletable,omitempty"`
 	// Short descriptions of the direct blocks that were used to create
 	// this block.

tsdb/compact_test.go

@@ -1235,7 +1235,7 @@ func TestCancelCompactions(t *testing.T) {
 	}
 }
 
-// TestDeleteCompactionBlockAfterFailedReload ensures that a failed reload immediately after a compaction
+// TestDeleteCompactionBlockAfterFailedReload ensures that a failed reloadBlocks immediately after a compaction
 // deletes the resulting block to avoid creatings blocks with the same time range.
 func TestDeleteCompactionBlockAfterFailedReload(t *testing.T) {
 	tests := map[string]func(*DB) int{
@@ -1265,7 +1265,7 @@ func TestDeleteCompactionBlockAfterFailedReload(t *testing.T) {
 				createBlock(t, db.Dir(), genSeries(1, 1, m.MinTime, m.MaxTime))
 			}
 			testutil.Ok(t, db.reload())
-			testutil.Equals(t, len(blocks), len(db.Blocks()), "unexpected block count after a reload")
+			testutil.Equals(t, len(blocks), len(db.Blocks()), "unexpected block count after a reloadBlocks")
 
 			return len(blocks)
 		},
@@ -1281,7 +1281,7 @@ func TestDeleteCompactionBlockAfterFailedReload(t *testing.T) {
 
 			expBlocks := bootStrap(db)
 
-			// Create a block that will trigger the reload to fail.
+			// Create a block that will trigger the reloadBlocks to fail.
 			blockPath := createBlock(t, db.Dir(), genSeries(1, 1, 200, 300))
 			lastBlockIndex := path.Join(blockPath, indexFilename)
 			actBlocks, err := blockDirs(db.Dir())
@@ -1289,15 +1289,15 @@ func TestDeleteCompactionBlockAfterFailedReload(t *testing.T) {
 			testutil.Equals(t, expBlocks, len(actBlocks)-1) // -1 to exclude the corrupted block.
 			testutil.Ok(t, os.RemoveAll(lastBlockIndex))    // Corrupt the block by removing the index file.
 
-			testutil.Equals(t, 0.0, prom_testutil.ToFloat64(db.metrics.reloadsFailed), "initial 'failed db reload' count metrics mismatch")
+			testutil.Equals(t, 0.0, prom_testutil.ToFloat64(db.metrics.reloadsFailed), "initial 'failed db reloadBlocks' count metrics mismatch")
 			testutil.Equals(t, 0.0, prom_testutil.ToFloat64(db.compactor.(*LeveledCompactor).metrics.ran), "initial `compactions` count metric mismatch")
 			testutil.Equals(t, 0.0, prom_testutil.ToFloat64(db.metrics.compactionsFailed), "initial `compactions failed` count metric mismatch")
 
 			// Do the compaction and check the metrics.
-			// Compaction should succeed, but the reload should fail and
+			// Compaction should succeed, but the reloadBlocks should fail and
 			// the new block created from the compaction should be deleted.
 			testutil.NotOk(t, db.Compact())
-			testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.metrics.reloadsFailed), "'failed db reload' count metrics mismatch")
+			testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.metrics.reloadsFailed), "'failed db reloadBlocks' count metrics mismatch")
 			testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.compactor.(*LeveledCompactor).metrics.ran), "`compaction` count metric mismatch")
 			testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.metrics.compactionsFailed), "`compactions failed` count metric mismatch")
 

tsdb/db.go

@@ -216,7 +216,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 	})
 	m.reloadsFailed = prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "prometheus_tsdb_reloads_failures_total",
-		Help: "Number of times the database failed to reload block data from disk.",
+		Help: "Number of times the database failed to reloadBlocks block data from disk.",
 	})
 	m.compactionsTriggered = prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "prometheus_tsdb_compactions_triggered_total",
@@ -769,20 +769,26 @@ func (a dbAppender) Commit() error {
 }
 
