diff --git a/tsdb/chunks/chunks.go b/tsdb/chunks/chunks.go
index 1a11924d1..9b332f971 100644
--- a/tsdb/chunks/chunks.go
+++ b/tsdb/chunks/chunks.go
@@ -190,27 +190,7 @@ func (w *Writer) cut() error {
 		return err
 	}
 
-	p, _, err := nextSequenceFile(w.dirFile.Name())
-	if err != nil {
-		return err
-	}
-	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE, 0666)
-	if err != nil {
-		return err
-	}
-	if err = fileutil.Preallocate(f, w.segmentSize, true); err != nil {
-		return err
-	}
-	if err = w.dirFile.Sync(); err != nil {
-		return err
-	}
-
-	// Write header metadata for new file.
-	metab := make([]byte, SegmentHeaderSize)
-	binary.BigEndian.PutUint32(metab[:MagicChunksSize], MagicChunks)
-	metab[4] = chunksFormatV1
-
-	n, err := f.Write(metab)
+	n, f, _, err := cutSegmentFile(w.dirFile, chunksFormatV1, w.segmentSize)
 	if err != nil {
 		return err
 	}
@@ -226,6 +206,36 @@ func (w *Writer) cut() error {
 	return nil
 }
 
+func cutSegmentFile(dirFile *os.File, chunksFormat byte, segmentSize int64) (headerSize int, newFile *os.File, seq int, err error) {
+	p, seq, err := nextSequenceFile(dirFile.Name())
+	if err != nil {
+		return 0, nil, 0, err
+	}
+	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE, 0666)
+	if err != nil {
+		return 0, nil, 0, err
+	}
+	if segmentSize > 0 {
+		if err = fileutil.Preallocate(f, segmentSize, true); err != nil {
+			return 0, nil, 0, err
+		}
+	}
+	if err = dirFile.Sync(); err != nil {
+		return 0, nil, 0, err
+	}
+
+	// Write header metadata for new file.
+	metab := make([]byte, SegmentHeaderSize)
+	binary.BigEndian.PutUint32(metab[:MagicChunksSize], MagicChunks)
+	metab[4] = chunksFormat
+
+	n, err := f.Write(metab)
+	if err != nil {
+		return 0, nil, 0, err
+	}
+	return n, f, seq, nil
+}
+
 func (w *Writer) write(b []byte) error {
 	n, err := w.wbuf.Write(b)
 	w.n += int64(n)
@@ -464,8 +474,6 @@ type Reader struct {
 
 func newReader(bs []ByteSlice, cs []io.Closer, pool chunkenc.Pool) (*Reader, error) {
 	cr := Reader{pool: pool, bs: bs, cs: cs}
-	var totalSize int64
-
 	for i, b := range cr.bs {
 		if b.Len() < SegmentHeaderSize {
 			return nil, errors.Wrapf(errInvalidSize, "invalid segment header in segment %d", i)
@@ -479,9 +487,8 @@ func newReader(bs []ByteSlice, cs []io.Closer, pool chunkenc.Pool) (*Reader, err
 		if v := int(b.Range(MagicChunksSize, MagicChunksSize+ChunksFormatVersionSize)[0]); v != chunksFormatV1 {
 			return nil, errors.Errorf("invalid chunk format version %d", v)
 		}
-		totalSize += int64(b.Len())
+		cr.size += int64(b.Len())
 	}
-	cr.size = totalSize
 	return &cr, nil
 }
 
@@ -594,9 +601,18 @@ func nextSequenceFile(dir string) (string, int, error) {
 		if err != nil {
 			continue
 		}
-		i = j
+		// It is not necessary that we find the files in number order,
+		// for example with '1000000' and '200000', '1000000' would come first.
+		// Though this is a very very race case, we check anyway for the max id.
+		if j > i {
+			i = j
+		}
 	}
-	return filepath.Join(dir, fmt.Sprintf("%0.6d", i+1)), int(i + 1), nil
+	return segmentFile(dir, int(i+1)), int(i + 1), nil
+}
+
+func segmentFile(baseDir string, index int) string {
+	return filepath.Join(baseDir, fmt.Sprintf("%0.6d", index))
 }
 
 func sequenceFiles(dir string) ([]string, error) {
@@ -605,7 +621,6 @@ func sequenceFiles(dir string) ([]string, error) {
 		return nil, err
 	}
 	var res []string
-
 	for _, fi := range files {
 		if _, err := strconv.ParseUint(fi.Name(), 10, 64); err != nil {
 			continue