diff --git a/tsdb/block.go b/tsdb/block.go
index 399a2eed1f..3c820fc626 100644
--- a/tsdb/block.go
+++ b/tsdb/block.go
@@ -169,6 +169,9 @@ type BlockMeta struct {
 
 	// Version of the index format.
 	Version int `json:"version"`
+
+	// OutOfOrder is true if the block was directly created from out-of-order samples.
+	OutOfOrder bool `json:"out_of_order"`
 }
 
 // BlockStats contains stats about contents of a block.
diff --git a/tsdb/compact.go b/tsdb/compact.go
index 80d3f62536..2db02c340e 100644
--- a/tsdb/compact.go
+++ b/tsdb/compact.go
@@ -608,9 +608,10 @@ func (c *LeveledCompactor) compactOOO(dest string, oooHead *OOOCompactionHead, s
 		for jx := range outBlocks[ix] {
 			uid := ulid.MustNew(outBlocksTime, rand.Reader)
 			meta := &BlockMeta{
-				ULID:    uid,
-				MinTime: mint,
-				MaxTime: maxt,
+				ULID:       uid,
+				MinTime:    mint,
+				MaxTime:    maxt,
+				OutOfOrder: true,
 			}
 			meta.Compaction.Level = 1
 			meta.Compaction.Sources = []ulid.ULID{uid}
diff --git a/tsdb/db.go b/tsdb/db.go
index 305559e222..2670118bea 100644
--- a/tsdb/db.go
+++ b/tsdb/db.go
@@ -838,10 +838,13 @@ func open(dir string, l log.Logger, r prometheus.Registerer, opts *Options, rngs
 	}
 	// Set the min valid time for the ingested samples
 	// to be no lower than the maxt of the last block.
-	blocks := db.Blocks()
 	minValidTime := int64(math.MinInt64)
-	if len(blocks) > 0 {
-		minValidTime = blocks[len(blocks)-1].Meta().MaxTime
+	// We do not consider blocks created from out-of-order samples for Head's minValidTime
+	// since minValidTime is only for the in-order data and we do not want to discard unnecessary
+	// samples from the Head.
+	inOrderMaxTime, ok := db.inOrderBlocksMaxTime()
+	if ok {
+		minValidTime = inOrderMaxTime
 	}
 
 	if initErr := db.head.Init(minValidTime); initErr != nil {
@@ -858,7 +861,6 @@ func open(dir string, l log.Logger, r prometheus.Registerer, opts *Options, rngs
 				return nil, errors.Wrap(err, "repair corrupted WAL")
 			}
 		}
-
 	}
 
 	go db.run()
@@ -991,6 +993,7 @@ func (db *DB) ApplyConfig(conf *config.Config) error {
 		}
 	}
 
+	db.opts.OutOfOrderTimeWindow = oooTimeWindow
 	db.head.ApplyConfig(conf, wblog)
 
 	if !db.oooWasEnabled.Load() {
@@ -1237,10 +1240,11 @@ func (db *DB) reload() error {
 	if err := db.reloadBlocks(); err != nil {
 		return errors.Wrap(err, "reloadBlocks")
 	}
-	if len(db.blocks) == 0 {
+	maxt, ok := db.inOrderBlocksMaxTime()
+	if !ok {
 		return nil
 	}
-	if err := db.head.Truncate(db.blocks[len(db.blocks)-1].MaxTime()); err != nil {
+	if err := db.head.Truncate(maxt); err != nil {
 		return errors.Wrap(err, "head truncate")
 	}
 	return nil
@@ -1636,6 +1640,30 @@ func (db *DB) Blocks() []*Block {
 	return db.blocks
 }
 
