diff --git a/chunks.go b/chunks.go
new file mode 100644
index 000000000..c0c7954a6
--- /dev/null
+++ b/chunks.go
@@ -0,0 +1,299 @@
+package tsdb
+
+import (
+	"bufio"
+	"encoding/binary"
+	"fmt"
+	"hash"
+	"hash/crc32"
+	"io"
+	"os"
+
+	"github.com/coreos/etcd/pkg/fileutil"
+	"github.com/fabxc/tsdb/chunks"
+	"github.com/pkg/errors"
+)
+
+const (
+	// MagicSeries 4 bytes at the head of series file.
+	MagicChunks = 0x85BD40DD
+)
+
+// ChunkMeta holds information about a chunk of data.
+type ChunkMeta struct {
+	// Ref and Chunk hold either a reference that can be used to retrieve
+	// chunk data or the data itself.
+	// Generally, only one of them is set.
+	Ref   uint64
+	Chunk chunks.Chunk
+
+	MinTime, MaxTime int64 // time range the data covers
+}
+
+// ChunkWriter serializes a time block of chunked series data.
+type ChunkWriter interface {
+	// WriteChunks writes several chunks. The data field of the ChunkMetas
+	// must be populated.
+	// After returning successfully, the Ref fields in the ChunkMetas
+	// is set and can be used to retrieve the chunks from the written data.
+	WriteChunks(chunks ...ChunkMeta) error
+
+	// Close writes any required finalization and closes the resources
+	// associated with the underlying writer.
+	Close() error
+}
+
+// chunkWriter implements the ChunkWriter interface for the standard
+// serialization format.
+type chunkWriter struct {
+	dirFile *os.File
+	files   []*os.File
+	wbuf    *bufio.Writer
+	n       int64
+	crc32   hash.Hash
+
+	segmentSize int64
+}
+
+const (
+	defaultChunkSegmentSize = 512 * 1024 * 1024
+
+	chunksFormatV1 = 1
+)
+
+func newChunkWriter(dir string) (*chunkWriter, error) {
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		return nil, err
+	}
+	dirFile, err := fileutil.OpenDir(dir)
+	if err != nil {
+		return nil, err
+	}
+	cw := &chunkWriter{
+		dirFile:     dirFile,
+		n:           0,
+		crc32:       crc32.New(crc32.MakeTable(crc32.Castagnoli)),
+		segmentSize: defaultChunkSegmentSize,
+	}
+	return cw, nil
+}
+
+func (w *chunkWriter) tail() *os.File {
+	if len(w.files) == 0 {
+		return nil
+	}
+	return w.files[len(w.files)-1]
+}
+
+// finalizeTail writes all pending data to the current tail file,
+// truncates its size, and closes it.
+func (w *chunkWriter) finalizeTail() error {
+	tf := w.tail()
+	if tf == nil {
+		return nil
+	}
+
+	if err := w.wbuf.Flush(); err != nil {
+		return err
+	}
+	if err := fileutil.Fsync(tf); err != nil {
+		return err
+	}
+	// As the file was pre-allocated, we truncate any superfluous zero bytes.
+	off, err := tf.Seek(0, os.SEEK_CUR)
+	if err != nil {
+		return err
+	}
+	if err := tf.Truncate(off); err != nil {
+		return err
+	}
+	return tf.Close()
+}
+
+func (w *chunkWriter) cut() error {
+	// Sync current tail to disk and close.
+	w.finalizeTail()
+
+	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, 8)
+	binary.BigEndian.PutUint32(metab[:4], MagicChunks)
+	metab[4] = chunksFormatV1
+
+	if _, err := f.Write(metab); err != nil {
+		return err
+	}
+
+	w.files = append(w.files, f)
+	if w.wbuf != nil {
+		w.wbuf.Reset(f)
+	} else {
+		w.wbuf = bufio.NewWriterSize(f, 8*1024*1024)
+	}
+	w.n = 8
+
+	return nil
+}
+
+func (w *chunkWriter) write(wr io.Writer, b []byte) error {
+	n, err := wr.Write(b)
+	w.n += int64(n)
+	return err
+}
+
+func (w *chunkWriter) WriteChunks(chks ...ChunkMeta) error {
+	// Calculate maximum space we need and cut a new segment in case
+	// we don't fit into the current one.
+	maxLen := int64(binary.MaxVarintLen32)
+	for _, c := range chks {
+		maxLen += binary.MaxVarintLen32 + 1
+		maxLen += int64(len(c.Chunk.Bytes()))
+	}
+	newsz := w.n + maxLen
+
+	if w.wbuf == nil || w.n > w.segmentSize || newsz > w.segmentSize && maxLen <= w.segmentSize {
+		if err := w.cut(); err != nil {
+			return err
+		}
+	}
+
+	// Write chunks sequentially and set the reference field in the ChunkMeta.
