diff --git a/Documentation/format/index.md b/Documentation/format/index.md
index 5fbdb176e..606de597a 100644
--- a/Documentation/format/index.md
+++ b/Documentation/format/index.md
@@ -18,14 +18,14 @@ It is terminated by a table of contents which serves as an entry point into the
 │ ├──────────────────────────────────────────────┤ │
 │ │                 Label Index N                │ │
 │ ├──────────────────────────────────────────────┤ │
-│ │               Label Index Table              │ │
-│ ├──────────────────────────────────────────────┤ │
 │ │                   Postings 1                 │ │
 │ ├──────────────────────────────────────────────┤ │
 │ │                      ...                     │ │
 │ ├──────────────────────────────────────────────┤ │
 │ │                   Postings N                 │ │
 │ ├──────────────────────────────────────────────┤ │
+│ │               Label Index Table              │ │
+│ ├──────────────────────────────────────────────┤ │
 │ │                 Postings Table               │ │
 │ ├──────────────────────────────────────────────┤ │
 │ │                      TOC                     │ │
diff --git a/block.go b/block.go
index 232e64e67..90915a1f0 100644
--- a/block.go
+++ b/block.go
@@ -26,6 +26,7 @@ import (
 	"github.com/prometheus/tsdb/labels"
 )
 
+// DiskBlock represents a data block backed by on-disk data.
 type DiskBlock interface {
 	BlockReader
 
@@ -42,6 +43,7 @@ type DiskBlock interface {
 	Close() error
 }
 
+// BlockReader provides reading access to a data block.
 type BlockReader interface {
 	// Index returns an IndexReader over the block's data.
 	Index() IndexReader
@@ -53,11 +55,6 @@ type BlockReader interface {
 	Tombstones() TombstoneReader
 }
 
-// Snapshottable defines an entity that can be backedup online.
-type Snapshottable interface {
-	Snapshot(dir string) error
-}
-
 // Appendable defines an entity to which data can be appended.
 type Appendable interface {
 	// Appender returns a new Appender against an underlying store.
diff --git a/encoding_helpers.go b/encoding_helpers.go
index 17c3ff081..9aa4ba409 100644
--- a/encoding_helpers.go
+++ b/encoding_helpers.go
@@ -77,22 +77,6 @@ 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()) }
 
-// uvarintTempStr decodes like uvarintStr but the returned string is
-// not safe to use if the underyling buffer changes.
-func (d *decbuf) uvarintTempStr() string {
-	l := d.uvarint64()
-	if d.e != nil {
-		return ""
-	}
-	if len(d.b) < int(l) {
-		d.e = errInvalidSize
-		return ""
-	}
-	s := yoloString(d.b[:l])
-	d.b = d.b[l:]
-	return s
-}
-
 func (d *decbuf) uvarintStr() string {
 	l := d.uvarint64()
 	if d.e != nil {
diff --git a/index.go b/index.go
index 3cdaad74d..9beb916f4 100644
--- a/index.go
+++ b/index.go
@@ -232,13 +232,13 @@ func (w *indexWriter) ensureStage(s indexWriterStage) error {
 		w.toc.labelIndices = w.pos
 
 	case idxStagePostings:
+		w.toc.postings = w.pos
+
+	case idxStageDone:
 		w.toc.labelIndicesTable = w.pos
 		if err := w.writeOffsetTable(w.labelIndexes); err != nil {
 			return err
 		}
-		w.toc.postings = w.pos
-
-	case idxStageDone:
 		w.toc.postingsTable = w.pos
 		if err := w.writeOffsetTable(w.postings); err != nil {
 			return err
@@ -404,10 +404,8 @@ func (w *indexWriter) WriteLabelIndex(names []string, values []string) error {
 
 // writeOffsetTable writes a sequence of readable hash entries.
 func (w *indexWriter) writeOffsetTable(entries []hashEntry) error {
-	w.buf1.reset()
-	w.buf1.putBE32int(len(entries))
-
 	w.buf2.reset()
+	w.buf2.putBE32int(len(entries))
 
 	for _, e := range entries {
 		w.buf2.putUvarint(len(e.keys))
@@ -417,6 +415,7 @@ func (w *indexWriter) writeOffsetTable(entries []hashEntry) error {
 		w.buf2.putUvarint64(e.offset)
 	}
 
+	w.buf1.reset()
 	w.buf1.putBE32int(w.buf2.len())
 	w.buf2.putHash(w.crc32)
 
