diff --git a/app/vmstorage/main.go b/app/vmstorage/main.go
index 1e08b323a..a78a17946 100644
--- a/app/vmstorage/main.go
+++ b/app/vmstorage/main.go
@@ -285,6 +285,18 @@ func registerStorageMetrics(strg *storage.Storage) {
 	metrics.NewGauge(`vm_missing_tsids_for_metric_id_total`, func() float64 {
 		return float64(idbm().MissingTSIDsForMetricID)
 	})
+	metrics.NewGauge(`vm_recent_hour_metric_ids_search_calls`, func() float64 {
+		return float64(idbm().RecentHourMetricIDsSearchCalls)
+	})
+	metrics.NewGauge(`vm_recent_hour_metric_ids_search_hits`, func() float64 {
+		return float64(idbm().RecentHourMetricIDsSearchHits)
+	})
+	metrics.NewGauge(`vm_date_metric_ids_search_calls`, func() float64 {
+		return float64(idbm().DateMetricIDsSearchCalls)
+	})
+	metrics.NewGauge(`vm_date_metric_ids_search_hits`, func() float64 {
+		return float64(idbm().DateMetricIDsSearchHits)
+	})
 
 	metrics.NewGauge(`vm_assisted_merges_total{type="storage/small"}`, func() float64 {
 		return float64(tm().SmallAssistedMerges)
diff --git a/lib/storage/index_db.go b/lib/storage/index_db.go
index d6aa7e514..e0069771a 100644
--- a/lib/storage/index_db.go
+++ b/lib/storage/index_db.go
@@ -71,15 +71,33 @@ type indexDB struct {
 	deletedMetricIDs           atomic.Value
 	deletedMetricIDsUpdateLock sync.Mutex
 
+	// Global lists of metric ids for the current and the previous hours.
+	// They are used for fast lookups on small time ranges covering
+	// up to two last hours.
+	currHourMetricIDs *atomic.Value
+	prevHourMetricIDs *atomic.Value
+
 	// The number of missing MetricID -> TSID entries.
 	// High rate for this value means corrupted indexDB.
 	missingTSIDsForMetricID uint64
 
+	// The number of calls to search for metric ids for recent hours.
+	recentHourMetricIDsSearchCalls uint64
+
+	// The number of cache hits during search for metric ids in recent hours.
+	recentHourMetricIDsSearchHits uint64
+
+	// The number of searches for metric ids by days.
+	dateMetricIDsSearchCalls uint64
+
+	// The number of successful searches for metric ids by days.
+	dateMetricIDsSearchHits uint64
+
 	mustDrop uint64
 }
 
 // openIndexDB opens index db from the given path with the given caches.
-func openIndexDB(path string, metricIDCache, metricNameCache *fastcache.Cache) (*indexDB, error) {
+func openIndexDB(path string, metricIDCache, metricNameCache *fastcache.Cache, currHourMetricIDs, prevHourMetricIDs *atomic.Value) (*indexDB, error) {
 	tb, err := mergeset.OpenTable(path)
 	if err != nil {
 		return nil, fmt.Errorf("cannot open indexDB %q: %s", path, err)
@@ -99,6 +117,9 @@ func openIndexDB(path string, metricIDCache, metricNameCache *fastcache.Cache) (
 		tagCache:        tagCache,
 		metricIDCache:   metricIDCache,
 		metricNameCache: metricNameCache,
+
+		currHourMetricIDs: currHourMetricIDs,
+		prevHourMetricIDs: prevHourMetricIDs,
 	}
 
 	is := db.getIndexSearch()
@@ -125,6 +146,11 @@ type IndexDBMetrics struct {
 
 	MissingTSIDsForMetricID uint64
 
+	RecentHourMetricIDsSearchCalls uint64
+	RecentHourMetricIDsSearchHits  uint64
+	DateMetricIDsSearchCalls       uint64
+	DateMetricIDsSearchHits        uint64
+
 	mergeset.TableMetrics
 }
 
