diff --git a/cmd/tsdb/main.go b/cmd/tsdb/main.go
index 491c2ac35..960c9c05e 100644
--- a/cmd/tsdb/main.go
+++ b/cmd/tsdb/main.go
@@ -139,7 +139,7 @@ func (b *writeBenchmark) run(cmd *cobra.Command, args []string) {
 
 	dur := measureTime("ingestScrapes", func() {
 		b.startProfiling()
-		total, err = b.ingestScrapes(metrics, 2000)
+		total, err = b.ingestScrapes(metrics, 15000)
 		if err != nil {
 			exitWithError(err)
 		}
@@ -147,6 +147,7 @@ func (b *writeBenchmark) run(cmd *cobra.Command, args []string) {
 
 	fmt.Println(" > total samples:", total)
 	fmt.Println(" > samples/sec:", float64(total)/dur.Seconds())
+	select {}
 
 	measureTime("stopStorage", func() {
 		if err := b.storage.Close(); err != nil {
diff --git a/head.go b/head.go
index 4e1e85877..b2d6e9842 100644
--- a/head.go
+++ b/head.go
@@ -403,11 +403,12 @@ func (a *headAppender) AddFast(ref uint64, t int64, v float64) error {
 		return errors.Wrap(ErrNotFound, "unknown series")
 	}
 	s.Lock()
-	if err := s.appendable(t, v); err != nil {
-		return err
-	}
+	err := s.appendable(t, v)
 	s.Unlock()
 
+	if err != nil {
+		return err
+	}
 	if t < a.mint {
 		return ErrOutOfBounds
 	}
diff --git a/wal.go b/wal.go
index 747510fd6..9af9a1853 100644
--- a/wal.go
+++ b/wal.go
@@ -417,10 +417,6 @@ func (w *SegmentWAL) LogSeries(series []RefSeries) error {
 			tf.minSeries = s.Ref
 		}
 	}
-
-	if w.flushInterval <= 0 {
-		return errors.Wrap(w.Sync(), "sync")
-	}
 	return nil
 }
 
@@ -447,10 +443,6 @@ func (w *SegmentWAL) LogSamples(samples []RefSample) error {
 			tf.maxTime = s.T
 		}
 	}
-
-	if w.flushInterval <= 0 {
-		return errors.Wrap(w.Sync(), "sync")
-	}
 	return nil
 }
 
@@ -479,10 +471,6 @@ func (w *SegmentWAL) LogDeletes(stones []Stone) error {
 			}
 		}
 	}
-
-	if w.flushInterval <= 0 {
-		return errors.Wrap(w.Sync(), "sync")
-	}
 	return nil
 }
 
@@ -537,19 +525,26 @@ func (w *SegmentWAL) createSegmentFile(name string) (*os.File, error) {
 func (w *SegmentWAL) cut() error {
 	// Sync current head to disk and close.
 	if hf := w.head(); hf != nil {
-		if err := w.sync(); err != nil {
-			return err
-		}
-		off, err := hf.Seek(0, os.SEEK_CUR)
-		if err != nil {
-			return err
-		}
-		if err := hf.Truncate(off); err != nil {
-			return err
-		}
-		if err := hf.Close(); err != nil {
+		if err := w.flush(); err != nil {
 			return err
 		}
+		// Finish last segment asynchronously to not block the WAL moving along
+		// in the new segment.
+		go func() {
+			off, err := hf.Seek(0, os.SEEK_CUR)
+			if err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+			if err := hf.Truncate(off); err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+			if err := hf.Sync(); err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+			if err := hf.Close(); err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+		}()
 	}
 
 	p, _, err := nextSequenceFile(w.dirFile.Name())
@@ -561,9 +556,11 @@ func (w *SegmentWAL) cut() error {
 		return err
 	}
 
-	if err = w.dirFile.Sync(); err != nil {
-		return err
-	}
+	go func() {
+		if err = w.dirFile.Sync(); err != nil {
+			w.logger.Log("msg", "sync WAL directory", "err", err)
+		}
+	}()
 
 	w.files = append(w.files, newSegmentFile(f))
 
diff --git a/wal_test.go b/wal_test.go
index 45279b1b5..180623dd0 100644
--- a/wal_test.go
+++ b/wal_test.go
@@ -91,9 +91,8 @@ func TestSegmentWAL_cut(t *testing.T) {
 
 	require.NoError(t, w.cut(), "cut failed")
 
-	// Cutting creates a new file and close the previous tail file.
+	// Cutting creates a new file.
 	require.Equal(t, 2, len(w.files))
-	require.Error(t, w.files[0].Close())
 
 	require.NoError(t, w.write(WALEntrySeries, 1, []byte("Hello World!!")))