+// inOrderBlocksMaxTime returns the max time among the blocks that were not totally created
+// out of out-of-order data. If the returned boolean is true, it means there is at least
+// one such block.
+func (db *DB) inOrderBlocksMaxTime() (maxt int64, ok bool) {
+	maxt, ok, hasOOO := int64(math.MinInt64), false, false
+	// If blocks are overlapping, last block might not have the max time. So check all blocks.
+	for _, b := range db.Blocks() {
+		hasOOO = hasOOO || b.meta.OutOfOrder
+		if !b.meta.OutOfOrder && b.meta.MaxTime > maxt {
+			ok = true
+			maxt = b.meta.MaxTime
+		}
+	}
+	if !hasOOO && ok && db.opts.OutOfOrderTimeWindow > 0 {
+		// Temporary patch. To be removed by mid July 2022.
+		// Before this patch, blocks did not have "out_of_order" in their meta, so we cannot
+		// say which block has the out_of_order data. In that case the out-of-order block can be
+		// up to 2 block ranges ahead of the latest in-order block.
+		// Note: if hasOOO was true, it means the latest block has the new meta and is taken care in inOrderBlocksMaxTime().
+		maxt -= 2 * db.opts.MinBlockDuration
+	}
+	return maxt, ok
+}
+
 // Head returns the databases's head.
 func (db *DB) Head() *Head {
 	return db.head
diff --git a/tsdb/db_test.go b/tsdb/db_test.go
index a629843b45..20dea2642b 100644
--- a/tsdb/db_test.go
+++ b/tsdb/db_test.go
@@ -5192,3 +5192,133 @@ func TestOutOfOrderRuntimeConfig(t *testing.T) {
 		require.Nil(t, db.head.wbl)
 	})
 }
+
+func TestNoGapAfterRestartWithOOO(t *testing.T) {
+	series1 := labels.FromStrings("foo", "bar1")
+	addSamples := func(t *testing.T, db *DB, fromMins, toMins int64, success bool) {
+		app := db.Appender(context.Background())
+		for min := fromMins; min <= toMins; min++ {
+			ts := min * time.Minute.Milliseconds()
+			_, err := app.Append(0, series1, ts, float64(ts))
+			if success {
+				require.NoError(t, err)
+			} else {
+				require.Error(t, err)
+			}
+		}
+		require.NoError(t, app.Commit())
+	}
+
+	verifySamples := func(t *testing.T, db *DB, fromMins, toMins int64) {
+		var expSamples []tsdbutil.Sample
+		for min := fromMins; min <= toMins; min++ {
+			ts := min * time.Minute.Milliseconds()
+			expSamples = append(expSamples, sample{t: ts, v: float64(ts)})
+		}
+
+		expRes := map[string][]tsdbutil.Sample{
+			series1.String(): expSamples,
+		}
+
+		q, err := db.Querier(context.Background(), math.MinInt64, math.MaxInt64)
+		require.NoError(t, err)
+
+		actRes := query(t, q, labels.MustNewMatcher(labels.MatchRegexp, "foo", "bar.*"))
+		require.Equal(t, expRes, actRes)
+	}
+
+	cases := []struct {
+		inOrderMint, inOrderMaxt int64
+		oooMint, oooMaxt         int64
+		// After compaction.
+		blockRanges        [][2]int64
+		headMint, headMaxt int64
+		// Head time ranges after restart for old blocks.
+		legacyHeadMint, legacyHeadMaxt int64
+	}{
+		{
+			300, 490,
+			489, 489,
+			[][2]int64{{300, 360}, {480, 600}},
+			360, 490,
+			360, 490, // OOO blocks is already 2 ranges ahead of the in-order block.
+		},
+		{
+			300, 490,
+			479, 479,
+			[][2]int64{{300, 360}, {360, 480}},
+			360, 490,
+			240, 490, // OOO block was only 1 range ahead of in-order block.
+		},
+	}
+
+	for i, c := range cases {
+		// legacy = true means the out-of-order blocks don't have the `out_of_order: true` metadata.
+		for _, legacy := range []bool{false, true} {
+			t.Run(fmt.Sprintf("case=%d,legacy=%t", i, legacy), func(t *testing.T) {
+				dir := t.TempDir()
+
+				opts := DefaultOptions()
+				opts.OutOfOrderTimeWindow = 30 * time.Minute.Milliseconds()
+
+				db, err := Open(dir, nil, nil, opts, nil)
+				require.NoError(t, err)
+				db.DisableCompactions()
+				t.Cleanup(func() {
+					require.NoError(t, db.Close())
+				})
+
+				// 3h10m=190m worth in-order data.
+				addSamples(t, db, c.inOrderMint, c.inOrderMaxt, true)
+				verifySamples(t, db, c.inOrderMint, c.inOrderMaxt)
+
+				// One ooo samples.
+				addSamples(t, db, c.oooMint, c.oooMaxt, true)
+				verifySamples(t, db, c.inOrderMint, c.inOrderMaxt)
+
+				// We get 2 blocks. 1 from OOO, 1 from in-order.
+				require.NoError(t, db.Compact())
+				verifyBlockRanges := func() {
+					blocks := db.Blocks()
+					require.Equal(t, len(c.blockRanges), len(blocks))
+					for j, br := range c.blockRanges {
+						require.Equal(t, br[0]*time.Minute.Milliseconds(), blocks[j].MinTime())
+						require.Equal(t, br[1]*time.Minute.Milliseconds(), blocks[j].MaxTime())
+					}
+				}
+				verifyBlockRanges()
+				require.Equal(t, c.headMint*time.Minute.Milliseconds(), db.head.MinTime())
+				require.Equal(t, c.headMaxt*time.Minute.Milliseconds(), db.head.MaxTime())
+
+				if legacy {
+					// In the legacy version, the blocks from out-of-order data did not write a
+					// "out_of_order: true" to the meta. So we remove it here.
+					for _, b := range db.Blocks() {
+						m, _, err := readMetaFile(b.Dir())
+						require.NoError(t, err)
+						m.OutOfOrder = false
+						_, err = writeMetaFile(log.NewNopLogger(), b.Dir(), m)
+						require.NoError(t, err)
+					}
+				}
+
+				// Restart and expect all samples to be present.
+				require.NoError(t, db.Close())
+
+				db, err = Open(dir, nil, nil, opts, nil)
+				require.NoError(t, err)
+				db.DisableCompactions()
+
+				verifyBlockRanges()
+				if legacy {
+					require.Equal(t, c.legacyHeadMint*time.Minute.Milliseconds(), db.head.MinTime())
+					require.Equal(t, c.legacyHeadMaxt*time.Minute.Milliseconds(), db.head.MaxTime())
+				} else {
+					require.Equal(t, c.headMint*time.Minute.Milliseconds(), db.head.MinTime())
+					require.Equal(t, c.headMaxt*time.Minute.Milliseconds(), db.head.MaxTime())
+				}
+				verifySamples(t, db, c.inOrderMint, c.inOrderMaxt)
+			})
+		}
+	}
+}
diff --git a/tsdb/head_wal.go b/tsdb/head_wal.go
index e92bcc8b88..6b6e835769 100644
--- a/tsdb/head_wal.go
+++ b/tsdb/head_wal.go
@@ -464,9 +464,9 @@ func (wp *walSubsetProcessor) waitUntilIdle() {
 }
 
