diff --git a/block.go b/block.go
index 2bd1fd364..034f33673 100644
--- a/block.go
+++ b/block.go
@@ -142,8 +142,8 @@ type Block struct {
 	dir  string
 	meta BlockMeta
 
-	chunkr *chunkReader
-	indexr *indexReader
+	chunkr ChunkReader
+	indexr IndexReader
 
 	tombstones tombstoneReader
 }
@@ -156,11 +156,11 @@ func OpenBlock(dir string, pool chunks.Pool) (*Block, error) {
 		return nil, err
 	}
 
-	cr, err := newChunkReader(chunkDir(dir), pool)
+	cr, err := NewDirChunkReader(chunkDir(dir), pool)
 	if err != nil {
 		return nil, err
 	}
-	ir, err := newIndexReader(dir)
+	ir, err := NewFileIndexReader(filepath.Join(dir, "index"))
 	if err != nil {
 		return nil, err
 	}
diff --git a/chunks.go b/chunks.go
index f6e329b79..30ab93f80 100644
--- a/chunks.go
+++ b/chunks.go
@@ -298,7 +298,7 @@ type ChunkReader interface {
 // of series data.
 type chunkReader struct {
 	// The underlying bytes holding the encoded series data.
-	bs [][]byte
+	bs []ByteSlice
 
 	// Closers for resources behind the byte slices.
 	cs []io.Closer
@@ -306,8 +306,32 @@ type chunkReader struct {
 	pool chunks.Pool
 }
 
-// newChunkReader returns a new chunkReader based on mmaped files found in dir.
-func newChunkReader(dir string, pool chunks.Pool) (*chunkReader, error) {
+func newChunkReader(bs []ByteSlice, cs []io.Closer, pool chunks.Pool) (*chunkReader, error) {
+	cr := chunkReader{pool: pool, bs: bs, cs: cs}
+
+	for i, b := range cr.bs {
+		if b.Len() < 4 {
+			return nil, errors.Wrapf(errInvalidSize, "validate magic in segment %d", i)
+		}
+		// Verify magic number.
+		if m := binary.BigEndian.Uint32(b.Range(0, 4)); m != MagicChunks {
+			return nil, fmt.Errorf("invalid magic number %x", m)
+		}
+	}
+	return &cr, nil
+}
+
+// NewChunkReader returns a new chunk reader against the given byte slices.
+func NewChunkReader(bs []ByteSlice, pool chunks.Pool) (ChunkReader, error) {
+	if pool == nil {
+		pool = chunks.NewPool()
+	}
+	return newChunkReader(bs, nil, pool)
+}
+
+// NewDirChunkReader returns a new ChunkReader against sequentially numbered files in the
+// given directory.
+func NewDirChunkReader(dir string, pool chunks.Pool) (ChunkReader, error) {
 	files, err := sequenceFiles(dir)
 	if err != nil {
 		return nil, err
@@ -315,27 +339,19 @@ func newChunkReader(dir string, pool chunks.Pool) (*chunkReader, error) {
 	if pool == nil {
 		pool = chunks.NewPool()
 	}
-	cr := chunkReader{pool: pool}
+
+	var bs []ByteSlice
+	var cs []io.Closer
 
 	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)
+		cs = append(cs, f)
+		bs = append(bs, realByteSlice(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
+	return newChunkReader(bs, cs, pool)
 }
 
 func (s *chunkReader) Close() error {
@@ -352,16 +368,18 @@ func (s *chunkReader) Chunk(ref uint64) (chunks.Chunk, error) {
 	}
 	b := s.bs[seq]
 
-	if int(off) >= len(b) {
-		return nil, errors.Errorf("offset %d beyond data size %d", off, len(b))
+	if int(off) >= b.Len() {
+		return nil, errors.Errorf("offset %d beyond data size %d", off, b.Len())
 	}
-	b = b[off:]
+	// With the minimum chunk length this should never cause us reading
+	// over the end of the slice.
+	r := b.Range(off, off+binary.MaxVarintLen32)
 
