diff --git a/CHANGELOG.md b/CHANGELOG.md
index 93650e7b8f..87d8c82db3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#4508](https://github.com/thanos-io/thanos/pull/4508) Adjust and rename `ThanosSidecarUnhealthy` to `ThanosSidecarNoConnectionToStartedPrometheus`; Remove `ThanosSidecarPrometheusDown` alert; Remove unused `thanos_sidecar_last_heartbeat_success_time_seconds` metrics.
 - [#4663](https://github.com/thanos-io/thanos/pull/4663) Fetcher: Fix discovered data races.
 - [#4754](https://github.com/thanos-io/thanos/pull/4754) Query: Fix possible panic on stores endpoint.
+- [#4753](https://github.com/thanos-io/thanos/pull/4753) Store: validate block sync concurrency parameter
 
 ## [v0.23.1](https://github.com/thanos-io/thanos/tree/release-0.23) - 2021.10.1
 
diff --git a/cmd/thanos/store.go b/cmd/thanos/store.go
index 19b6dbf837..25310f9ef3 100644
--- a/cmd/thanos/store.go
+++ b/cmd/thanos/store.go
@@ -113,7 +113,7 @@ func (sc *storeConfig) registerFlag(cmd extkingpin.FlagClause) {
 	cmd.Flag("sync-block-duration", "Repeat interval for syncing the blocks between local and remote view.").
 		Default("3m").DurationVar(&sc.syncInterval)
 
-	cmd.Flag("block-sync-concurrency", "Number of goroutines to use when constructing index-cache.json blocks from object storage.").
+	cmd.Flag("block-sync-concurrency", "Number of goroutines to use when constructing index-cache.json blocks from object storage. Must be equal or greater than 1.").
 		Default("20").IntVar(&sc.blockSyncConcurrency)
 
 	cmd.Flag("block-meta-fetch-concurrency", "Number of goroutines to use when fetching block metadata from object storage.").
diff --git a/docs/components/store.md b/docs/components/store.md
index 3ae037beb9..cdc8605052 100644
--- a/docs/components/store.md
+++ b/docs/components/store.md
@@ -34,6 +34,7 @@ Flags:
       --block-sync-concurrency=20  
                                  Number of goroutines to use when constructing
                                  index-cache.json blocks from object storage.
+                                 Must be equal or greater than 1.
       --chunk-pool-size=2GB      Maximum size of concurrently allocatable bytes
                                  reserved strictly to reuse for chunks in
                                  memory.
diff --git a/pkg/block/fetcher.go b/pkg/block/fetcher.go
index f9f45202d9..77091ecbff 100644
--- a/pkg/block/fetcher.go
+++ b/pkg/block/fetcher.go
@@ -660,9 +660,9 @@ func (f *DeduplicateFilter) DuplicateIDs() []ulid.ULID {
 
 func addNodeBySources(root, add *Node) bool {
 	var rootNode *Node
+	childSources := add.Compaction.Sources
 	for _, node := range root.Children {
 		parentSources := node.Compaction.Sources
-		childSources := add.Compaction.Sources
 
 		// Block exists with same sources, add as child.
 		if contains(parentSources, childSources) && contains(childSources, parentSources) {
diff --git a/pkg/reloader/reloader_test.go b/pkg/reloader/reloader_test.go
index 6659d20cc7..25a0af5ae9 100644
--- a/pkg/reloader/reloader_test.go
+++ b/pkg/reloader/reloader_test.go
@@ -247,6 +247,84 @@ func TestReloader_DirectoriesApply(t *testing.T) {
 	testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-source.yaml"), path.Join(dir2, "rule3-001.yaml")))
 	testutil.Ok(t, ioutil.WriteFile(path.Join(dir2, "rule-dir", "rule4.yaml"), []byte("rule4"), os.ModePerm))
 