+	w.crc32.Reset()
+	wr := io.MultiWriter(w.crc32, w.wbuf)
+
+	b := make([]byte, binary.MaxVarintLen32)
+	n := binary.PutUvarint(b, uint64(len(chks)))
+
+	if err := w.write(wr, b[:n]); err != nil {
+		return err
+	}
+	seq := uint64(w.seq()) << 32
+
+	for i := range chks {
+		chk := &chks[i]
+
+		chk.Ref = seq | uint64(w.n)
+
+		n = binary.PutUvarint(b, uint64(len(chk.Chunk.Bytes())))
+
+		if err := w.write(wr, b[:n]); err != nil {
+			return err
+		}
+		if err := w.write(wr, []byte{byte(chk.Chunk.Encoding())}); err != nil {
+			return err
+		}
+		if err := w.write(wr, chk.Chunk.Bytes()); err != nil {
+			return err
+		}
+		chk.Chunk = nil
+	}
+
+	if err := w.write(w.wbuf, w.crc32.Sum(nil)); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (w *chunkWriter) seq() int {
+	return len(w.files) - 1
+}
+
+func (w *chunkWriter) Close() error {
+	return w.finalizeTail()
+}
+
+// ChunkReader provides reading access of serialized time series data.
+type ChunkReader interface {
+	// Chunk returns the series data chunk with the given reference.
+	Chunk(ref uint64) (chunks.Chunk, error)
+
+	// Close releases all underlying resources of the reader.
+	Close() error
+}
+
+// chunkReader implements a SeriesReader for a serialized byte stream
+// of series data.
+type chunkReader struct {
+	// The underlying bytes holding the encoded series data.
+	bs [][]byte
+
+	// Closers for resources behind the byte slices.
+	cs []io.Closer
+}
+
+// newChunkReader returns a new chunkReader based on mmaped files found in dir.
+func newChunkReader(dir string) (*chunkReader, error) {
+	files, err := sequenceFiles(dir, "")
+	if err != nil {
+		return nil, err
+	}
+	var cr chunkReader
+
+	for _, fn := range files {
+		f, err := openMmapFile(fn)
+		if err != nil {
+			return nil, errors.Wrapf(err, "mmap files")
+		}
+		cr.cs = append(cr.cs, f)
+		cr.bs = append(cr.bs, f.b)
+	}
+
+	for i, b := range cr.bs {
+		if len(b) < 4 {
+			return nil, errors.Wrapf(errInvalidSize, "validate magic in segment %d", i)
+		}
+		// Verify magic number.
+		if m := binary.BigEndian.Uint32(b[:4]); m != MagicChunks {
+			return nil, fmt.Errorf("invalid magic number %x", m)
+		}
+	}
+	return &cr, nil
+}
+
+func (s *chunkReader) Close() error {
+	return closeAll(s.cs...)
+}
+
+func (s *chunkReader) Chunk(ref uint64) (chunks.Chunk, error) {
+	var (
+		seq = int(ref >> 32)
+		off = int((ref << 32) >> 32)
+	)
+	if seq >= len(s.bs) {
+		return nil, errors.Errorf("reference sequence %d out of range", seq)
+	}
+	b := s.bs[seq]
+
+	if int(off) >= len(b) {
+		return nil, errors.Errorf("offset %d beyond data size %d", off, len(b))
+	}
+	b = b[off:]
+
+	l, n := binary.Uvarint(b)
+	if n < 0 {
+		return nil, fmt.Errorf("reading chunk length failed")
+	}
+	b = b[n:]
+	enc := chunks.Encoding(b[0])
+
+	c, err := chunks.FromData(enc, b[1:1+l])
+	if err != nil {
+		return nil, err
+	}
+	return c, nil
+}
diff --git a/chunks_test.go b/chunks_test.go
new file mode 100644
index 000000000..3255ffe3e
--- /dev/null
+++ b/chunks_test.go
@@ -0,0 +1,16 @@
+package tsdb
+
+import "github.com/fabxc/tsdb/chunks"
+
+type mockChunkReader struct {
+	chunk func(ref uint64) (chunks.Chunk, error)
+	close func() error
+}
+
+func (cr *mockChunkReader) Chunk(ref uint64) (chunks.Chunk, error) {
+	return cr.chunk(ref)
+}
+
+func (cr *mockChunkReader) Close() error {
+	return cr.close()
+}
diff --git a/writer.go b/index.go
similarity index 55%
rename from writer.go
rename to index.go
index 264052d32..23771f035 100644
--- a/writer.go
+++ b/index.go
@@ -3,6 +3,7 @@ package tsdb
 import (
 	"bufio"
 	"encoding/binary"
+	"fmt"
 	"hash"
 	"hash/crc32"
 	"io"
@@ -12,220 +13,19 @@ import (
 	"strings"
 
 	"github.com/coreos/etcd/pkg/fileutil"
-	"github.com/fabxc/tsdb/chunks"
 	"github.com/fabxc/tsdb/labels"
 	"github.com/pkg/errors"
 )
 
 const (
-	// MagicSeries 4 bytes at the head of series file.
-	MagicSeries = 0x85BD40DD
-
 	// MagicIndex 4 bytes at the head of an index file.
 	MagicIndex = 0xBAAAD700
+
+	indexFormatV1 = 1
 )
 
 const compactionPageBytes = minSectorSize * 64
 
-// ChunkWriter serializes a time block of chunked series data.
-type ChunkWriter interface {
-	// WriteChunks writes several chunks. The data field of the ChunkMetas
-	// must be populated.
-	// After returning successfully, the Ref fields in the ChunkMetas
-	// is set and can be used to retrieve the chunks from the written data.
-	WriteChunks(chunks ...ChunkMeta) error
-
-	// Close writes any required finalization and closes the resources
-	// associated with the underlying writer.