 // Compact data if possible. After successful compaction blocks are reloaded
-// which will also trigger blocks to be deleted that fall out of the retention
+// which will also delete the blocks that fall out of the retention window.
-// window.
+// Old blocks are only deleted on reloadBlocks based on the new block's parent information.
-// If no blocks are compacted, the retention window state doesn't change. Thus,
+// See DB.reloadBlocks documentation for further information.
-// this is sufficient to reliably delete old data.
+func (db *DB) Compact() (returnErr error) {
-// Old blocks are only deleted on reload based on the new block's parent information.
-// See DB.reload documentation for further information.
-func (db *DB) Compact() (err error) {
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
 	defer func() {
-		if err != nil {
+		if returnErr != nil {
 			db.metrics.compactionsFailed.Inc()
 		}
 	}()
+
+	lastBlockMaxt := int64(math.MinInt64)
+	defer func() {
+		var merr tsdb_errors.MultiError
+		merr.Add(returnErr)
+		merr.Add(errors.Wrap(db.head.truncateWAL(lastBlockMaxt), "WAL truncation in Compact defer"))
+		returnErr = merr.Err()
+	}()
+
 	// Check whether we have pending head blocks that are ready to be persisted.
 	// They have the highest priority.
 	for {
@@ -804,55 +810,59 @@ func (db *DB) Compact() (err error) {
 		// so in order to make sure that overlaps are evaluated
 		// consistently, we explicitly remove the last value
 		// from the block interval here.
-		head := NewRangeHead(db.head, mint, maxt-1)
+		if err := db.compactHead(NewRangeHead(db.head, mint, maxt-1)); err != nil {
-		if err := db.compactHead(head); err != nil {
+			return errors.Wrap(err, "compact head")
-			return err
 		}
+		// Consider only successful compactions for WAL truncation.
+		lastBlockMaxt = maxt
+	}
+
+	// Clear some disk space before compacting blocks, especially important
+	// when Head compaction happened over a long time range.
+	if err := db.head.truncateWAL(lastBlockMaxt); err != nil {
+		return errors.Wrap(err, "WAL truncation in Compact")
 	}
 
 	return db.compactBlocks()
 }
 
-// CompactHead compacts the given the RangeHead.
+// CompactHead compacts the given RangeHead.
-func (db *DB) CompactHead(head *RangeHead) (err error) {
+func (db *DB) CompactHead(head *RangeHead) error {
 	db.cmtx.Lock()
 	defer db.cmtx.Unlock()
 
-	return db.compactHead(head)
+	if err := db.compactHead(head); err != nil {
+		return errors.Wrap(err, "compact head")
+	}
+
+	if err := db.head.truncateWAL(head.BlockMaxTime()); err != nil {
+		return errors.Wrap(err, "WAL truncation")
+	}
+	return nil
 }
 
-// compactHead compacts the given the RangeHead.
+// compactHead compacts the given RangeHead.
 // The compaction mutex should be held before calling this method.
-func (db *DB) compactHead(head *RangeHead) (err error) {
+func (db *DB) compactHead(head *RangeHead) error {
-	// Add +1 millisecond to block maxt because block intervals are half-open: [b.MinTime, b.MaxTime).
+	uid, err := db.compactor.Write(db.dir, head, head.MinTime(), head.BlockMaxTime(), nil)
-	// Because of this block intervals are always +1 than the total samples it includes.
-	maxt := head.MaxTime() + 1
-	uid, err := db.compactor.Write(db.dir, head, head.MinTime(), maxt, nil)
 	if err != nil {
 		return errors.Wrap(err, "persist head block")
 	}
 