+	stepFunc := func(rel int) {
+		t.Log("Performing step number", rel)
+		switch rel {
+		case 0:
+			// Create rule2.yaml.
+			//
+			// dir
+			// ├─ rule-dir -> dir2/rule-dir
+			// ├─ rule1.yaml
+			// └─ rule2.yaml (*)
+			// dir2
+			// ├─ rule-dir
+			// │  └─ rule4.yaml
+			// ├─ rule3-001.yaml -> rule3-source.yaml
+			// └─ rule3-source.yaml
+			testutil.Ok(t, ioutil.WriteFile(path.Join(dir, "rule2.yaml"), []byte("rule2"), os.ModePerm))
+		case 1:
+			// Update rule1.yaml.
+			//
+			// dir
+			// ├─ rule-dir -> dir2/rule-dir
+			// ├─ rule1.yaml (*)
+			// └─ rule2.yaml
+			// dir2
+			// ├─ rule-dir
+			// │  └─ rule4.yaml
+			// ├─ rule3-001.yaml -> rule3-source.yaml
+			// └─ rule3-source.yaml
+			testutil.Ok(t, os.Rename(tempRule1File, path.Join(dir, "rule1.yaml")))
+		case 2:
+			// Create dir/rule3.yaml (symlink to rule3-001.yaml).
+			//
+			// dir
+			// ├─ rule-dir -> dir2/rule-dir
+			// ├─ rule1.yaml
+			// ├─ rule2.yaml
+			// └─ rule3.yaml -> dir2/rule3-001.yaml (*)
+			// dir2
+			// ├─ rule-dir
+			// │  └─ rule4.yaml
+			// ├─ rule3-001.yaml -> rule3-source.yaml
+			// └─ rule3-source.yaml
+			testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-001.yaml"), path.Join(dir2, "rule3.yaml")))
+			testutil.Ok(t, os.Rename(path.Join(dir2, "rule3.yaml"), path.Join(dir, "rule3.yaml")))
+		case 3:
+			// Update the symlinked file and replace the symlink file to trigger fsnotify.
+			//
+			// dir
+			// ├─ rule-dir -> dir2/rule-dir
+			// ├─ rule1.yaml
+			// ├─ rule2.yaml
+			// └─ rule3.yaml -> dir2/rule3-002.yaml (*)
+			// dir2
+			// ├─ rule-dir
+			// │  └─ rule4.yaml
+			// ├─ rule3-002.yaml -> rule3-source.yaml (*)
+			// └─ rule3-source.yaml (*)
+			testutil.Ok(t, os.Rename(tempRule3File, path.Join(dir2, "rule3-source.yaml")))
+			testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-source.yaml"), path.Join(dir2, "rule3-002.yaml")))
+			testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-002.yaml"), path.Join(dir2, "rule3.yaml")))
+			testutil.Ok(t, os.Rename(path.Join(dir2, "rule3.yaml"), path.Join(dir, "rule3.yaml")))
+			testutil.Ok(t, os.Remove(path.Join(dir2, "rule3-001.yaml")))
+		case 4:
+			// Update rule4.yaml in the symlinked directory.
+			//
+			// dir
+			// ├─ rule-dir -> dir2/rule-dir
+			// ├─ rule1.yaml
+			// ├─ rule2.yaml
+			// └─ rule3.yaml -> rule3-source.yaml
+			// dir2
+			// ├─ rule-dir
+			// │  └─ rule4.yaml (*)
+			// └─ rule3-source.yaml
+			testutil.Ok(t, os.Rename(tempRule4File, path.Join(dir2, "rule-dir", "rule4.yaml")))
+		}
+	}
+
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	g := sync.WaitGroup{}
 	g.Add(1)
@@ -267,90 +345,21 @@ func TestReloader_DirectoriesApply(t *testing.T) {
 
 			reloadsMtx.Lock()
 			rel := reloads
+			reloadsMtx.Unlock()
 			if init && rel <= reloadsSeen {
-				reloadsMtx.Unlock()
 				continue
 			}
-			reloadsMtx.Unlock()
-			init = true
-			reloadsSeen = rel
 
-			t.Log("Performing step number", rel)
-			switch rel {
-			case 0:
-				// Create rule2.yaml.
-				//
-				// dir
-				// ├─ rule-dir -> dir2/rule-dir
-				// ├─ rule1.yaml
-				// └─ rule2.yaml (*)
-				// dir2
-				// ├─ rule-dir
-				// │  └─ rule4.yaml
-				// ├─ rule3-001.yaml -> rule3-source.yaml
-				// └─ rule3-source.yaml
-				testutil.Ok(t, ioutil.WriteFile(path.Join(dir, "rule2.yaml"), []byte("rule2"), os.ModePerm))
-			case 1:
-				// Update rule1.yaml.
-				//
-				// dir
-				// ├─ rule-dir -> dir2/rule-dir
-				// ├─ rule1.yaml (*)
-				// └─ rule2.yaml
-				// dir2
-				// ├─ rule-dir
-				// │  └─ rule4.yaml
-				// ├─ rule3-001.yaml -> rule3-source.yaml
-				// └─ rule3-source.yaml
-				testutil.Ok(t, os.Rename(tempRule1File, path.Join(dir, "rule1.yaml")))
-			case 2:
-				// Create dir/rule3.yaml (symlink to rule3-001.yaml).
-				//
-				// dir
-				// ├─ rule-dir -> dir2/rule-dir
-				// ├─ rule1.yaml
-				// ├─ rule2.yaml
-				// └─ rule3.yaml -> dir2/rule3-001.yaml (*)
-				// dir2
-				// ├─ rule-dir
-				// │  └─ rule4.yaml
-				// ├─ rule3-001.yaml -> rule3-source.yaml
-				// └─ rule3-source.yaml
-				testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-001.yaml"), path.Join(dir2, "rule3.yaml")))
-				testutil.Ok(t, os.Rename(path.Join(dir2, "rule3.yaml"), path.Join(dir, "rule3.yaml")))
-			case 3:
-				// Update the symlinked file and replace the symlink file to trigger fsnotify.
-				//
-				// dir
-				// ├─ rule-dir -> dir2/rule-dir
-				// ├─ rule1.yaml
-				// ├─ rule2.yaml
-				// └─ rule3.yaml -> dir2/rule3-002.yaml (*)
-				// dir2
-				// ├─ rule-dir
-				// │  └─ rule4.yaml
-				// ├─ rule3-002.yaml -> rule3-source.yaml (*)
-				// └─ rule3-source.yaml (*)
-				testutil.Ok(t, os.Rename(tempRule3File, path.Join(dir2, "rule3-source.yaml")))
-				testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-source.yaml"), path.Join(dir2, "rule3-002.yaml")))
-				testutil.Ok(t, os.Symlink(path.Join(dir2, "rule3-002.yaml"), path.Join(dir2, "rule3.yaml")))
-				testutil.Ok(t, os.Rename(path.Join(dir2, "rule3.yaml"), path.Join(dir, "rule3.yaml")))
-				testutil.Ok(t, os.Remove(path.Join(dir2, "rule3-001.yaml")))
-			case 4:
-				// Update rule4.yaml in the symlinked directory.
-				//
-				// dir
-				// ├─ rule-dir -> dir2/rule-dir
-				// ├─ rule1.yaml
-				// ├─ rule2.yaml
-				// └─ rule3.yaml -> rule3-source.yaml
-				// dir2
-				// ├─ rule-dir
-				// │  └─ rule4.yaml (*)
-				// └─ rule3-source.yaml
-				testutil.Ok(t, os.Rename(tempRule4File, path.Join(dir2, "rule-dir", "rule4.yaml")))
+			// Catch up if reloader is step(s) ahead.
+			for skipped := rel - reloadsSeen - 1; skipped > 0; skipped-- {
+				stepFunc(rel - skipped)
 			}
 