-	l, n := binary.Uvarint(b)
+	l, n := binary.Uvarint(r)
 	if n < 0 {
 		return nil, fmt.Errorf("reading chunk length failed")
 	}
-	b = b[n:]
+	r = b.Range(off+n, off+n+int(l))
 
-	return s.pool.Get(chunks.Encoding(b[0]), b[1:1+l])
+	return s.pool.Get(chunks.Encoding(r[0]), r[1:1+l])
 }
diff --git a/encoding_helpers.go b/encoding_helpers.go
index 9aa4ba409..b55c7fda9 100644
--- a/encoding_helpers.go
+++ b/encoding_helpers.go
@@ -3,6 +3,7 @@ package tsdb
 import (
 	"encoding/binary"
 	"hash"
+	"hash/crc32"
 	"unsafe"
 )
 
@@ -77,6 +78,11 @@ func (d *decbuf) uvarint32() uint32 { return uint32(d.uvarint64()) }
 func (d *decbuf) be32int() int      { return int(d.be32()) }
 func (d *decbuf) be64int64() int64  { return int64(d.be64()) }
 
+// crc32 returns a CRC32 checksum over the remaining bytes.
+func (d *decbuf) crc32() uint32 {
+	return crc32.Checksum(d.b, castagnoliTable)
+}
+
 func (d *decbuf) uvarintStr() string {
 	l := d.uvarint64()
 	if d.e != nil {
diff --git a/index.go b/index.go
index 36507f645..6895c16f4 100644
--- a/index.go
+++ b/index.go
@@ -560,7 +560,7 @@ type StringTuples interface {
 
 type indexReader struct {
 	// The underlying byte slice holding the encoded series data.
-	b   []byte
+	b   ByteSlice
 	toc indexTOC
 
 	// Close that releases the underlying resources of the byte slice.
@@ -585,27 +585,52 @@ var (
 	errInvalidChecksum = fmt.Errorf("invalid checksum")
 )
 
-// NewIndexReader returns a new IndexReader on the given directory.
-func NewIndexReader(dir string) (IndexReader, error) { return newIndexReader(dir) }
+// ByteSlice abstracts a byte slice.
+type ByteSlice interface {
+	Len() int
+	Range(start, end int) []byte
+}
 
-// newIndexReader returns a new indexReader on the given directory.
-func newIndexReader(dir string) (*indexReader, error) {
-	f, err := openMmapFile(filepath.Join(dir, "index"))
+type realByteSlice []byte
+
+func (b realByteSlice) Len() int {
+	return len(b)
+}
+
+func (b realByteSlice) Range(start, end int) []byte {
+	return b[start:end]
+}
+
+func (b realByteSlice) Sub(start, end int) ByteSlice {
+	return b[start:end]
+}
+
+// NewIndexReader returns a new IndexReader on the given byte slice.
+func NewIndexReader(b ByteSlice) (IndexReader, error) {
+	return newIndexReader(b, nil)
+}
+
+// NewFileIndexReader returns a new index reader against the given index file.
+func NewFileIndexReader(path string) (IndexReader, error) {
+	f, err := openMmapFile(path)
 	if err != nil {
 		return nil, err
 	}
+	return newIndexReader(realByteSlice(f.b), f)
+}
+
+func newIndexReader(b ByteSlice, c io.Closer) (*indexReader, error) {
 	r := &indexReader{
-		b:       f.b,
-		c:       f,
+		b:       b,
+		c:       c,
 		symbols: map[uint32]string{},
 		crc32:   newCRC32(),
 	}
-
 	// Verify magic number.
-	if len(f.b) < 4 {
+	if b.Len() < 4 {
 		return nil, errors.Wrap(errInvalidSize, "index header")
 	}
-	if m := binary.BigEndian.Uint32(r.b[:4]); m != MagicIndex {
+	if m := binary.BigEndian.Uint32(r.b.Range(0, 4)); m != MagicIndex {
 		return nil, errors.Errorf("invalid magic number %x", m)
 	}
 