 	runtime.GC()
-
+	if err := db.reloadBlocks(); err != nil {
-	if err := db.reload(); err != nil {
 		if errRemoveAll := os.RemoveAll(filepath.Join(db.dir, uid.String())); errRemoveAll != nil {
 			var merr tsdb_errors.MultiError
-			merr.Add(errors.Wrap(err, "reload blocks"))
+			merr.Add(errors.Wrap(err, "reloadBlocks blocks"))
-			merr.Add(errors.Wrapf(errRemoveAll, "delete persisted head block after failed db reload:%s", uid))
+			merr.Add(errors.Wrapf(errRemoveAll, "delete persisted head block after failed db reloadBlocks:%s", uid))
 			return merr.Err()
 		}
-		return errors.Wrap(err, "reload blocks")
+		return errors.Wrap(err, "reloadBlocks blocks")
 	}
-	if (uid == ulid.ULID{}) {
+	if err = db.head.truncateMemory(head.BlockMaxTime()); err != nil {
-		// Compaction resulted in an empty block.
+		return errors.Wrap(err, "head memory truncate")
-		// Head truncating during db.reload() depends on the persisted blocks and
-		// in this case no new block will be persisted so manually truncate the head.
-		if err = db.head.Truncate(maxt); err != nil {
-			return errors.Wrap(err, "head truncate failed (in compact)")
-		}
 	}
 	runtime.GC()
-
 	return nil
 }
 
@@ -881,11 +891,11 @@ func (db *DB) compactBlocks() (err error) {
 		}
 		runtime.GC()
 
-		if err := db.reload(); err != nil {
+		if err := db.reloadBlocks(); err != nil {
 			if err := os.RemoveAll(filepath.Join(db.dir, uid.String())); err != nil {
-				return errors.Wrapf(err, "delete compacted block after failed db reload:%s", uid)
+				return errors.Wrapf(err, "delete compacted block after failed db reloadBlocks:%s", uid)
 			}
-			return errors.Wrap(err, "reload blocks")
+			return errors.Wrap(err, "reloadBlocks blocks")
 		}
 		runtime.GC()
 	}
@@ -904,9 +914,23 @@ func getBlock(allBlocks []*Block, id ulid.ULID) (*Block, bool) {
 	return nil, false
 }
 
-// reload blocks and trigger head truncation if new blocks appeared.
+// reload reloads blocks and truncates the head and its WAL.
+func (db *DB) reload() error {
+	if err := db.reloadBlocks(); err != nil {
+		return errors.Wrap(err, "reloadBlocks")
+	}
+	if len(db.blocks) == 0 {
+		return nil
+	}
+	if err := db.head.Truncate(db.blocks[len(db.blocks)-1].MaxTime()); err != nil {
+		return errors.Wrap(err, "head truncate")
+	}
+	return nil
+}
+
+// reloadBlocks reloads blocks without touching head.
 // Blocks that are obsolete due to replacement or retention will be deleted.
-func (db *DB) reload() (err error) {
+func (db *DB) reloadBlocks() (err error) {
 	defer func() {
 		if err != nil {
 			db.metrics.reloadsFailed.Inc()
@@ -989,7 +1013,7 @@ func (db *DB) reload() (err error) {
 		blockMetas = append(blockMetas, b.Meta())
 	}
 	if overlaps := OverlappingBlocks(blockMetas); len(overlaps) > 0 {
-		level.Warn(db.logger).Log("msg", "Overlapping blocks found during reload", "detail", overlaps.String())
+		level.Warn(db.logger).Log("msg", "Overlapping blocks found during reloadBlocks", "detail", overlaps.String())
 	}
 
 	// Append blocks to old, deletable blocks, so we can close them.
@@ -999,15 +1023,9 @@ func (db *DB) reload() (err error) {
 		}
 	}
 	if err := db.deleteBlocks(deletable); err != nil {
-		return err
+		return errors.Wrapf(err, "delete %v blocks", len(deletable))
 	}
-
+	return nil
-	// Garbage collect data in the head if the most recent persisted block
-	// covers data of its current time range.
-	if len(toLoad) == 0 {
-		return nil
-	}
-	return errors.Wrap(db.head.Truncate(toLoad[len(toLoad)-1].Meta().MaxTime), "head truncate failed")
 }
 