-	Close() error
-}
-
-// chunkWriter implements the ChunkWriter interface for the standard
-// serialization format.
-type chunkWriter struct {
-	dirFile *os.File
-	files   []*os.File
-	wbuf    *bufio.Writer
-	n       int64
-	crc32   hash.Hash
-
-	segmentSize int64
-}
-
-const (
-	defaultChunkSegmentSize = 512 * 1024 * 1024
-
-	chunksFormatV1 = 1
-	indexFormatV1  = 1
-)
-
-func newChunkWriter(dir string) (*chunkWriter, error) {
-	if err := os.MkdirAll(dir, 0777); err != nil {
-		return nil, err
-	}
-	dirFile, err := fileutil.OpenDir(dir)
-	if err != nil {
-		return nil, err
-	}
-	cw := &chunkWriter{
-		dirFile:     dirFile,
-		n:           0,
-		crc32:       crc32.New(crc32.MakeTable(crc32.Castagnoli)),
-		segmentSize: defaultChunkSegmentSize,
-	}
-	return cw, nil
-}
-
-func (w *chunkWriter) tail() *os.File {
-	if len(w.files) == 0 {
-		return nil
-	}
-	return w.files[len(w.files)-1]
-}
-
-// finalizeTail writes all pending data to the current tail file,
-// truncates its size, and closes it.
-func (w *chunkWriter) finalizeTail() error {
-	tf := w.tail()
-	if tf == nil {
-		return nil
-	}
-
-	if err := w.wbuf.Flush(); err != nil {
-		return err
-	}
-	if err := fileutil.Fsync(tf); err != nil {
-		return err
-	}
-	// As the file was pre-allocated, we truncate any superfluous zero bytes.
-	off, err := tf.Seek(0, os.SEEK_CUR)
-	if err != nil {
-		return err
-	}
-	if err := tf.Truncate(off); err != nil {
-		return err
-	}
-	return tf.Close()
-}
-
-func (w *chunkWriter) cut() error {
-	// Sync current tail to disk and close.
-	w.finalizeTail()
-
-	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, 8)
-	binary.BigEndian.PutUint32(metab[:4], MagicSeries)
-	metab[4] = chunksFormatV1
-
-	if _, err := f.Write(metab); err != nil {
-		return err
-	}
-
-	w.files = append(w.files, f)
-	if w.wbuf != nil {
-		w.wbuf.Reset(f)
-	} else {
-		w.wbuf = bufio.NewWriterSize(f, 8*1024*1024)
-	}
-	w.n = 8
-
-	return nil
-}
-
-func (w *chunkWriter) write(wr io.Writer, b []byte) error {
-	n, err := wr.Write(b)
-	w.n += int64(n)
-	return err
-}
-
-func (w *chunkWriter) WriteChunks(chks ...ChunkMeta) error {
-	// Calculate maximum space we need and cut a new segment in case
-	// we don't fit into the current one.
-	maxLen := int64(binary.MaxVarintLen32)
-	for _, c := range chks {
-		maxLen += binary.MaxVarintLen32 + 1
-		maxLen += int64(len(c.Chunk.Bytes()))
-	}
-	newsz := w.n + maxLen
-
-	if w.wbuf == nil || w.n > w.segmentSize || newsz > w.segmentSize && maxLen <= w.segmentSize {
-		if err := w.cut(); err != nil {
-			return err
-		}
-	}
-
-	// Write chunks sequentially and set the reference field in the ChunkMeta.
-	w.crc32.Reset()
-	wr := io.MultiWriter(w.crc32, w.wbuf)
-
-	b := make([]byte, binary.MaxVarintLen32)
-	n := binary.PutUvarint(b, uint64(len(chks)))
-
-	if err := w.write(wr, b[:n]); err != nil {
-		return err
-	}
-	seq := uint64(w.seq()) << 32
-
-	for i := range chks {
-		chk := &chks[i]
-
-		chk.Ref = seq | uint64(w.n)
-
-		n = binary.PutUvarint(b, uint64(len(chk.Chunk.Bytes())))
-
-		if err := w.write(wr, b[:n]); err != nil {
-			return err
-		}
-		if err := w.write(wr, []byte{byte(chk.Chunk.Encoding())}); err != nil {
-			return err
-		}
-		if err := w.write(wr, chk.Chunk.Bytes()); err != nil {
-			return err
-		}
-		chk.Chunk = nil
-	}
-
-	if err := w.write(w.wbuf, w.crc32.Sum(nil)); err != nil {
-		return err
-	}
-	return nil
-}
-
-func (w *chunkWriter) seq() int {
-	return len(w.files) - 1
-}
-
-func (w *chunkWriter) Close() error {
-	return w.finalizeTail()
-}
-
-// ChunkMeta holds information about a chunk of data.
-type ChunkMeta struct {
-	// Ref and Chunk hold either a reference that can be used to retrieve
-	// chunk data or the data itself.
-	// Generally, only one of them is set.
-	Ref   uint64
-	Chunk chunks.Chunk
-
-	MinTime, MaxTime int64 // time range the data covers
-}
-
 // IndexWriter serialized the index for a block of series data.
 // The methods must generally be called in order they are specified.
 type IndexWriter interface {
@@ -609,3 +409,368 @@ func (w *indexWriter) Close() error {
 	}
 	return w.f.Close()
 }
+
+// IndexReader provides reading access of serialized index data.