@@ -615,6 +640,7 @@ func newIndexReader(dir string) (*indexReader, error) {
 	if err := r.readSymbols(int(r.toc.symbols)); err != nil {
 		return nil, errors.Wrap(err, "read symbols")
 	}
+	var err error
 
 	r.labels, err = r.readOffsetTable(r.toc.labelIndicesTable)
 	if err != nil {
@@ -625,7 +651,17 @@ func newIndexReader(dir string) (*indexReader, error) {
 }
 
 func (r *indexReader) readTOC() error {
-	d := r.decbufAt(len(r.b) - indexTOCLen)
+	if r.b.Len() < indexTOCLen {
+		return errInvalidSize
+	}
+	b := r.b.Range(r.b.Len()-indexTOCLen, r.b.Len())
+
+	expCRC := binary.BigEndian.Uint32(b[len(b)-4:])
+	d := decbuf{b: b[:len(b)-4]}
+
+	if d.crc32() != expCRC {
+		return errInvalidChecksum
+	}
 
 	r.toc.symbols = d.be64()
 	r.toc.series = d.be64()
@@ -634,32 +670,61 @@ func (r *indexReader) readTOC() error {
 	r.toc.postings = d.be64()
 	r.toc.postingsTable = d.be64()
 
-	if valid, err := r.checkCRC(d.be32(), len(r.b)-indexTOCLen, indexTOCLen-4); !valid {
-		return errors.Wrap(err, "TOC checksum")
-	}
-
 	return d.err()
 }
 
+// decbufAt returns a new decoding buffer. It expects the first 4 bytes
+// after offset to hold the big endian encoded content length, followed by the contents and the expected
+// checksum.
 func (r *indexReader) decbufAt(off int) decbuf {
-	if len(r.b) < off {
+	if r.b.Len() < off+4 {
 		return decbuf{e: errInvalidSize}
 	}
-	return decbuf{b: r.b[off:]}
+	b := r.b.Range(off, off+4)
+	l := int(binary.BigEndian.Uint32(b))
+
+	if r.b.Len() < off+4+l+4 {
+		return decbuf{e: errInvalidSize}
+	}
+
+	// Load bytes holding the contents plus a CRC32 checksum.
+	b = r.b.Range(off+4, off+4+l+4)
+	dec := decbuf{b: b[:len(b)-4]}
+
+	if exp := binary.BigEndian.Uint32(b[len(b)-4:]); dec.crc32() != exp {
+		return decbuf{e: errInvalidChecksum}
+	}
+	return dec
 }
 
-func (r *indexReader) checkCRC(crc uint32, off, cnt int) (bool, error) {
-	r.crc32.Reset()
-	if len(r.b) < off+cnt {
-		return false, errInvalidSize
+// decbufUvarintAt returns a new decoding buffer. It expects the first bytes
+// after offset to hold the uvarint-encoded buffers length, followed by the contents and the expected
+// checksum.
+func (r *indexReader) decbufUvarintAt(off int) decbuf {
+	// We never have to access this method at the far end of the byte slice. Thus just checking
+	// against the MaxVarintLen32 is sufficient.
+	if r.b.Len() < off+binary.MaxVarintLen32 {
+		return decbuf{e: errInvalidSize}
 	}
-	if _, err := r.crc32.Write(r.b[off : off+cnt]); err != nil {
-		return false, errors.Wrap(err, "write to hash")
+	b := r.b.Range(off, off+binary.MaxVarintLen32)
+
+	l, n := binary.Uvarint(b)
+	if n > binary.MaxVarintLen32 {
+		return decbuf{e: errors.New("invalid uvarint")}
 	}
-	if r.crc32.Sum32() != crc {
-		return false, errInvalidChecksum
+
+	if r.b.Len() < off+n+int(l)+4 {
+		return decbuf{e: errInvalidSize}
 	}
-	return true, nil
+
+	// Load bytes holding the contents plus a CRC32 checksum.
+	b = r.b.Range(off+n, off+n+int(l)+4)
+	dec := decbuf{b: b[:len(b)-4]}
+
+	if dec.crc32() != binary.BigEndian.Uint32(b[len(b)-4:]) {
+		return decbuf{e: errInvalidChecksum}
+	}
+	return dec
 }
 