 func openBlocks(l log.Logger, dir string, loaded []*Block, chunkPool chunkenc.Pool) (blocks []*Block, corrupted map[ulid.ULID]error, err error) {
@@ -1020,7 +1038,7 @@ func openBlocks(l log.Logger, dir string, loaded []*Block, chunkPool chunkenc.Po
 	for _, bDir := range bDirs {
 		meta, _, err := readMetaFile(bDir)
 		if err != nil {
-			level.Error(l).Log("msg", "Failed to read meta.json for a block during reload. Skipping", "dir", bDir, "err", err)
+			level.Error(l).Log("msg", "Failed to read meta.json for a block during reloadBlocks. Skipping", "dir", bDir, "err", err)
 			continue
 		}
 
@@ -1502,7 +1520,11 @@ func (db *DB) CleanTombstones() (err error) {
 			newUIDs = append(newUIDs, *uid)
 		}
 	}
-	return errors.Wrap(db.reload(), "reload blocks")
+
+	if err := db.reloadBlocks(); err != nil {
+		return errors.Wrap(err, "reload blocks")
+	}
+	return nil
 }
 
 func isBlockDir(fi os.FileInfo) bool {

tsdb/db_test.go

@@ -151,7 +151,7 @@ func TestDB_reloadOrder(t *testing.T) {
 		createBlock(t, db.Dir(), genSeries(1, 1, m.MinTime, m.MaxTime))
 	}
 
-	testutil.Ok(t, db.reload())
+	testutil.Ok(t, db.reloadBlocks())
 	blocks := db.Blocks()
 	testutil.Equals(t, 3, len(blocks))
 	testutil.Equals(t, metas[1].MinTime, blocks[0].Meta().MinTime)
@@ -1195,11 +1195,11 @@ func TestTimeRetention(t *testing.T) {
 		createBlock(t, db.Dir(), genSeries(10, 10, m.MinTime, m.MaxTime))
 	}
 
-	testutil.Ok(t, db.reload())                       // Reload the db to register the new blocks.
+	testutil.Ok(t, db.reloadBlocks())                 // Reload the db to register the new blocks.
 	testutil.Equals(t, len(blocks), len(db.Blocks())) // Ensure all blocks are registered.
 
 	db.opts.RetentionDuration = blocks[2].MaxTime - blocks[1].MinTime
-	testutil.Ok(t, db.reload())
+	testutil.Ok(t, db.reloadBlocks())
 
 	expBlocks := blocks[1:]
 	actBlocks := db.Blocks()
@@ -1249,7 +1249,7 @@ func TestSizeRetention(t *testing.T) {
 	testutil.Ok(t, headApp.Commit())
 
 	// Test that registered size matches the actual disk size.
-	testutil.Ok(t, db.reload())                                         // Reload the db to register the new db size.
+	testutil.Ok(t, db.reloadBlocks())                                   // Reload the db to register the new db size.
 	testutil.Equals(t, len(blocks), len(db.Blocks()))                   // Ensure all blocks are registered.
 	blockSize := int64(prom_testutil.ToFloat64(db.metrics.blocksBytes)) // Use the actual internal metrics.
 	walSize, err := db.Head().wal.Size()
@@ -1277,8 +1277,8 @@ func TestSizeRetention(t *testing.T) {
 	// Check total size, total count and check that the oldest block was deleted.
 	firstBlockSize := db.Blocks()[0].Size()
 	sizeLimit := actSize - firstBlockSize
-	db.opts.MaxBytes = sizeLimit // Set the new db size limit one block smaller that the actual size.
+	db.opts.MaxBytes = sizeLimit      // Set the new db size limit one block smaller that the actual size.
-	testutil.Ok(t, db.reload())  // Reload the db to register the new db size.
+	testutil.Ok(t, db.reloadBlocks()) // Reload the db to register the new db size.
 