@@ -145,6 +171,10 @@ func (db *indexDB) UpdateMetrics(m *IndexDBMetrics) {
 
 	m.IndexDBRefCount += atomic.LoadUint64(&db.refCount)
 	m.MissingTSIDsForMetricID += atomic.LoadUint64(&db.missingTSIDsForMetricID)
+	m.RecentHourMetricIDsSearchCalls += atomic.LoadUint64(&db.recentHourMetricIDsSearchCalls)
+	m.RecentHourMetricIDsSearchHits += atomic.LoadUint64(&db.recentHourMetricIDsSearchHits)
+	m.DateMetricIDsSearchCalls += atomic.LoadUint64(&db.dateMetricIDsSearchCalls)
+	m.DateMetricIDsSearchHits += atomic.LoadUint64(&db.dateMetricIDsSearchHits)
 
 	db.tb.UpdateMetrics(&m.TableMetrics)
 	db.doExtDB(func(extDB *indexDB) {
@@ -1541,6 +1571,16 @@ func (is *indexSearch) getMetricIDsForTimeRange(tr TimeRange, maxMetrics int) (m
 	if tr.isZero() {
 		return nil, errMissingMetricIDsForDate
 	}
+	atomic.AddUint64(&is.db.recentHourMetricIDsSearchCalls, 1)
+	if metricIDs, ok := is.getMetricIDsForRecentHours(tr, maxMetrics); ok {
+		// Fast path: tr covers the current and / or the previous hour.
+		// Return the full list of metric ids for this time range.
+		atomic.AddUint64(&is.db.recentHourMetricIDsSearchHits, 1)
+		return metricIDs, nil
+	}
+
+	// Slow path: collect the metric ids for all the days covering the given tr.
+	atomic.AddUint64(&is.db.dateMetricIDsSearchCalls, 1)
 	minDate := tr.MinTimestamp / msecPerDay
 	maxDate := tr.MaxTimestamp / msecPerDay
 	if maxDate-minDate > 40 {
@@ -1554,9 +1594,64 @@ func (is *indexSearch) getMetricIDsForTimeRange(tr TimeRange, maxMetrics int) (m
 		}
 		minDate++
 	}
+	atomic.AddUint64(&is.db.dateMetricIDsSearchHits, 1)
 	return metricIDs, nil
 }
 
+func (is *indexSearch) getMetricIDsForRecentHours(tr TimeRange, maxMetrics int) (map[uint64]struct{}, bool) {
+	minHour := uint64(tr.MinTimestamp) / msecPerHour
+	maxHour := uint64(tr.MaxTimestamp) / msecPerHour
+	if is.db.currHourMetricIDs == nil {
+		return nil, false
+	}
+	hmCurr := is.db.currHourMetricIDs.Load().(*hourMetricIDs)
+	if maxHour == hmCurr.hour && minHour == maxHour && hmCurr.isFull {
+		// The tr fits the current hour.
+		// Return a copy of hmCurr.m, because the caller may modify
+		// the returned map.
+		if len(hmCurr.m) > maxMetrics {
+			return nil, false
+		}
+		return getMetricIDsCopy(hmCurr.m), true
+	}
+	if is.db.prevHourMetricIDs == nil {
+		return nil, false
+	}
+	hmPrev := is.db.prevHourMetricIDs.Load().(*hourMetricIDs)
+	if maxHour == hmPrev.hour && minHour == maxHour && hmPrev.isFull {
+		// The tr fits the previous hour.
+		// Return a copy of hmPrev.m, because the caller may modify
+		// the returned map.
+		if len(hmPrev.m) > maxMetrics {
+			return nil, false
+		}
+		return getMetricIDsCopy(hmPrev.m), true
+	}
+	if maxHour == hmCurr.hour && minHour == hmPrev.hour && hmCurr.isFull && hmPrev.isFull {
+		// The tr spans the previous and the current hours.
+		if len(hmCurr.m)+len(hmPrev.m) > maxMetrics {
+			return nil, false
+		}
+		metricIDs := make(map[uint64]struct{}, len(hmCurr.m)+len(hmPrev.m))
+		for metricID := range hmCurr.m {
+			metricIDs[metricID] = struct{}{}
+		}
+		for metricID := range hmPrev.m {
+			metricIDs[metricID] = struct{}{}
+		}
+		return metricIDs, true
+	}
+	return nil, false
+}
+
+func getMetricIDsCopy(src map[uint64]struct{}) map[uint64]struct{} {
+	dst := make(map[uint64]struct{}, len(src))
+	for metricID := range src {
+		dst[metricID] = struct{}{}
+	}
+	return dst
+}
+
 func (db *indexDB) storeDateMetricID(date, metricID uint64) error {
 	is := db.getIndexSearch()
 	ok, err := is.hasDateMetricID(date, metricID)
diff --git a/lib/storage/index_db_test.go b/lib/storage/index_db_test.go
index 071859119..1944a7aef 100644
--- a/lib/storage/index_db_test.go
+++ b/lib/storage/index_db_test.go
@@ -61,7 +61,7 @@ func TestIndexDBOpenClose(t *testing.T) {
 	defer metricIDCache.Reset()
 	defer metricNameCache.Reset()
 	for i := 0; i < 5; i++ {
-		db, err := openIndexDB("test-index-db", metricIDCache, metricNameCache)
+		db, err := openIndexDB("test-index-db", metricIDCache, metricNameCache, nil, nil)
 		if err != nil {
 			t.Fatalf("cannot open indexDB: %s", err)
 		}
@@ -83,7 +83,7 @@ func TestIndexDB(t *testing.T) {
 		defer metricIDCache.Reset()
 		defer metricNameCache.Reset()
 		dbName := "test-index-db-serial"
-		db, err := openIndexDB(dbName, metricIDCache, metricNameCache)
+		db, err := openIndexDB(dbName, metricIDCache, metricNameCache, nil, nil)
 		if err != nil {
 			t.Fatalf("cannot open indexDB: %s", err)
 		}
@@ -113,7 +113,7 @@ func TestIndexDB(t *testing.T) {
 