+type IndexReader interface {
+	// LabelValues returns the possible label values
+	LabelValues(names ...string) (StringTuples, error)
+
+	// Postings returns the postings list iterator for the label pair.
+	Postings(name, value string) (Postings, error)
+
+	// Series returns the series for the given reference.
+	Series(ref uint32) (labels.Labels, []ChunkMeta, error)
+
+	// LabelIndices returns the label pairs for which indices exist.
+	LabelIndices() ([][]string, error)
+
+	// Close released the underlying resources of the reader.
+	Close() error
+}
+
+// StringTuples provides access to a sorted list of string tuples.
+type StringTuples interface {
+	// Total number of tuples in the list.
+	Len() int
+	// At returns the tuple at position i.
+	At(i int) ([]string, error)
+}
+
+type indexReader struct {
+	// The underlying byte slice holding the encoded series data.
+	b []byte
+
+	// Close that releases the underlying resources of the byte slice.
+	c io.Closer
+
+	// Cached hashmaps of section offsets.
+	labels   map[string]uint32
+	postings map[string]uint32
+}
+
+var (
+	errInvalidSize = fmt.Errorf("invalid size")
+	errInvalidFlag = fmt.Errorf("invalid flag")
+)
+
+// newIndexReader returns a new indexReader on the given directory.
+func newIndexReader(dir string) (*indexReader, error) {
+	f, err := openMmapFile(filepath.Join(dir, "index"))
+	if err != nil {
+		return nil, err
+	}
+	r := &indexReader{b: f.b, c: f}
+
+	// Verify magic number.
+	if len(f.b) < 4 {
+		return nil, errors.Wrap(errInvalidSize, "index header")
+	}
+	if m := binary.BigEndian.Uint32(r.b[:4]); m != MagicIndex {
+		return nil, errors.Errorf("invalid magic number %x", m)
+	}
+
+	// The last two 4 bytes hold the pointers to the hashmaps.
+	loff := binary.BigEndian.Uint32(r.b[len(r.b)-8 : len(r.b)-4])
+	poff := binary.BigEndian.Uint32(r.b[len(r.b)-4:])
+
+	flag, b, err := r.section(loff)
+	if err != nil {
+		return nil, errors.Wrapf(err, "label index hashmap section at %d", loff)
+	}
+	if r.labels, err = readHashmap(flag, b); err != nil {
+		return nil, errors.Wrap(err, "read label index hashmap")
+	}
+	flag, b, err = r.section(poff)
+	if err != nil {
+		return nil, errors.Wrapf(err, "postings hashmap section at %d", loff)
+	}
+	if r.postings, err = readHashmap(flag, b); err != nil {
+		return nil, errors.Wrap(err, "read postings hashmap")
+	}
+
+	return r, nil
+}
+
+func readHashmap(flag byte, b []byte) (map[string]uint32, error) {
+	if flag != flagStd {
+		return nil, errInvalidFlag
+	}
+	h := make(map[string]uint32, 512)
+
+	for len(b) > 0 {
+		l, n := binary.Uvarint(b)
+		if n < 1 {
+			return nil, errors.Wrap(errInvalidSize, "read key length")
+		}
+		b = b[n:]
+
+		if len(b) < int(l) {
+			return nil, errors.Wrap(errInvalidSize, "read key")
+		}
+		s := string(b[:l])
+		b = b[l:]
+
+		o, n := binary.Uvarint(b)
+		if n < 1 {
+			return nil, errors.Wrap(errInvalidSize, "read offset value")
+		}
+		b = b[n:]
+
+		h[s] = uint32(o)
+	}
+
+	return h, nil
+}
+
+func (r *indexReader) Close() error {
+	return r.c.Close()
+}
+
+func (r *indexReader) section(o uint32) (byte, []byte, error) {
+	b := r.b[o:]
+
+	if len(b) < 5 {
+		return 0, nil, errors.Wrap(errInvalidSize, "read header")
+	}
+
+	flag := b[0]
+	l := binary.BigEndian.Uint32(b[1:5])
+
+	b = b[5:]
+
+	// b must have the given length plus 4 bytes for the CRC32 checksum.