 	expBlocks := blocks[1:]
 	actBlocks := db.Blocks()
@@ -1811,7 +1811,7 @@ func TestNoEmptyBlocks(t *testing.T) {
 		}
 
 		oldBlocks := db.Blocks()
-		testutil.Ok(t, db.reload())                                      // Reload the db to register the new blocks.
+		testutil.Ok(t, db.reloadBlocks())                                // Reload the db to register the new blocks.
 		testutil.Equals(t, len(blocks)+len(oldBlocks), len(db.Blocks())) // Ensure all blocks are registered.
 		testutil.Ok(t, db.Delete(math.MinInt64, math.MaxInt64, defaultMatcher))
 		testutil.Ok(t, db.Compact())
@@ -2838,3 +2838,124 @@ func TestOpen_VariousBlockStates(t *testing.T) {
 	}
 	testutil.Equals(t, len(expectedIgnoredDirs), ignored)
 }
+
+func TestOneCheckpointPerCompactCall(t *testing.T) {
+	blockRange := int64(1000)
+	tsdbCfg := &Options{
+		RetentionDuration: blockRange * 1000,
+		NoLockfile:        true,
+		MinBlockDuration:  blockRange,
+		MaxBlockDuration:  blockRange,
+	}
+
+	tmpDir, err := ioutil.TempDir("", "test")
+	testutil.Ok(t, err)
+	t.Cleanup(func() {
+		testutil.Ok(t, os.RemoveAll(tmpDir))
+	})
+
+	db, err := Open(tmpDir, log.NewNopLogger(), prometheus.NewRegistry(), tsdbCfg)
+	testutil.Ok(t, err)
+	t.Cleanup(func() {
+		testutil.Ok(t, db.Close())
+	})
+	db.DisableCompactions()
+
+	// Case 1: Lot's of uncompacted data in Head.
+
+	lbls := labels.Labels{labels.Label{Name: "foo_d", Value: "choco_bar"}}
+	// Append samples spanning 59 block ranges.
+	app := db.Appender(context.Background())
+	for i := int64(0); i < 60; i++ {
+		_, err := app.Add(lbls, blockRange*i, rand.Float64())
+		testutil.Ok(t, err)
+		_, err = app.Add(lbls, (blockRange*i)+blockRange/2, rand.Float64())
+		testutil.Ok(t, err)
+		// Rotate the WAL file so that there is >3 files for checkpoint to happen.
+		testutil.Ok(t, db.head.wal.NextSegment())
+	}
+	testutil.Ok(t, app.Commit())
+
+	// Check the existing WAL files.
+	first, last, err := wal.Segments(db.head.wal.Dir())
+	testutil.Ok(t, err)
+	testutil.Equals(t, 0, first)
+	testutil.Equals(t, 60, last)
+
+	testutil.Equals(t, 0.0, prom_testutil.ToFloat64(db.head.metrics.checkpointCreationTotal))
+	testutil.Ok(t, db.Compact())
+	testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.head.metrics.checkpointCreationTotal))
+
+	// As the data spans for 59 blocks, 58 go to disk and 1 remains in Head.
+	testutil.Equals(t, 58, len(db.Blocks()))
+	// Though WAL was truncated only once, head should be truncated after each compaction.
+	testutil.Equals(t, 58.0, prom_testutil.ToFloat64(db.head.metrics.headTruncateTotal))
+
+	// The compaction should have only truncated first 2/3 of WAL (while also rotating the files).
+	first, last, err = wal.Segments(db.head.wal.Dir())
+	testutil.Ok(t, err)
+	testutil.Equals(t, 40, first)
+	testutil.Equals(t, 61, last)
+
+	// The first checkpoint would be for first 2/3rd of WAL, hence till 39.
+	// That should be the last checkpoint.
+	_, cno, err := wal.LastCheckpoint(db.head.wal.Dir())
+	testutil.Ok(t, err)
+	testutil.Equals(t, 39, cno)
+
+	// Case 2: Old blocks on disk.
+	// The above blocks will act as old blocks.
+
+	// Creating a block to cover the data in the Head so that
+	// Head will skip the data during replay and start fresh.
+	blocks := db.Blocks()
+	newBlockMint := blocks[len(blocks)-1].Meta().MaxTime
+	newBlockMaxt := db.Head().MaxTime() + 1
+	testutil.Ok(t, db.Close())
+
+	createBlock(t, db.dir, genSeries(1, 1, newBlockMint, newBlockMaxt))
+
+	db, err = Open(db.dir, log.NewNopLogger(), prometheus.NewRegistry(), tsdbCfg)
+	testutil.Ok(t, err)
+	db.DisableCompactions()
+
+	// 1 block more.
+	testutil.Equals(t, 59, len(db.Blocks()))
+	// No series in Head because of this new block.
+	testutil.Equals(t, 0, int(db.head.NumSeries()))
+
+	// Adding sample way into the future.
+	app = db.Appender(context.Background())
+	_, err = app.Add(lbls, blockRange*120, rand.Float64())
+	testutil.Ok(t, err)
+	testutil.Ok(t, app.Commit())
+
+	// The mint of head is the last block maxt, that means the gap between mint and maxt
+	// of Head is too large. This will trigger many compactions.
+	testutil.Equals(t, newBlockMaxt, db.head.MinTime())
+
+	// Another WAL file was rotated.
+	first, last, err = wal.Segments(db.head.wal.Dir())
+	testutil.Ok(t, err)
+	testutil.Equals(t, 40, first)
+	testutil.Equals(t, 62, last)
+
+	testutil.Equals(t, 0.0, prom_testutil.ToFloat64(db.head.metrics.checkpointCreationTotal))
+	testutil.Ok(t, db.Compact())
+	testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.head.metrics.checkpointCreationTotal))
+
+	// No new blocks should be created as there was not data in between the new samples and the blocks.
+	testutil.Equals(t, 59, len(db.Blocks()))
+
+	// The compaction should have only truncated first 2/3 of WAL (while also rotating the files).
+	first, last, err = wal.Segments(db.head.wal.Dir())
+	testutil.Ok(t, err)
+	testutil.Equals(t, 55, first)
+	testutil.Equals(t, 63, last)
+
+	// The first checkpoint would be for first 2/3rd of WAL, hence till 54.
+	// That should be the last checkpoint.
+	_, cno, err = wal.LastCheckpoint(db.head.wal.Dir())
+	testutil.Ok(t, err)
+	testutil.Equals(t, 54, cno)
+}