+			stepFunc(rel)
+
+			init = true
+			reloadsSeen = rel
+
 			if rel > 4 {
 				// All good.
 				return
diff --git a/pkg/store/bucket.go b/pkg/store/bucket.go
index 39d78b257f..3af0c179ec 100644
--- a/pkg/store/bucket.go
+++ b/pkg/store/bucket.go
@@ -92,6 +92,12 @@ const (
 	// Labels for metrics.
 	labelEncode = "encode"
 	labelDecode = "decode"
+
+	minBlockSyncConcurrency = 1
+)
+
+var (
+	errBlockSyncConcurrencyNotValid = errors.New("the block sync concurrency must be equal or greater than 1.")
 )
 
 type bucketStoreMetrics struct {
@@ -298,6 +304,13 @@ type BucketStore struct {
 	enableSeriesResponseHints bool
 }
 
+func (b *BucketStore) validate() error {
+	if b.blockSyncConcurrency < minBlockSyncConcurrency {
+		return errBlockSyncConcurrencyNotValid
+	}
+	return nil
+}
+
 type noopCache struct{}
 
 func (noopCache) StorePostings(context.Context, ulid.ULID, labels.Label, []byte) {}
@@ -407,6 +420,10 @@ func NewBucketStore(
 	s.indexReaderPool = indexheader.NewReaderPool(s.logger, lazyIndexReaderEnabled, lazyIndexReaderIdleTimeout, indexReaderPoolMetrics)
 	s.metrics = newBucketStoreMetrics(s.reg) // TODO(metalmatze): Might be possible via Option too
 
+	if err := s.validate(); err != nil {
+		return nil, errors.Wrap(err, "validate config")
+	}
+
 	if err := os.MkdirAll(dir, 0750); err != nil {
 		return nil, errors.Wrap(err, "create dir")
 	}
diff --git a/pkg/store/bucket_test.go b/pkg/store/bucket_test.go
index e635ab22c1..adc5701740 100644
--- a/pkg/store/bucket_test.go
+++ b/pkg/store/bucket_test.go
@@ -556,6 +556,32 @@ func TestGapBasedPartitioner_Partition(t *testing.T) {
 	}
 }
 
+func TestBucketStoreConfig_validate(t *testing.T) {
+	tests := map[string]struct {
+		config   *BucketStore
+		expected error
+	}{
+		"should pass on valid config": {
+			config: &BucketStore{
+				blockSyncConcurrency: 1,
+			},
+			expected: nil,
+		},
+		"should fail on blockSyncConcurrency < 1": {
+			config: &BucketStore{
+				blockSyncConcurrency: 0,
+			},
+			expected: errBlockSyncConcurrencyNotValid,
+		},
+	}
+
+	for testName, testData := range tests {
+		t.Run(testName, func(t *testing.T) {
+			testutil.Equals(t, testData.expected, testData.config.validate())
+		})
+	}
+}
+
 func TestBucketStore_Info(t *testing.T) {
 	defer testutil.TolerantVerifyLeak(t)