@@ -563,6 +562,12 @@ type indexReader struct {
 	// Cached hashmaps of section offsets.
 	labels   map[string]uint32
 	postings map[string]uint32
+	// Cache of read symbols. Strings that are returned when reading from the
+	// block are always backed by true strings held in here rather than
+	// strings that are backed by byte slices from the mmap'd index file. This
+	// prevents memory faults when applications work with read symbols after
+	// the block has been unmapped.
+	symbols map[uint32]string
 }
 
 var (
@@ -579,7 +584,11 @@ func newIndexReader(dir string) (*indexReader, error) {
 	if err != nil {
 		return nil, err
 	}
-	r := &indexReader{b: f.b, c: f}
+	r := &indexReader{
+		b:       f.b,
+		c:       f,
+		symbols: map[uint32]string{},
+	}
 
 	// Verify magic number.
 	if len(f.b) < 4 {
@@ -592,6 +601,9 @@ func newIndexReader(dir string) (*indexReader, error) {
 	if err := r.readTOC(); err != nil {
 		return nil, errors.Wrap(err, "read TOC")
 	}
+	if err := r.readSymbols(int(r.toc.symbols)); err != nil {
+		return nil, errors.Wrap(err, "read symbols")
+	}
 
 	r.labels, err = r.readOffsetTable(r.toc.labelIndicesTable)
 	if err != nil {
@@ -623,21 +635,40 @@ func (r *indexReader) decbufAt(off int) decbuf {
 	return decbuf{b: r.b[off:]}
 }
 
+// readSymbols reads the symbol table fully into memory and allocates proper strings for them.
+// Strings backed by the mmap'd memory would cause memory faults if applications keep using them
+// after the reader is closed.
+func (r *indexReader) readSymbols(off int) error {
+	if off == 0 {
+		return nil
+	}
+	var (
+		d1      = r.decbufAt(int(off))
+		d2      = d1.decbuf(d1.be32int())
+		origLen = d2.len()
+		cnt     = d2.be32int()
+		basePos = uint32(off) + 4
+		nextPos = basePos + uint32(origLen-d2.len())
+	)
+	for d2.err() == nil && d2.len() > 0 && cnt > 0 {
+		s := d2.uvarintStr()
+		r.symbols[uint32(nextPos)] = s
+
+		nextPos = basePos + uint32(origLen-d2.len())
+		cnt--
+	}
+	return d2.err()
+}
+
 // readOffsetTable reads an offset table at the given position and returns a map
 // with the key strings concatenated by the 0xff unicode non-character.
 func (r *indexReader) readOffsetTable(off uint64) (map[string]uint32, error) {
-	// A table might not have been written at all, in which case the position
-	// is zeroed out.
-	if off == 0 {
-		return nil, nil
-	}
-
 	const sep = "\xff"
 
 	var (
 		d1  = r.decbufAt(int(off))
-		cnt = d1.be32()
 		d2  = d1.decbuf(d1.be32int())
+		cnt = d2.be32()
 	)
 
 	res := make(map[string]uint32, 512)
@@ -647,7 +678,7 @@ func (r *indexReader) readOffsetTable(off uint64) (map[string]uint32, error) {
 		keys := make([]string, 0, keyCount)
 
 		for i := 0; i < keyCount; i++ {
-			keys = append(keys, d2.uvarintTempStr())
+			keys = append(keys, d2.uvarintStr())
 		}
 		res[strings.Join(keys, sep)] = uint32(d2.uvarint())
 
@@ -682,28 +713,20 @@ func (r *indexReader) section(o uint32) (byte, []byte, error) {
 }
 
 func (r *indexReader) lookupSymbol(o uint32) (string, error) {
-	d := r.decbufAt(int(o))
-
-	s := d.uvarintTempStr()
-	if d.err() != nil {
-		return "", errors.Wrapf(d.err(), "read symbol at %d", o)
+	s, ok := r.symbols[o]
+	if !ok {
+		return "", errors.Errorf("unknown symbol offset %d", o)
 	}
 	return s, nil
 }
 
 func (r *indexReader) Symbols() (map[string]struct{}, error) {
-	d1 := r.decbufAt(int(r.toc.symbols))
-	d2 := d1.decbuf(d1.be32int())
+	res := make(map[string]struct{}, len(r.symbols))
 
-	count := d2.be32int()
-	sym := make(map[string]struct{}, count)
-
-	for ; count > 0; count-- {
-		s := d2.uvarintTempStr()
-		sym[s] = struct{}{}
+	for _, s := range r.symbols {
+		res[s] = struct{}{}
 	}
-
-	return sym, d2.err()
+	return res, nil
 }
 
 func (r *indexReader) LabelValues(names ...string) (StringTuples, error) {
diff --git a/index_test.go b/index_test.go
index 12329b560..c97cf380c 100644
--- a/index_test.go
+++ b/index_test.go
@@ -221,7 +221,7 @@ func TestIndexRW_Postings(t *testing.T) {
 
 		require.NoError(t, err)
 		require.Equal(t, 0, len(c))
-		require.Equal(t, l, series[i])
+		require.Equal(t, series[i], l)
 	}
 	require.NoError(t, p.Err())