+	if len(b) < int(l)+4 {
+		return 0, nil, errors.Wrap(errInvalidSize, "section content")
+	}
+	return flag, b[:l], nil
+}
+
+func (r *indexReader) lookupSymbol(o uint32) (string, error) {
+	if int(o) > len(r.b) {
+		return "", errors.Errorf("invalid symbol offset %d", o)
+	}
+	l, n := binary.Uvarint(r.b[o:])
+	if n < 0 {
+		return "", errors.New("reading symbol length failed")
+	}
+
+	end := int(o) + n + int(l)
+	if end > len(r.b) {
+		return "", errors.New("invalid length")
+	}
+	b := r.b[int(o)+n : end]
+
+	return yoloString(b), nil
+}
+
+func (r *indexReader) LabelValues(names ...string) (StringTuples, error) {
+	key := strings.Join(names, string(sep))
+	off, ok := r.labels[key]
+	if !ok {
+		return nil, fmt.Errorf("label index doesn't exist")
+	}
+
+	flag, b, err := r.section(off)
+	if err != nil {
+		return nil, errors.Wrapf(err, "section at %d", off)
+	}
+	if flag != flagStd {
+		return nil, errInvalidFlag
+	}
+	l, n := binary.Uvarint(b)
+	if n < 1 {
+		return nil, errors.Wrap(errInvalidSize, "read label index size")
+	}
+
+	st := &serializedStringTuples{
+		l:      int(l),
+		b:      b[n:],
+		lookup: r.lookupSymbol,
+	}
+	return st, nil
+}
+
+func (r *indexReader) LabelIndices() ([][]string, error) {
+	res := [][]string{}
+
+	for s := range r.labels {
+		res = append(res, strings.Split(s, string(sep)))
+	}
+	return res, nil
+}
+
+func (r *indexReader) Series(ref uint32) (labels.Labels, []ChunkMeta, error) {
+	k, n := binary.Uvarint(r.b[ref:])
+	if n < 1 {
+		return nil, nil, errors.Wrap(errInvalidSize, "number of labels")
+	}
+
+	b := r.b[int(ref)+n:]
+	lbls := make(labels.Labels, 0, k)
+
+	for i := 0; i < 2*int(k); i += 2 {
+		o, m := binary.Uvarint(b)
+		if m < 1 {
+			return nil, nil, errors.Wrap(errInvalidSize, "symbol offset")
+		}
+		n, err := r.lookupSymbol(uint32(o))
+		if err != nil {
+			return nil, nil, errors.Wrap(err, "symbol lookup")
+		}
+		b = b[m:]
+
+		o, m = binary.Uvarint(b)
+		if m < 1 {
+			return nil, nil, errors.Wrap(errInvalidSize, "symbol offset")
+		}
+		v, err := r.lookupSymbol(uint32(o))
+		if err != nil {
+			return nil, nil, errors.Wrap(err, "symbol lookup")
+		}
+		b = b[m:]
+
+		lbls = append(lbls, labels.Label{
+			Name:  n,
+			Value: v,
+		})
+	}
+
+	// Read the chunks meta data.
+	k, n = binary.Uvarint(b)
+	if n < 1 {
+		return nil, nil, errors.Wrap(errInvalidSize, "number of chunks")
+	}
+
+	b = b[n:]
+	chunks := make([]ChunkMeta, 0, k)
+
+	for i := 0; i < int(k); i++ {
+		firstTime, n := binary.Varint(b)
+		if n < 1 {
+			return nil, nil, errors.Wrap(errInvalidSize, "first time")
+		}
+		b = b[n:]
+
+		lastTime, n := binary.Varint(b)
+		if n < 1 {
+			return nil, nil, errors.Wrap(errInvalidSize, "last time")
+		}
+		b = b[n:]
+
+		o, n := binary.Uvarint(b)
+		if n < 1 {
+			return nil, nil, errors.Wrap(errInvalidSize, "chunk offset")
+		}
+		b = b[n:]
+
+		chunks = append(chunks, ChunkMeta{
+			Ref:     o,
+			MinTime: firstTime,
+			MaxTime: lastTime,
+		})
+	}
+
+	return lbls, chunks, nil
+}
+
+func (r *indexReader) Postings(name, value string) (Postings, error) {
+	key := name + string(sep) + value
+
+	off, ok := r.postings[key]
+	if !ok {
+		return nil, ErrNotFound
+	}
+
+	flag, b, err := r.section(off)
+	if err != nil {
+		return nil, errors.Wrapf(err, "section at %d", off)
+	}
+
+	if flag != flagStd {
+		return nil, errors.Wrapf(errInvalidFlag, "section at %d", off)
+	}
+
+	// TODO(fabxc): just read into memory as an intermediate solution.
+	// Add iterator over serialized data.
+	var l []uint32
+
+	for len(b) > 0 {
+		if len(b) < 4 {
+			return nil, errors.Wrap(errInvalidSize, "plain postings entry")
+		}
+		l = append(l, binary.BigEndian.Uint32(b[:4]))
+
+		b = b[4:]
+	}
+
+	return &listPostings{list: l, idx: -1}, nil
+}
+
+type stringTuples struct {
+	l int      // tuple length
+	s []string // flattened tuple entries
+}
+
+func newStringTuples(s []string, l int) (*stringTuples, error) {
+	if len(s)%l != 0 {
+		return nil, errors.Wrap(errInvalidSize, "string tuple list")
+	}
+	return &stringTuples{s: s, l: l}, nil
+}
+
+func (t *stringTuples) Len() int                   { return len(t.s) / t.l }
+func (t *stringTuples) At(i int) ([]string, error) { return t.s[i : i+t.l], nil }
+
+func (t *stringTuples) Swap(i, j int) {
+	c := make([]string, t.l)
+	copy(c, t.s[i:i+t.l])
+
+	for k := 0; k < t.l; k++ {
+		t.s[i+k] = t.s[j+k]
+		t.s[j+k] = c[k]
+	}
+}
+
+func (t *stringTuples) Less(i, j int) bool {
+	for k := 0; k < t.l; k++ {
+		d := strings.Compare(t.s[i+k], t.s[j+k])
+
+		if d < 0 {
+			return true
+		}
+		if d > 0 {
+			return false
+		}
+	}
+	return false
+}
+
+type serializedStringTuples struct {
+	l      int
+	b      []byte
+	lookup func(uint32) (string, error)
+}
+
+func (t *serializedStringTuples) Len() int {
+	// TODO(fabxc): Cache this?