tsdb/head.go

@@ -52,11 +52,12 @@ var (
 
 // Head handles reads and writes of time series data within a time window.
 type Head struct {
-	chunkRange       atomic.Int64
+	chunkRange            atomic.Int64
-	numSeries        atomic.Uint64
+	numSeries             atomic.Uint64
-	minTime, maxTime atomic.Int64 // Current min and max of the samples included in the head.
+	minTime, maxTime      atomic.Int64 // Current min and max of the samples included in the head.
-	minValidTime     atomic.Int64 // Mint allowed to be added to the head. It shouldn't be lower than the maxt of the last persisted block.
+	minValidTime          atomic.Int64 // Mint allowed to be added to the head. It shouldn't be lower than the maxt of the last persisted block.
-	lastSeriesID     atomic.Uint64
+	lastWALTruncationTime atomic.Int64
+	lastSeriesID          atomic.Uint64
 
 	metrics      *headMetrics
 	wal          *wal.WAL
@@ -323,6 +324,7 @@ func NewHead(r prometheus.Registerer, l log.Logger, wal *wal.WAL, chunkRange int
 	h.chunkRange.Store(chunkRange)
 	h.minTime.Store(math.MaxInt64)
 	h.maxTime.Store(math.MinInt64)
+	h.lastWALTruncationTime.Store(math.MinInt64)
 	h.metrics = newHeadMetrics(h, r)
 
 	if pool == nil {
@@ -776,8 +778,20 @@ func (h *Head) removeCorruptedMmappedChunks(err error) map[uint64][]*mmappedChun
 	return mmappedChunks
 }
 
