diff --git a/tsdb/compact_test.go b/tsdb/compact_test.go
index e24b720d2a..91024d2826 100644
--- a/tsdb/compact_test.go
+++ b/tsdb/compact_test.go
@@ -852,8 +852,10 @@ func BenchmarkCompaction(b *testing.B) {
 
 			b.ResetTimer()
 			b.ReportAllocs()
-			_, err = c.Compact(dir, blockDirs, blocks)
-			testutil.Ok(b, err)
+			for i := 0; i < b.N; i++ {
+				_, err = c.Compact(dir, blockDirs, blocks)
+				testutil.Ok(b, err)
+			}
 		})
 	}
 }
diff --git a/tsdb/encoding/encoding.go b/tsdb/encoding/encoding.go
index a732a6048e..3e2e2804a0 100644
--- a/tsdb/encoding/encoding.go
+++ b/tsdb/encoding/encoding.go
@@ -75,10 +75,20 @@ func (e *Encbuf) PutUvarintStr(s string) {
 // PutHash appends a hash over the buffers current contents to the buffer.
 func (e *Encbuf) PutHash(h hash.Hash) {
 	h.Reset()
+	e.WriteToHash(h)
+	e.PutHashSum(h)
+}
+
+// WriteToHash writes the current buffer contents to the given hash.
+func (e *Encbuf) WriteToHash(h hash.Hash) {
 	_, err := h.Write(e.B)
 	if err != nil {
 		panic(err) // The CRC32 implementation does not error
 	}
+}
+
+// PutHashSum writes the Sum of the given hash to the buffer.
+func (e *Encbuf) PutHashSum(h hash.Hash) {
 	e.B = h.Sum(e.B)
 }
 
diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index 1e601ccf32..592cb1f697 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -236,6 +236,14 @@ func (w *Writer) write(bufs ...[]byte) error {
 	return nil
 }
 
+func (w *Writer) writeAt(buf []byte, pos uint64) error {
+	if err := w.fbuf.Flush(); err != nil {
+		return err
+	}
+	_, err := w.f.WriteAt(buf, int64(pos))
+	return err
+}
+
 // addPadding adds zero byte padding until the file size is a multiple size.
 func (w *Writer) addPadding(size int) error {
 	p := w.pos % uint64(size)
@@ -382,23 +390,42 @@ func (w *Writer) AddSymbols(sym map[string]struct{}) error {
 	}
 	sort.Strings(symbols)
 
-	w.buf1.Reset()
-	w.buf2.Reset()
+	startPos := w.pos
+	// Leave 4 bytes of space for the length, which will be calculated later.
+	if err := w.write([]byte("alen")); err != nil {
+		return err
+	}
+	w.crc32.Reset()
 
-	w.buf2.PutBE32int(len(symbols))
+	w.buf1.Reset()
+	w.buf1.PutBE32int(len(symbols))
+	w.buf1.WriteToHash(w.crc32)
+	if err := w.write(w.buf1.Get()); err != nil {
+		return err
+	}
 
 	w.symbols = make(map[string]uint32, len(symbols))
 
 	for index, s := range symbols {
 		w.symbols[s] = uint32(index)
-		w.buf2.PutUvarintStr(s)
+		w.buf1.Reset()
+		w.buf1.PutUvarintStr(s)
+		w.buf1.WriteToHash(w.crc32)
+		if err := w.write(w.buf1.Get()); err != nil {
+			return err
+		}
 	}
 
-	w.buf1.PutBE32int(w.buf2.Len())
-	w.buf2.PutHash(w.crc32)
+	// Write out the length.
+	w.buf1.Reset()
+	w.buf1.PutBE32int(int(w.pos - startPos - 4))
+	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
+		return err
+	}
 
-	err := w.write(w.buf1.Get(), w.buf2.Get())
-	return errors.Wrap(err, "write symbols")
+	w.buf1.Reset()
+	w.buf1.PutHashSum(w.crc32)
+	return w.write(w.buf1.Get())
 }
 
 func (w *Writer) WriteLabelIndex(names []string, values []string) error {
@@ -425,9 +452,20 @@ func (w *Writer) WriteLabelIndex(names []string, values []string) error {
 		offset: w.pos,
 	})
 
-	w.buf2.Reset()
-	w.buf2.PutBE32int(len(names))
-	w.buf2.PutBE32int(valt.Len())
+	startPos := w.pos
+	// Leave 4 bytes of space for the length, which will be calculated later.
+	if err := w.write([]byte("alen")); err != nil {
+		return err
+	}
+	w.crc32.Reset()
+
+	w.buf1.Reset()
+	w.buf1.PutBE32int(len(names))
+	w.buf1.PutBE32int(valt.Len())
+	w.buf1.WriteToHash(w.crc32)
+	if err := w.write(w.buf1.Get()); err != nil {
+		return err
+	}
 
 	// here we have an index for the symbol file if v2, otherwise it's an offset
 	for _, v := range valt.entries {
@@ -435,55 +473,104 @@ func (w *Writer) WriteLabelIndex(names []string, values []string) error {
 		if !ok {
 			return errors.Errorf("symbol entry for %q does not exist", v)
 		}
-		w.buf2.PutBE32(index)
+		w.buf1.Reset()
+		w.buf1.PutBE32(index)
+		w.buf1.WriteToHash(w.crc32)
+		if err := w.write(w.buf1.Get()); err != nil {
+			return err
+		}
+	}
+
+	// Write out the length.
+	w.buf1.Reset()
+	w.buf1.PutBE32int(int(w.pos - startPos - 4))
+	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
+		return err
 	}
 
 	w.buf1.Reset()
-	w.buf1.PutBE32int(w.buf2.Len())
-
-	w.buf2.PutHash(w.crc32)
-
-	err = w.write(w.buf1.Get(), w.buf2.Get())
-	return errors.Wrap(err, "write label index")
+	w.buf1.PutHashSum(w.crc32)
+	return w.write(w.buf1.Get())
 }
 