+	return len(t.b) / (4 * t.l)
+}
+
+func (t *serializedStringTuples) At(i int) ([]string, error) {
+	if len(t.b) < (i+t.l)*4 {
+		return nil, errInvalidSize
+	}
+	res := make([]string, 0, t.l)
+
+	for k := 0; k < t.l; k++ {
+		offset := binary.BigEndian.Uint32(t.b[(i+k)*4:])
+
+		s, err := t.lookup(offset)
+		if err != nil {
+			return nil, errors.Wrap(err, "symbol lookup")
+		}
+		res = append(res, s)
+	}
+
+	return res, nil
+}
diff --git a/rw_test.go b/index_test.go
similarity index 90%
rename from rw_test.go
rename to index_test.go
index 4f6dad169..b65ce58a0 100644
--- a/rw_test.go
+++ b/index_test.go
@@ -40,18 +40,6 @@ func (ir *mockIndexReader) Close() error {
 	return ir.close()
 }
 
-type mockChunkReader struct {
-	chunk func(ref uint64) (chunks.Chunk, error)
-	close func() error
-}
-
-func (cr *mockChunkReader) Chunk(ref uint64) (chunks.Chunk, error) {
-	return cr.chunk(ref)
-}
-
-func (cr *mockChunkReader) Close() error {
-	return cr.close()
-}
 
 func TestPersistence_index_e2e(t *testing.T) {
 	dir, err := ioutil.TempDir("", "test_persistence_e2e")
@@ -152,8 +140,4 @@ func TestPersistence_index_e2e(t *testing.T) {
 
 	require.NoError(t, ir.Close())
 
-}
-
-func BenchmarkPersistence_index_write(b *testing.B) {
-
-}
+}
\ No newline at end of file
diff --git a/reader.go b/reader.go
deleted file mode 100644
index d4d816c1f..000000000
--- a/reader.go
+++ /dev/null
@@ -1,459 +0,0 @@
-package tsdb
-
-import (
-	"encoding/binary"
-	"fmt"
-	"io"
-	"path/filepath"
-	"strings"
-
-	"github.com/fabxc/tsdb/chunks"
-	"github.com/fabxc/tsdb/labels"
-	"github.com/pkg/errors"
-)
-
-// ChunkReader provides reading access of serialized time series data.
-type ChunkReader interface {
-	// Chunk returns the series data chunk with the given reference.
-	Chunk(ref uint64) (chunks.Chunk, error)
-
-	// Close releases all underlying resources of the reader.
-	Close() error
-}
-
-// chunkReader implements a SeriesReader for a serialized byte stream
-// of series data.
-type chunkReader struct {
-	// The underlying bytes holding the encoded series data.
-	bs [][]byte
-
-	// Closers for resources behind the byte slices.
-	cs []io.Closer
-}
-
-// newChunkReader returns a new chunkReader based on mmaped files found in dir.
-func newChunkReader(dir string) (*chunkReader, error) {
-	files, err := sequenceFiles(dir, "")
-	if err != nil {
-		return nil, err
-	}
-	var cr chunkReader
-
-	for _, fn := range files {
-		f, err := openMmapFile(fn)
-		if err != nil {
-			return nil, errors.Wrapf(err, "mmap files")
-		}
-		cr.cs = append(cr.cs, f)
-		cr.bs = append(cr.bs, f.b)
-	}
-
-	for i, b := range cr.bs {
-		if len(b) < 4 {
-			return nil, errors.Wrapf(errInvalidSize, "validate magic in segment %d", i)
-		}
-		// Verify magic number.
-		if m := binary.BigEndian.Uint32(b[:4]); m != MagicSeries {
-			return nil, fmt.Errorf("invalid magic number %x", m)
-		}
-	}
-	return &cr, nil
-}
-
-func (s *chunkReader) Close() error {
-	return closeAll(s.cs...)
-}
-
-func (s *chunkReader) Chunk(ref uint64) (chunks.Chunk, error) {
-	var (
-		seq = int(ref >> 32)
-		off = int((ref << 32) >> 32)
-	)
-	if seq >= len(s.bs) {
-		return nil, errors.Errorf("reference sequence %d out of range", seq)
-	}
-	b := s.bs[seq]
-
-	if int(off) >= len(b) {
-		return nil, errors.Errorf("offset %d beyond data size %d", off, len(b))
-	}
-	b = b[off:]
-
-	l, n := binary.Uvarint(b)
-	if n < 0 {
-		return nil, fmt.Errorf("reading chunk length failed")
-	}
-	b = b[n:]
-	enc := chunks.Encoding(b[0])
-
-	c, err := chunks.FromData(enc, b[1:1+l])
-	if err != nil {
-		return nil, err
-	}
-	return c, nil
-}
-
-// IndexReader provides reading access of serialized index data.
-type IndexReader interface {
-	// LabelValues returns the possible label values
-	LabelValues(names ...string) (StringTuples, error)
-
-	// Postings returns the postings list iterator for the label pair.
-	Postings(name, value string) (Postings, error)
-
-	// Series returns the series for the given reference.
-	Series(ref uint32) (labels.Labels, []ChunkMeta, error)
-
-	// LabelIndices returns the label pairs for which indices exist.
-	LabelIndices() ([][]string, error)
-
-	// Close released the underlying resources of the reader.
-	Close() error
-}
-
-// StringTuples provides access to a sorted list of string tuples.