 // readSymbols reads the symbol table fully into memory and allocates proper strings for them.
@@ -669,26 +734,22 @@ func (r *indexReader) readSymbols(off int) error {
 	if off == 0 {
 		return nil
 	}
+	d := r.decbufAt(off)
+
 	var (
-		d1      = r.decbufAt(int(off))
-		l       = d1.be32int()
-		d2      = d1.decbuf(l)
-		origLen = d2.len()
-		cnt     = d2.be32int()
+		origLen = d.len()
+		cnt     = d.be32int()
 		basePos = uint32(off) + 4
-		nextPos = basePos + uint32(origLen-d2.len())
+		nextPos = basePos + uint32(origLen-d.len())
 	)
-	for d2.err() == nil && d2.len() > 0 && cnt > 0 {
-		s := d2.uvarintStr()
+	for d.err() == nil && d.len() > 0 && cnt > 0 {
+		s := d.uvarintStr()
 		r.symbols[uint32(nextPos)] = s
 
-		nextPos = basePos + uint32(origLen-d2.len())
+		nextPos = basePos + uint32(origLen-d.len())
 		cnt--
 	}
-	if valid, err := r.checkCRC(d1.be32(), int(off)+4, l); !valid {
-		return errors.Wrap(err, "symbol table checksum")
-	}
-	return d2.err()
+	return d.err()
 }
 
 // readOffsetTable reads an offset table at the given position and returns a map
@@ -696,57 +757,29 @@ func (r *indexReader) readSymbols(off int) error {
 func (r *indexReader) readOffsetTable(off uint64) (map[string]uint32, error) {
 	const sep = "\xff"
 
-	var (
-		d1  = r.decbufAt(int(off))
-		l   = d1.be32int()
-		d2  = d1.decbuf(l)
-		cnt = d2.be32()
-	)
+	d := r.decbufAt(int(off))
+	cnt := d.be32()
 
-	res := make(map[string]uint32, 512)
+	res := make(map[string]uint32, cnt)
 
-	for d2.err() == nil && d2.len() > 0 && cnt > 0 {
-		keyCount := int(d2.uvarint())
+	for d.err() == nil && d.len() > 0 && cnt > 0 {
+		keyCount := int(d.uvarint())
 		keys := make([]string, 0, keyCount)
 
 		for i := 0; i < keyCount; i++ {
-			keys = append(keys, d2.uvarintStr())
+			keys = append(keys, d.uvarintStr())
 		}
-		res[strings.Join(keys, sep)] = uint32(d2.uvarint())
+		res[strings.Join(keys, sep)] = uint32(d.uvarint())
 
 		cnt--
 	}
-
-	if valid, err := r.checkCRC(d1.be32(), int(off)+4, l); !valid {
-		return res, errors.Wrap(err, "offset table checksum")
-	}
-
-	return res, d2.err()
+	return res, d.err()
 }
 
 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) {
 	s, ok := r.symbols[o]
 	if !ok {
@@ -776,24 +809,17 @@ func (r *indexReader) LabelValues(names ...string) (StringTuples, error) {
 		//return nil, fmt.Errorf("label index doesn't exist")
 	}
 
-	d1 := r.decbufAt(int(off))
-	l := d1.be32int()
-	d2 := d1.decbuf(l)
+	d := r.decbufAt(int(off))
 
-	nc := d2.be32int()
-	d2.be32() // consume unused value entry count.
+	nc := d.be32int()
+	d.be32() // consume unused value entry count.
 