 		// Re-open the db and verify it works as expected.
 		db.MustClose()
-		db, err = openIndexDB(dbName, metricIDCache, metricNameCache)
+		db, err = openIndexDB(dbName, metricIDCache, metricNameCache, nil, nil)
 		if err != nil {
 			t.Fatalf("cannot open indexDB: %s", err)
 		}
@@ -134,7 +134,7 @@ func TestIndexDB(t *testing.T) {
 		defer metricIDCache.Reset()
 		defer metricNameCache.Reset()
 		dbName := "test-index-db-concurrent"
-		db, err := openIndexDB(dbName, metricIDCache, metricNameCache)
+		db, err := openIndexDB(dbName, metricIDCache, metricNameCache, nil, nil)
 		if err != nil {
 			t.Fatalf("cannot open indexDB: %s", err)
 		}
diff --git a/lib/storage/index_db_timing_test.go b/lib/storage/index_db_timing_test.go
index e830a3777..ae18371ad 100644
--- a/lib/storage/index_db_timing_test.go
+++ b/lib/storage/index_db_timing_test.go
@@ -17,7 +17,7 @@ func BenchmarkIndexDBAddTSIDs(b *testing.B) {
 	defer metricIDCache.Reset()
 	defer metricNameCache.Reset()
 	const dbName = "bench-index-db-add-tsids"
-	db, err := openIndexDB(dbName, metricIDCache, metricNameCache)
+	db, err := openIndexDB(dbName, metricIDCache, metricNameCache, nil, nil)
 	if err != nil {
 		b.Fatalf("cannot open indexDB: %s", err)
 	}
@@ -77,7 +77,7 @@ func BenchmarkIndexDBSearchTSIDs(b *testing.B) {
 	defer metricIDCache.Reset()
 	defer metricNameCache.Reset()
 	const dbName = "bench-index-db-search-tsids"
-	db, err := openIndexDB(dbName, metricIDCache, metricNameCache)
+	db, err := openIndexDB(dbName, metricIDCache, metricNameCache, nil, nil)
 	if err != nil {
 		b.Fatalf("cannot open indexDB: %s", err)
 	}
@@ -148,7 +148,7 @@ func BenchmarkIndexDBGetTSIDs(b *testing.B) {
 	defer metricIDCache.Reset()
 	defer metricNameCache.Reset()
 	const dbName = "bench-index-db-get-tsids"
-	db, err := openIndexDB(dbName, metricIDCache, metricNameCache)
+	db, err := openIndexDB(dbName, metricIDCache, metricNameCache, nil, nil)
 	if err != nil {
 		b.Fatalf("cannot open indexDB: %s", err)
 	}
diff --git a/lib/storage/storage.go b/lib/storage/storage.go
index 58b148805..31f5f6d4f 100644
--- a/lib/storage/storage.go
+++ b/lib/storage/storage.go
@@ -50,15 +50,20 @@ type Storage struct {
 	// dateMetricIDCache is (Date, MetricID) cache.
 	dateMetricIDCache *fastcache.Cache
 