-type StringTuples interface {
-	// Total number of tuples in the list.
-	Len() int
-	// At returns the tuple at position i.
-	At(i int) ([]string, error)
-}
-
-type indexReader struct {
-	// The underlying byte slice holding the encoded series data.
-	b []byte
-
-	// Close that releases the underlying resources of the byte slice.
-	c io.Closer
-
-	// Cached hashmaps of section offsets.
-	labels   map[string]uint32
-	postings map[string]uint32
-}
-
-var (
-	errInvalidSize = fmt.Errorf("invalid size")
-	errInvalidFlag = fmt.Errorf("invalid flag")
-)
-
-// newIndexReader returns a new indexReader on the given directory.
-func newIndexReader(dir string) (*indexReader, error) {
-	f, err := openMmapFile(filepath.Join(dir, "index"))
-	if err != nil {
-		return nil, err
-	}
-	r := &indexReader{b: f.b, c: f}
-
-	// Verify magic number.
-	if len(f.b) < 4 {
-		return nil, errors.Wrap(errInvalidSize, "index header")
-	}
-	if m := binary.BigEndian.Uint32(r.b[:4]); m != MagicIndex {
-		return nil, errors.Errorf("invalid magic number %x", m)
-	}
-
-	// The last two 4 bytes hold the pointers to the hashmaps.
-	loff := binary.BigEndian.Uint32(r.b[len(r.b)-8 : len(r.b)-4])
-	poff := binary.BigEndian.Uint32(r.b[len(r.b)-4:])
-
-	flag, b, err := r.section(loff)
-	if err != nil {
-		return nil, errors.Wrapf(err, "label index hashmap section at %d", loff)
-	}
-	if r.labels, err = readHashmap(flag, b); err != nil {
-		return nil, errors.Wrap(err, "read label index hashmap")
-	}
-	flag, b, err = r.section(poff)
-	if err != nil {
-		return nil, errors.Wrapf(err, "postings hashmap section at %d", loff)
-	}
-	if r.postings, err = readHashmap(flag, b); err != nil {
-		return nil, errors.Wrap(err, "read postings hashmap")
-	}
-
-	return r, nil
-}
-
-func readHashmap(flag byte, b []byte) (map[string]uint32, error) {
-	if flag != flagStd {
-		return nil, errInvalidFlag
-	}
-	h := make(map[string]uint32, 512)
-
-	for len(b) > 0 {
-		l, n := binary.Uvarint(b)
-		if n < 1 {
-			return nil, errors.Wrap(errInvalidSize, "read key length")
-		}
-		b = b[n:]
-
-		if len(b) < int(l) {
-			return nil, errors.Wrap(errInvalidSize, "read key")
-		}
-		s := string(b[:l])
-		b = b[l:]
-
-		o, n := binary.Uvarint(b)
-		if n < 1 {
-			return nil, errors.Wrap(errInvalidSize, "read offset value")
-		}
-		b = b[n:]
-
-		h[s] = uint32(o)
-	}
-
-	return h, nil
-}
-
-func (r *indexReader) Close() error {
-	return r.c.Close()
-}
-
-func (r *indexReader) section(o uint32) (byte, []byte, error) {
-	b := r.b[o:]
-
-	if len(b) < 5 {
-		return 0, nil, errors.Wrap(errInvalidSize, "read header")
-	}
-
-	flag := b[0]
-	l := binary.BigEndian.Uint32(b[1:5])
-
-	b = b[5:]
-
-	// b must have the given length plus 4 bytes for the CRC32 checksum.
-	if len(b) < int(l)+4 {
-		return 0, nil, errors.Wrap(errInvalidSize, "section content")
-	}
-	return flag, b[:l], nil
-}
-
-func (r *indexReader) lookupSymbol(o uint32) (string, error) {
-	if int(o) > len(r.b) {
-		return "", errors.Errorf("invalid symbol offset %d", o)
-	}
-	l, n := binary.Uvarint(r.b[o:])
-	if n < 0 {
-		return "", errors.New("reading symbol length failed")
-	}
-
-	end := int(o) + n + int(l)
-	if end > len(r.b) {
-		return "", errors.New("invalid length")
-	}
-	b := r.b[int(o)+n : end]
-
-	return yoloString(b), nil
-}
-
-func (r *indexReader) LabelValues(names ...string) (StringTuples, error) {
-	key := strings.Join(names, string(sep))
-	off, ok := r.labels[key]
-	if !ok {
-		return nil, fmt.Errorf("label index doesn't exist")
-	}
-
-	flag, b, err := r.section(off)
-	if err != nil {
-		return nil, errors.Wrapf(err, "section at %d", off)
-	}
-	if flag != flagStd {
-		return nil, errInvalidFlag
-	}
-	l, n := binary.Uvarint(b)
-	if n < 1 {
-		return nil, errors.Wrap(errInvalidSize, "read label index size")
-	}
-
-	st := &serializedStringTuples{
-		l:      int(l),
-		b:      b[n:],
-		lookup: r.lookupSymbol,
-	}
-	return st, nil
-}
-
-func (r *indexReader) LabelIndices() ([][]string, error) {
-	res := [][]string{}
-
-	for s := range r.labels {
-		res = append(res, strings.Split(s, string(sep)))
-	}
-	return res, nil
-}
-
-func (r *indexReader) Series(ref uint32) (labels.Labels, []ChunkMeta, error) {
-	k, n := binary.Uvarint(r.b[ref:])
-	if n < 1 {
-		return nil, nil, errors.Wrap(errInvalidSize, "number of labels")
-	}
-
-	b := r.b[int(ref)+n:]
-	lbls := make(labels.Labels, 0, k)
-
-	for i := 0; i < 2*int(k); i += 2 {
-		o, m := binary.Uvarint(b)
-		if m < 1 {
-			return nil, nil, errors.Wrap(errInvalidSize, "symbol offset")
-		}
-		n, err := r.lookupSymbol(uint32(o))
-		if err != nil {
-			return nil, nil, errors.Wrap(err, "symbol lookup")
-		}
-		b = b[m:]
-
-		o, m = binary.Uvarint(b)
-		if m < 1 {
-			return nil, nil, errors.Wrap(errInvalidSize, "symbol offset")
-		}
-		v, err := r.lookupSymbol(uint32(o))
-		if err != nil {
-			return nil, nil, errors.Wrap(err, "symbol lookup")
-		}
-		b = b[m:]
-
-		lbls = append(lbls, labels.Label{
-			Name:  n,
-			Value: v,
-		})
-	}
-
-	// Read the chunks meta data.