-	if d2.err() != nil {
-		return nil, errors.Wrap(d2.err(), "read label value index")
+	if d.err() != nil {
+		return nil, errors.Wrap(d.err(), "read label value index")
 	}
-
-	if valid, err := r.checkCRC(d1.be32(), int(off)+4, l); !valid {
-		return nil, errors.Wrap(err, "read label values checksum")
-	}
-
 	st := &serializedStringTuples{
 		l:      nc,
-		b:      d2.get(),
+		b:      d.get(),
 		lookup: r.lookupSymbol,
 	}
 	return st, nil
@@ -816,22 +842,19 @@ func (r *indexReader) LabelIndices() ([][]string, error) {
 }
 
 func (r *indexReader) Series(ref uint64, lbls *labels.Labels, chks *[]ChunkMeta) error {
-	d1 := r.decbufAt(int(ref))
-	l := d1.uvarint()
-	sl := len(r.b[ref:]) - d1.len() // # bytes in l
-	d2 := d1.decbuf(l)
+	d := r.decbufUvarintAt(int(ref))
 
 	*lbls = (*lbls)[:0]
 	*chks = (*chks)[:0]
 
-	k := int(d2.uvarint())
+	k := int(d.uvarint())
 
 	for i := 0; i < k; i++ {
-		lno := uint32(d2.uvarint())
-		lvo := uint32(d2.uvarint())
+		lno := uint32(d.uvarint())
+		lvo := uint32(d.uvarint())
 
-		if d2.err() != nil {
-			return errors.Wrap(d2.err(), "read series label offsets")
+		if d.err() != nil {
+			return errors.Wrap(d.err(), "read series label offsets")
 		}
 
 		ln, err := r.lookupSymbol(lno)
@@ -847,15 +870,15 @@ func (r *indexReader) Series(ref uint64, lbls *labels.Labels, chks *[]ChunkMeta)
 	}
 
 	// Read the chunks meta data.
-	k = int(d2.uvarint())
+	k = int(d.uvarint())
 
 	if k == 0 {
 		return nil
 	}
 
-	t0 := d2.varint64()
-	maxt := int64(d2.uvarint64()) + t0
-	ref0 := int64(d2.uvarint64())
+	t0 := d.varint64()
+	maxt := int64(d.uvarint64()) + t0
+	ref0 := int64(d.uvarint64())
 
 	*chks = append(*chks, ChunkMeta{
 		Ref:     uint64(ref0),
@@ -865,14 +888,14 @@ func (r *indexReader) Series(ref uint64, lbls *labels.Labels, chks *[]ChunkMeta)
 	t0 = maxt
 
 	for i := 1; i < k; i++ {
-		mint := int64(d2.uvarint64()) + t0
-		maxt := int64(d2.uvarint64()) + mint
+		mint := int64(d.uvarint64()) + t0
+		maxt := int64(d.uvarint64()) + mint
 
-		ref0 += d2.varint64()
+		ref0 += d.varint64()
 		t0 = maxt
 
-		if d2.err() != nil {
-			return errors.Wrapf(d2.err(), "read meta for chunk %d", i)
+		if d.err() != nil {
+			return errors.Wrapf(d.err(), "read meta for chunk %d", i)
 		}
 
 		*chks = append(*chks, ChunkMeta{
@@ -881,12 +904,7 @@ func (r *indexReader) Series(ref uint64, lbls *labels.Labels, chks *[]ChunkMeta)
 			MaxTime: maxt,
 		})
 	}
-
-	if valid, err := r.checkCRC(d1.be32(), int(ref)+sl, l); !valid {
-		return errors.Wrap(err, "series checksum")
-	}
-
-	return nil
+	return d.err()
 }
 
 func (r *indexReader) Postings(name, value string) (Postings, error) {
@@ -897,22 +915,10 @@ func (r *indexReader) Postings(name, value string) (Postings, error) {
 	if !ok {
 		return emptyPostings, nil
 	}
+	d := r.decbufAt(int(off))
+	d.be32() // consume unused postings list length.
 