-	// Fast cache for today MetricID values.
-	todayMetricIDs            atomic.Value
-	pendingTodayMetricIDsLock sync.Mutex
-	pendingTodayMetricIDs     map[uint64]struct{}
+	// Fast cache for MetricID values occured during the current hour.
+	currHourMetricIDs atomic.Value
+
+	// Fast cache for MetricID values occured during the previous hour.
+	prevHourMetricIDs atomic.Value
+
+	// Pending MetricID values to be added to currHourMetricIDs.
+	pendingHourMetricIDsLock sync.Mutex
+	pendingHourMetricIDs     map[uint64]struct{}
 
 	stop chan struct{}
 
-	todayMetricIDsUpdaterWG sync.WaitGroup
-	retentionWatcherWG      sync.WaitGroup
+	currHourMetricIDsUpdaterWG sync.WaitGroup
+	retentionWatcherWG         sync.WaitGroup
 }
 
 // OpenStorage opens storage on the given path with the given number of retention months.
@@ -113,7 +118,7 @@ func OpenStorage(path string, retentionMonths int) (*Storage, error) {
 	if err := fs.MkdirAllIfNotExist(idbSnapshotsPath); err != nil {
 		return nil, fmt.Errorf("cannot create %q: %s", idbSnapshotsPath, err)
 	}
-	idbCurr, idbPrev, err := openIndexDBTables(idbPath, s.metricIDCache, s.metricNameCache)
+	idbCurr, idbPrev, err := openIndexDBTables(idbPath, s.metricIDCache, s.metricNameCache, &s.currHourMetricIDs, &s.prevHourMetricIDs)
 	if err != nil {
 		return nil, fmt.Errorf("cannot open indexdb tables at %q: %s", idbPath, err)
 	}
@@ -129,9 +134,9 @@ func OpenStorage(path string, retentionMonths int) (*Storage, error) {
 	}
 	s.tb = tb
 
-	s.todayMetricIDs.Store(&todayMetricIDs{})
-	s.pendingTodayMetricIDs = make(map[uint64]struct{})
-	s.startTodayMetricIDsUpdater()
+	s.currHourMetricIDs.Store(&hourMetricIDs{})
+	s.pendingHourMetricIDs = make(map[uint64]struct{})
+	s.startCurrHourMetricIDsUpdater()
 
 	s.startRetentionWatcher()
 
@@ -352,25 +357,25 @@ func (s *Storage) retentionWatcher() {
 	}
 }
 
-func (s *Storage) startTodayMetricIDsUpdater() {
-	s.todayMetricIDsUpdaterWG.Add(1)
+func (s *Storage) startCurrHourMetricIDsUpdater() {
+	s.currHourMetricIDsUpdaterWG.Add(1)
 	go func() {
-		s.todayMetricIDsUpdater()
-		s.todayMetricIDsUpdaterWG.Done()
+		s.currHourMetricIDsUpdater()
+		s.currHourMetricIDsUpdaterWG.Done()
 	}()
 }
 
-var todayMetricIDsUpdateInterval = time.Second * 10
+var currHourMetricIDsUpdateInterval = time.Second * 10
 
-func (s *Storage) todayMetricIDsUpdater() {
-	t := time.NewTimer(todayMetricIDsUpdateInterval)
+func (s *Storage) currHourMetricIDsUpdater() {
+	t := time.NewTimer(currHourMetricIDsUpdateInterval)
 	for {
 		select {
 		case <-s.stop:
 			return
 		case <-t.C:
-			s.updateTodayMetricIDs()
-			t.Reset(todayMetricIDsUpdateInterval)
+			s.updateCurrHourMetricIDs()
+			t.Reset(currHourMetricIDsUpdateInterval)
 		}
 	}
 }
@@ -379,7 +384,7 @@ func (s *Storage) mustRotateIndexDB() {
 	// Create new indexdb table.
 	newTableName := nextIndexDBTableName()
 	idbNewPath := s.path + "/indexdb/" + newTableName
-	idbNew, err := openIndexDB(idbNewPath, s.metricIDCache, s.metricNameCache)
+	idbNew, err := openIndexDB(idbNewPath, s.metricIDCache, s.metricNameCache, &s.currHourMetricIDs, &s.prevHourMetricIDs)
 	if err != nil {
 		logger.Panicf("FATAL: cannot create new indexDB at %q: %s", idbNewPath, err)
 	}
@@ -413,7 +418,7 @@ func (s *Storage) MustClose() {
 	close(s.stop)
 