-	k, n = binary.Uvarint(b)
-	if n < 1 {
-		return nil, nil, errors.Wrap(errInvalidSize, "number of chunks")
-	}
-
-	b = b[n:]
-	chunks := make([]ChunkMeta, 0, k)
-
-	for i := 0; i < int(k); i++ {
-		firstTime, n := binary.Varint(b)
-		if n < 1 {
-			return nil, nil, errors.Wrap(errInvalidSize, "first time")
-		}
-		b = b[n:]
-
-		lastTime, n := binary.Varint(b)
-		if n < 1 {
-			return nil, nil, errors.Wrap(errInvalidSize, "last time")
-		}
-		b = b[n:]
-
-		o, n := binary.Uvarint(b)
-		if n < 1 {
-			return nil, nil, errors.Wrap(errInvalidSize, "chunk offset")
-		}
-		b = b[n:]
-
-		chunks = append(chunks, ChunkMeta{
-			Ref:     o,
-			MinTime: firstTime,
-			MaxTime: lastTime,
-		})
-	}
-
-	return lbls, chunks, nil
-}
-
-func (r *indexReader) Postings(name, value string) (Postings, error) {
-	key := name + string(sep) + value
-
-	off, ok := r.postings[key]
-	if !ok {
-		return nil, ErrNotFound
-	}
-
-	flag, b, err := r.section(off)
-	if err != nil {
-		return nil, errors.Wrapf(err, "section at %d", off)
-	}
-
-	if flag != flagStd {
-		return nil, errors.Wrapf(errInvalidFlag, "section at %d", off)
-	}
-
-	// TODO(fabxc): just read into memory as an intermediate solution.
-	// Add iterator over serialized data.
-	var l []uint32
-
-	for len(b) > 0 {
-		if len(b) < 4 {
-			return nil, errors.Wrap(errInvalidSize, "plain postings entry")
-		}
-		l = append(l, binary.BigEndian.Uint32(b[:4]))
-
-		b = b[4:]
-	}
-
-	return &listPostings{list: l, idx: -1}, nil
-}
-
-type stringTuples struct {
-	l int      // tuple length
-	s []string // flattened tuple entries
-}
-
-func newStringTuples(s []string, l int) (*stringTuples, error) {
-	if len(s)%l != 0 {
-		return nil, errors.Wrap(errInvalidSize, "string tuple list")
-	}
-	return &stringTuples{s: s, l: l}, nil
-}
-
-func (t *stringTuples) Len() int                   { return len(t.s) / t.l }
-func (t *stringTuples) At(i int) ([]string, error) { return t.s[i : i+t.l], nil }
-
-func (t *stringTuples) Swap(i, j int) {
-	c := make([]string, t.l)
-	copy(c, t.s[i:i+t.l])
-
-	for k := 0; k < t.l; k++ {
-		t.s[i+k] = t.s[j+k]
-		t.s[j+k] = c[k]
-	}
-}
-
-func (t *stringTuples) Less(i, j int) bool {
-	for k := 0; k < t.l; k++ {
-		d := strings.Compare(t.s[i+k], t.s[j+k])
-
-		if d < 0 {
-			return true
-		}
-		if d > 0 {
-			return false
-		}
-	}
-	return false
-}
-
-type serializedStringTuples struct {
-	l      int
-	b      []byte
-	lookup func(uint32) (string, error)
-}
-
-func (t *serializedStringTuples) Len() int {
-	// TODO(fabxc): Cache this?
-	return len(t.b) / (4 * t.l)
-}
-
-func (t *serializedStringTuples) At(i int) ([]string, error) {
-	if len(t.b) < (i+t.l)*4 {
-		return nil, errInvalidSize
-	}
-	res := make([]string, 0, t.l)
-
-	for k := 0; k < t.l; k++ {
-		offset := binary.BigEndian.Uint32(t.b[(i+k)*4:])
-
-		s, err := t.lookup(offset)
-		if err != nil {
-			return nil, errors.Wrap(err, "symbol lookup")
-		}
-		res = append(res, s)
-	}
-
-	return res, nil
-}