-	d1 := r.decbufAt(int(off))
-	l := d1.be32int()
-	d2 := d1.decbuf(l)
-
-	d2.be32() // consume unused postings list length.
-
-	if d2.err() != nil {
-		return nil, errors.Wrap(d2.err(), "get postings bytes")
-	}
-
-	if valid, err := r.checkCRC(d1.be32(), int(off)+4, l); !valid {
-		return nil, errors.Wrap(err, "postings checksum")
-	}
-
-	return newBigEndianPostings(d2.get()), nil
+	return newBigEndianPostings(d.get()), errors.Wrap(d.err(), "get postings bytes")
 }
 
 func (r *indexReader) SortedPostings(p Postings) Postings {
diff --git a/index_test.go b/index_test.go
index 7b908e39a..f18aba0eb 100644
--- a/index_test.go
+++ b/index_test.go
@@ -159,7 +159,7 @@ func TestIndexRW_Create_Open(t *testing.T) {
 	require.NoError(t, err, "create index writer")
 	require.NoError(t, iw.Close(), "close index writer")
 
-	ir, err := newIndexReader(dir)
+	ir, err := NewFileIndexReader(filepath.Join(dir, "index"))
 	require.NoError(t, err, "open index reader")
 	require.NoError(t, ir.Close(), "close index reader")
 
@@ -169,7 +169,7 @@ func TestIndexRW_Create_Open(t *testing.T) {
 	_, err = f.WriteAt([]byte{0, 0}, 0)
 	require.NoError(t, err)
 
-	_, err = newIndexReader(dir)
+	_, err = NewFileIndexReader(dir)
 	require.Error(t, err)
 }
 
@@ -210,7 +210,7 @@ func TestIndexRW_Postings(t *testing.T) {
 
 	require.NoError(t, iw.Close())
 
-	ir, err := newIndexReader(dir)
+	ir, err := NewFileIndexReader(filepath.Join(dir, "index"))
 	require.NoError(t, err, "open index reader")
 
 	p, err := ir.Postings("a", "1")
@@ -325,7 +325,7 @@ func TestPersistence_index_e2e(t *testing.T) {
 	err = iw.Close()
 	require.NoError(t, err)
 
-	ir, err := newIndexReader(dir)
+	ir, err := NewFileIndexReader(filepath.Join(dir, "index"))
 	require.NoError(t, err)
 
 	for p := range mi.postings.m {
diff --git a/test/hash_test.go b/test/hash_test.go
index fa448b4c6..117616fd7 100644
--- a/test/hash_test.go
+++ b/test/hash_test.go
@@ -14,6 +14,9 @@
 package test
 
 import (
+	"crypto/rand"
+	"fmt"
+	"hash/crc32"
 	"testing"
 
 	"github.com/cespare/xxhash"
@@ -76,3 +79,46 @@ func fnv64a(b []byte) uint64 {
 	}
 	return h
 }
+
+func BenchmarkCRC32_diff(b *testing.B) {
+
+	data := [][]byte{}
+
+	for i := 0; i < 1000; i++ {
+		b := make([]byte, 512)
+		rand.Read(b)
+		data = append(data, b)
+	}
+
+	ctab := crc32.MakeTable(crc32.Castagnoli)
+	total := uint32(0)
+
+	b.Run("direct", func(b *testing.B) {
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			total += crc32.Checksum(data[i%1000], ctab)
+		}
+	})
+	b.Run("hash-reuse", func(b *testing.B) {
+		b.ReportAllocs()
+		h := crc32.New(ctab)
+
+		for i := 0; i < b.N; i++ {
+			h.Reset()
+			h.Write(data[i%1000])
+			total += h.Sum32()
+		}
+	})
+	b.Run("hash-new", func(b *testing.B) {
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			h := crc32.New(ctab)
+			h.Write(data[i%1000])
+			total += h.Sum32()
+		}
+	})
+
+	fmt.Println(total)
+}