 	s.retentionWatcherWG.Wait()
-	s.todayMetricIDsUpdaterWG.Wait()
+	s.currHourMetricIDsUpdaterWG.Wait()
 
 	s.tb.MustClose()
 	s.idb().MustClose()
@@ -700,6 +705,7 @@ func (s *Storage) add(rows []rawRow, mrs []MetricRow, precisionBits uint8) ([]ra
 
 func (s *Storage) updateDateMetricIDCache(rows []rawRow, errors []error) []error {
 	var date uint64
+	var hour uint64
 	var prevTimestamp int64
 	kb := kbPool.Get()
 	defer kbPool.Put(kb)
@@ -711,18 +717,20 @@ func (s *Storage) updateDateMetricIDCache(rows []rawRow, errors []error) []error
 		r := &rows[i]
 		if r.Timestamp != prevTimestamp {
 			date = uint64(r.Timestamp) / msecPerDay
+			hour = uint64(r.Timestamp) / msecPerHour
 			prevTimestamp = r.Timestamp
 		}
 		metricID := r.TSID.MetricID
-		tm := s.todayMetricIDs.Load().(*todayMetricIDs)
-		if date == tm.date {
-			if _, ok := tm.m[metricID]; ok {
-				// Fast path: the (date, metricID) entry is in the fast cache with today's metric ids.
+		hm := s.currHourMetricIDs.Load().(*hourMetricIDs)
+		if hour == hm.hour {
+			// The r belongs to the current hour. Check for the current hour cache.
+			if _, ok := hm.m[metricID]; ok {
+				// Fast path: the metricID is in the current hour cache.
 				continue
 			}
-			s.pendingTodayMetricIDsLock.Lock()
-			s.pendingTodayMetricIDs[metricID] = struct{}{}
-			s.pendingTodayMetricIDsLock.Unlock()
+			s.pendingHourMetricIDsLock.Lock()
+			s.pendingHourMetricIDs[metricID] = struct{}{}
+			s.pendingHourMetricIDsLock.Unlock()
 		}
 
 		// Slower path: check global cache for (date, metricID) entry.
@@ -744,51 +752,52 @@ func (s *Storage) updateDateMetricIDCache(rows []rawRow, errors []error) []error
 	return errors
 }
 
-func (s *Storage) updateTodayMetricIDs() {
-	tm := s.todayMetricIDs.Load().(*todayMetricIDs)
-	s.pendingTodayMetricIDsLock.Lock()
-	newMetricIDsLen := len(s.pendingTodayMetricIDs)
-	s.pendingTodayMetricIDsLock.Unlock()
-	today := uint64(timestampFromTime(time.Now())) / msecPerDay
-	if newMetricIDsLen == 0 {
-		if tm.date == today {
-			// Fast path: nothing to update.
-			return
-		}
-		// Reset tm because of new day.
-		tmNew := &todayMetricIDs{
-			m:    make(map[uint64]struct{}, len(tm.m)),
-			date: today,
-		}
-		s.todayMetricIDs.Store(tmNew)
+func (s *Storage) updateCurrHourMetricIDs() {
+	hm := s.currHourMetricIDs.Load().(*hourMetricIDs)
+	s.pendingHourMetricIDsLock.Lock()
+	newMetricIDsLen := len(s.pendingHourMetricIDs)
+	s.pendingHourMetricIDsLock.Unlock()
+	hour := uint64(timestampFromTime(time.Now())) / msecPerHour
+	if newMetricIDsLen == 0 && hm.hour == hour {
+		// Fast path: nothing to update.
 		return
 	}
 