 func (h *Head) loadWbl(r *wal.Reader, multiRef map[chunks.HeadSeriesRef]chunks.HeadSeriesRef, lastMmapRef chunks.ChunkDiskMapperRef) (err error) {
-	// Track number of samples that referenced a series we don't know about
+	// Track number of samples, m-map markers, that referenced a series we don't know about
 	// for error reporting.
-	var unknownRefs atomic.Uint64
+	var unknownRefs, mmapMarkerUnknownRefs atomic.Uint64
 
 	lastSeq, lastOff := lastMmapRef.Unpack()
 	// Start workers that each process samples for a partition of the series ID space.
@@ -593,9 +593,13 @@ func (h *Head) loadWbl(r *wal.Reader, multiRef map[chunks.HeadSeriesRef]chunks.H
 					continue
 				}
 
+				if r, ok := multiRef[rm.Ref]; ok {
+					rm.Ref = r
+				}
+
 				ms := h.series.getByID(rm.Ref)
 				if ms == nil {
-					unknownRefs.Inc()
+					mmapMarkerUnknownRefs.Inc()
 					continue
 				}
 
@@ -635,8 +639,8 @@ func (h *Head) loadWbl(r *wal.Reader, multiRef map[chunks.HeadSeriesRef]chunks.H
 		return errors.Wrap(r.Err(), "read records")
 	}
 
-	if unknownRefs.Load() > 0 {
-		level.Warn(h.logger).Log("msg", "Unknown series references for ooo WAL replay", "samples", unknownRefs.Load())
+	if unknownRefs.Load() > 0 || mmapMarkerUnknownRefs.Load() > 0 {
+		level.Warn(h.logger).Log("msg", "Unknown series references for ooo WAL replay", "samples", unknownRefs.Load(), "mmap_markers", mmapMarkerUnknownRefs.Load())
 	}
 	return nil
 }