-// Truncate removes old data before mint from the head.
+// Truncate removes old data before mint from the head and WAL.
 func (h *Head) Truncate(mint int64) (err error) {
+	initialize := h.MinTime() == math.MaxInt64
+	if err := h.truncateMemory(mint); err != nil {
+		return err
+	}
+	if initialize {
+		return nil
+	}
+	return h.truncateWAL(mint)
+}
+
+// truncateMemory removes old data before mint from the head.
+func (h *Head) truncateMemory(mint int64) (err error) {
 	defer func() {
 		if err != nil {
 			h.metrics.headTruncateFail.Inc()
@@ -813,11 +827,16 @@ func (h *Head) Truncate(mint int64) (err error) {
 	if err := h.chunkDiskMapper.Truncate(mint); err != nil {
 		return errors.Wrap(err, "truncate chunks.HeadReadWriter")
 	}
+	return nil
+}
 
-	if h.wal == nil {
+// truncateWAL removes old data before mint from the WAL.
+func (h *Head) truncateWAL(mint int64) error {
+	if h.wal == nil || mint <= h.lastWALTruncationTime.Load() {
 		return nil
 	}
-	start = time.Now()
+	start := time.Now()
+	h.lastWALTruncationTime.Store(mint)
 
 	first, last, err := wal.Segments(h.wal.Dir())
 	if err != nil {
@@ -825,8 +844,7 @@ func (h *Head) Truncate(mint int64) (err error) {
 	}
 	// Start a new segment, so low ingestion volume TSDB don't have more WAL than
 	// needed.
-	err = h.wal.NextSegment()
+	if err := h.wal.NextSegment(); err != nil {
-	if err != nil {
 		return errors.Wrap(err, "next segment")
 	}
 	last-- // Never consider last segment for checkpoint.
@@ -950,10 +968,19 @@ func (h *RangeHead) MinTime() int64 {
 	return h.mint
 }
 
+// MaxTime returns the max time of actual data fetch-able from the head.
+// This controls the chunks time range which is closed [b.MinTime, b.MaxTime].
 func (h *RangeHead) MaxTime() int64 {
 	return h.maxt
 }
 
+// BlockMaxTime returns the max time of the potential block created from this head.
+// It's different to MaxTime as we need to add +1 millisecond to block maxt because block
+// intervals are half-open: [b.MinTime, b.MaxTime). Block intervals are always +1 than the total samples it includes.
+func (h *RangeHead) BlockMaxTime() int64 {
+	return h.MaxTime() + 1
+}
+
 func (h *RangeHead) NumSeries() uint64 {
 	return h.head.NumSeries()
 }

tsdb/head_test.go

@@ -579,14 +579,14 @@ func TestHeadDeleteSimple(t *testing.T) {
 				}
 				testutil.Ok(t, app.Commit())
 
-				// Compare the samples for both heads - before and after the reload.
+				// Compare the samples for both heads - before and after the reloadBlocks.
-				reloadedW, err := wal.New(nil, nil, w.Dir(), compress) // Use a new wal to ensure deleted samples are gone even after a reload.
+				reloadedW, err := wal.New(nil, nil, w.Dir(), compress) // Use a new wal to ensure deleted samples are gone even after a reloadBlocks.
 				testutil.Ok(t, err)
 				reloadedHead, err := NewHead(nil, nil, reloadedW, 1000, reloadedW.Dir(), nil, DefaultStripeSize, nil)
 				testutil.Ok(t, err)
 				testutil.Ok(t, reloadedHead.Init(0))
 
-				// Compare the query results for both heads - before and after the reload.
+				// Compare the query results for both heads - before and after the reloadBlocks.
 			Outer:
 				for _, h := range []*Head{head, reloadedHead} {
 					q, err := NewBlockQuerier(h, h.MinTime(), h.MaxTime())