-	// Slow path: tm.m must be updated with non-empty s.pendingTodayMetricIDs.
-	m := make(map[uint64]struct{}, len(tm.m)+newMetricIDsLen)
-	if tm.date == today {
-		for metricID := range tm.m {
+	// Slow path: hm.m must be updated with non-empty s.pendingHourMetricIDs.
+	var m map[uint64]struct{}
+	isFull := hm.isFull
+	if hm.hour == hour {
+		m = make(map[uint64]struct{}, len(hm.m)+newMetricIDsLen)
+		for metricID := range hm.m {
 			m[metricID] = struct{}{}
 		}
+	} else {
+		m = make(map[uint64]struct{}, newMetricIDsLen)
+		isFull = true
 	}
-	s.pendingTodayMetricIDsLock.Lock()
-	newMetricIDs := s.pendingTodayMetricIDs
-	s.pendingTodayMetricIDs = make(map[uint64]struct{}, len(newMetricIDs))
-	s.pendingTodayMetricIDsLock.Unlock()
+	s.pendingHourMetricIDsLock.Lock()
+	newMetricIDs := s.pendingHourMetricIDs
+	s.pendingHourMetricIDs = make(map[uint64]struct{}, len(newMetricIDs))
+	s.pendingHourMetricIDsLock.Unlock()
 	for metricID := range newMetricIDs {
 		m[metricID] = struct{}{}
 	}
 
-	tmNew := &todayMetricIDs{
-		m:    m,
-		date: today,
+	hmNew := &hourMetricIDs{
+		m:      m,
+		hour:   hour,
+		isFull: isFull,
+	}
+	s.currHourMetricIDs.Store(hmNew)
+	if hm.hour != hour {
+		s.prevHourMetricIDs.Store(hm)
 	}
-	s.todayMetricIDs.Store(tmNew)
 }
 
-type todayMetricIDs struct {
-	m    map[uint64]struct{}
-	date uint64
+type hourMetricIDs struct {
+	m      map[uint64]struct{}
+	hour   uint64
+	isFull bool
 }
 
 func (s *Storage) getTSIDFromCache(dst *TSID, metricName []byte) bool {
@@ -802,7 +811,7 @@ func (s *Storage) putTSIDToCache(tsid *TSID, metricName []byte) {
 	s.tsidCache.Set(metricName, buf)
 }
 
-func openIndexDBTables(path string, metricIDCache, metricNameCache *fastcache.Cache) (curr, prev *indexDB, err error) {
+func openIndexDBTables(path string, metricIDCache, metricNameCache *fastcache.Cache, currHourMetricIDs, prevHourMetricIDs *atomic.Value) (curr, prev *indexDB, err error) {
 	if err := fs.MkdirAllIfNotExist(path); err != nil {
 		return nil, nil, fmt.Errorf("cannot create directory %q: %s", path, err)
 	}
@@ -863,12 +872,12 @@ func openIndexDBTables(path string, metricIDCache, metricNameCache *fastcache.Ca
 	// Open the last two tables.
 	currPath := path + "/" + tableNames[len(tableNames)-1]
 
-	curr, err = openIndexDB(currPath, metricIDCache, metricNameCache)
+	curr, err = openIndexDB(currPath, metricIDCache, metricNameCache, currHourMetricIDs, prevHourMetricIDs)
 	if err != nil {
 		return nil, nil, fmt.Errorf("cannot open curr indexdb table at %q: %s", currPath, err)
 	}
 	prevPath := path + "/" + tableNames[len(tableNames)-2]
-	prev, err = openIndexDB(prevPath, metricIDCache, metricNameCache)
+	prev, err = openIndexDB(prevPath, metricIDCache, metricNameCache, currHourMetricIDs, prevHourMetricIDs)
 	if err != nil {
 		curr.MustClose()
 		return nil, nil, fmt.Errorf("cannot open prev indexdb table at %q: %s", prevPath, err)
diff --git a/lib/storage/storage_test.go b/lib/storage/storage_test.go
index 5dc2b1dd6..18e9adf99 100644
--- a/lib/storage/storage_test.go
+++ b/lib/storage/storage_test.go
@@ -11,6 +11,184 @@ import (
 	"time"
 )
 
+func TestUpdateCurrHourMetricIDs(t *testing.T) {
+	newStorage := func() *Storage {
+		var s Storage
+		s.currHourMetricIDs.Store(&hourMetricIDs{})
+		s.prevHourMetricIDs.Store(&hourMetricIDs{})
+		s.pendingHourMetricIDs = make(map[uint64]struct{})
+		return &s
+	}
+	t.Run("empty_pedning_metric_ids_stale_curr_hour", func(t *testing.T) {
+		s := newStorage()
+		hour := uint64(timestampFromTime(time.Now())) / msecPerHour
+		hmOrig := &hourMetricIDs{
+			m: map[uint64]struct{}{
+				12: {},
+				34: {},
+			},
+			hour: 123,
+		}
+		s.currHourMetricIDs.Store(hmOrig)
+		s.updateCurrHourMetricIDs()
+		hmCurr := s.currHourMetricIDs.Load().(*hourMetricIDs)
+		if hmCurr.hour != hour {
+			// It is possible new hour occured. Update the hour and verify it again.
+			hour = uint64(timestampFromTime(time.Now())) / msecPerHour
+			if hmCurr.hour != hour {
+				t.Fatalf("unexpected hmCurr.hour; got %d; want %d", hmCurr.hour, hour)
+			}
+		}
+		if len(hmCurr.m) != 0 {
+			t.Fatalf("unexpected length of hm.m; got %d; want %d", len(hmCurr.m), 0)
+		}
+		if !hmCurr.isFull {
+			t.Fatalf("unexpected hmCurr.isFull; got %v; want %v", hmCurr.isFull, true)
+		}
+
+		hmPrev := s.prevHourMetricIDs.Load().(*hourMetricIDs)
+		if !reflect.DeepEqual(hmPrev, hmOrig) {
+			t.Fatalf("unexpected hmPrev; got %v; want %v", hmPrev, hmOrig)
+		}
+
+		if len(s.pendingHourMetricIDs) != 0 {
+			t.Fatalf("unexpected len(s.pendingHourMetricIDs); got %d; want %d", len(s.pendingHourMetricIDs), 0)
+		}
+	})
+	t.Run("empty_pedning_metric_ids_valid_curr_hour", func(t *testing.T) {
+		s := newStorage()
+		hour := uint64(timestampFromTime(time.Now())) / msecPerHour
+		hmOrig := &hourMetricIDs{
+			m: map[uint64]struct{}{
+				12: {},
+				34: {},
+			},
+			hour: hour,
+		}
+		s.currHourMetricIDs.Store(hmOrig)
+		s.updateCurrHourMetricIDs()
+		hmCurr := s.currHourMetricIDs.Load().(*hourMetricIDs)
+		if hmCurr.hour != hour {
+			// It is possible new hour occured. Update the hour and verify it again.
+			hour = uint64(timestampFromTime(time.Now())) / msecPerHour
+			if hmCurr.hour != hour {
+				t.Fatalf("unexpected hmCurr.hour; got %d; want %d", hmCurr.hour, hour)
+			}
+			// Do not run other checks, since they may fail.
+			return
+		}
+		if !reflect.DeepEqual(hmCurr, hmOrig) {
+			t.Fatalf("unexpected hmCurr; got %v; want %v", hmCurr, hmOrig)
+		}
+		if hmCurr.isFull {
+			t.Fatalf("unexpected hmCurr.isFull; got %v; want %v", hmCurr.isFull, false)
+		}
+
+		hmPrev := s.prevHourMetricIDs.Load().(*hourMetricIDs)
+		hmEmpty := &hourMetricIDs{}
+		if !reflect.DeepEqual(hmPrev, hmEmpty) {
+			t.Fatalf("unexpected hmPrev; got %v; want %v", hmPrev, hmEmpty)
+		}
+
+		if len(s.pendingHourMetricIDs) != 0 {
+			t.Fatalf("unexpected len(s.pendingHourMetricIDs); got %d; want %d", len(s.pendingHourMetricIDs), 0)
+		}
+	})
+	t.Run("nonempty_pending_metric_ids_stale_curr_hour", func(t *testing.T) {
+		s := newStorage()
+		pendingHourMetricIDs := map[uint64]struct{}{
+			343:     {},
+			32424:   {},
+			8293432: {},
+		}
+		s.pendingHourMetricIDs = pendingHourMetricIDs
+
+		hour := uint64(timestampFromTime(time.Now())) / msecPerHour
+		hmOrig := &hourMetricIDs{
+			m: map[uint64]struct{}{
+				12: {},
+				34: {},
+			},
+			hour: 123,
+		}
+		s.currHourMetricIDs.Store(hmOrig)
+		s.updateCurrHourMetricIDs()
+		hmCurr := s.currHourMetricIDs.Load().(*hourMetricIDs)
+		if hmCurr.hour != hour {
+			// It is possible new hour occured. Update the hour and verify it again.
+			hour = uint64(timestampFromTime(time.Now())) / msecPerHour
+			if hmCurr.hour != hour {
+				t.Fatalf("unexpected hmCurr.hour; got %d; want %d", hmCurr.hour, hour)
+			}
+		}
+		if !reflect.DeepEqual(hmCurr.m, pendingHourMetricIDs) {
+			t.Fatalf("unexpected hm.m; got %v; want %v", hmCurr.m, pendingHourMetricIDs)
+		}
+		if !hmCurr.isFull {
+			t.Fatalf("unexpected hmCurr.isFull; got %v; want %v", hmCurr.isFull, true)
+		}
+
+		hmPrev := s.prevHourMetricIDs.Load().(*hourMetricIDs)
+		if !reflect.DeepEqual(hmPrev, hmOrig) {
+			t.Fatalf("unexpected hmPrev; got %v; want %v", hmPrev, hmOrig)
+		}
+
+		if len(s.pendingHourMetricIDs) != 0 {
+			t.Fatalf("unexpected len(s.pendingHourMetricIDs); got %d; want %d", len(s.pendingHourMetricIDs), 0)
+		}
+	})
+	t.Run("nonempty_pending_metric_ids_valid_curr_hour", func(t *testing.T) {
+		s := newStorage()
+		pendingHourMetricIDs := map[uint64]struct{}{
+			343:     {},
+			32424:   {},
+			8293432: {},
+		}
+		s.pendingHourMetricIDs = pendingHourMetricIDs
+
+		hour := uint64(timestampFromTime(time.Now())) / msecPerHour
+		hmOrig := &hourMetricIDs{
+			m: map[uint64]struct{}{
+				12: {},
+				34: {},
+			},
+			hour: hour,
+		}
+		s.currHourMetricIDs.Store(hmOrig)
+		s.updateCurrHourMetricIDs()
+		hmCurr := s.currHourMetricIDs.Load().(*hourMetricIDs)
+		if hmCurr.hour != hour {
+			// It is possible new hour occured. Update the hour and verify it again.
+			hour = uint64(timestampFromTime(time.Now())) / msecPerHour
+			if hmCurr.hour != hour {
+				t.Fatalf("unexpected hmCurr.hour; got %d; want %d", hmCurr.hour, hour)
+			}
+			// Do not run other checks, since they may fail.
+			return
+		}
+		m := getMetricIDsCopy(pendingHourMetricIDs)
+		for metricID := range hmOrig.m {
+			m[metricID] = struct{}{}
+		}
+		if !reflect.DeepEqual(hmCurr.m, m) {
+			t.Fatalf("unexpected hm.m; got %v; want %v", hmCurr.m, m)
+		}
+		if hmCurr.isFull {
+			t.Fatalf("unexpected hmCurr.isFull; got %v; want %v", hmCurr.isFull, false)
+		}
+
+		hmPrev := s.prevHourMetricIDs.Load().(*hourMetricIDs)
+		hmEmpty := &hourMetricIDs{}
+		if !reflect.DeepEqual(hmPrev, hmEmpty) {
+			t.Fatalf("unexpected hmPrev; got %v; want %v", hmPrev, hmEmpty)
+		}
+
+		if len(s.pendingHourMetricIDs) != 0 {
+			t.Fatalf("unexpected len(s.pendingHourMetricIDs); got %d; want %d", len(s.pendingHourMetricIDs), 0)
+		}
+	})
+}
+
 func TestMetricRowMarshalUnmarshal(t *testing.T) {
 	var buf []byte
 	typ := reflect.TypeOf(&MetricRow{})
diff --git a/lib/storage/time.go b/lib/storage/time.go
index 7593b1959..eaa2a3f29 100644
--- a/lib/storage/time.go
+++ b/lib/storage/time.go
@@ -67,3 +67,5 @@ func (tr *TimeRange) fromPartitionTime(t time.Time) {
 }
 
 const msecPerDay = 24 * 3600 * 1000
+
+const msecPerHour = 3600 * 1000