 // writeLabelIndexesOffsetTable writes the label indices offset table.
 func (w *Writer) writeLabelIndexesOffsetTable() error {
-	w.buf2.Reset()
-	w.buf2.PutBE32int(len(w.labelIndexes))
+	startPos := w.pos
+	// Leave 4 bytes of space for the length, which will be calculated later.
+	if err := w.write([]byte("alen")); err != nil {
+		return err
+	}
+	w.crc32.Reset()
+
+	w.buf1.Reset()
+	w.buf1.PutBE32int(len(w.labelIndexes))
+	w.buf1.WriteToHash(w.crc32)
+	if err := w.write(w.buf1.Get()); err != nil {
+		return err
+	}
 
 	for _, e := range w.labelIndexes {
-		w.buf2.PutUvarint(len(e.keys))
+		w.buf1.Reset()
+		w.buf1.PutUvarint(len(e.keys))
 		for _, k := range e.keys {
-			w.buf2.PutUvarintStr(k)
+			w.buf1.PutUvarintStr(k)
 		}
-		w.buf2.PutUvarint64(e.offset)
+		w.buf1.PutUvarint64(e.offset)
+		w.buf1.WriteToHash(w.crc32)
+		if err := w.write(w.buf1.Get()); err != nil {
+			return err
+		}
+	}
+	// Write out the length.
+	w.buf1.Reset()
+	w.buf1.PutBE32int(int(w.pos - startPos - 4))
+	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
+		return err
 	}
 
 	w.buf1.Reset()
-	w.buf1.PutBE32int(w.buf2.Len())
-	w.buf2.PutHash(w.crc32)
-
-	return w.write(w.buf1.Get(), w.buf2.Get())
+	w.buf1.PutHashSum(w.crc32)
+	return w.write(w.buf1.Get())
 }
 
 // writePostingsOffsetTable writes the postings offset table.
 func (w *Writer) writePostingsOffsetTable() error {
-	w.buf2.Reset()
-	w.buf2.PutBE32int(len(w.postings))
+	startPos := w.pos
+	// Leave 4 bytes of space for the length, which will be calculated later.
+	if err := w.write([]byte("alen")); err != nil {
+		return err
+	}
+	w.crc32.Reset()
+
+	w.buf1.Reset()
+	w.buf1.PutBE32int(len(w.postings))
+	w.buf1.WriteToHash(w.crc32)
+	if err := w.write(w.buf1.Get()); err != nil {
+		return err
+	}
 
 	for _, e := range w.postings {
-		w.buf2.PutUvarint(2)
-		w.buf2.PutUvarintStr(e.name)
-		w.buf2.PutUvarintStr(e.value)
-		w.buf2.PutUvarint64(e.offset)
+		w.buf1.Reset()
+		w.buf1.PutUvarint(2)
+		w.buf1.PutUvarintStr(e.name)
+		w.buf1.PutUvarintStr(e.value)
+		w.buf1.PutUvarint64(e.offset)
+		w.buf1.WriteToHash(w.crc32)
+		if err := w.write(w.buf1.Get()); err != nil {
+			return err
+		}
+	}
+
+	// Write out the length.
+	w.buf1.Reset()
+	w.buf1.PutBE32int(int(w.pos - startPos - 4))
+	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
+		return err
 	}
 
 	w.buf1.Reset()
-	w.buf1.PutBE32int(w.buf2.Len())
-	w.buf2.PutHash(w.crc32)
-
-	return w.write(w.buf1.Get(), w.buf2.Get())
+	w.buf1.PutHashSum(w.crc32)
+	return w.write(w.buf1.Get())
 }
 
 const indexTOCLen = 6*8 + 4
@@ -539,21 +626,40 @@ func (w *Writer) WritePostings(name, value string, it Postings) error {
 	}
 	sort.Sort(uint32slice(refs))
 
-	w.buf2.Reset()
-	w.buf2.PutBE32int(len(refs))
+	startPos := w.pos
+	// Leave 4 bytes of space for the length, which will be calculated later.
+	if err := w.write([]byte("alen")); err != nil {
+		return err
+	}
+	w.crc32.Reset()
+
+	w.buf1.Reset()
+	w.buf1.PutBE32int(len(refs))
+	w.buf1.WriteToHash(w.crc32)
+	if err := w.write(w.buf1.Get()); err != nil {
+		return err
+	}
 
 	for _, r := range refs {
-		w.buf2.PutBE32(r)
+		w.buf1.Reset()
+		w.buf1.PutBE32(r)
+		w.buf1.WriteToHash(w.crc32)
+		if err := w.write(w.buf1.Get()); err != nil {
+			return err
+		}
 	}
 	w.uint32s = refs
 
+	// Write out the length.
 	w.buf1.Reset()
-	w.buf1.PutBE32int(w.buf2.Len())
+	w.buf1.PutBE32int(int(w.pos - startPos - 4))
+	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
+		return err
+	}
 
-	w.buf2.PutHash(w.crc32)
-
-	err := w.write(w.buf1.Get(), w.buf2.Get())
-	return errors.Wrap(err, "write postings")
+	w.buf1.Reset()
+	w.buf1.PutHashSum(w.crc32)
+	return w.write(w.buf1.Get())
 }
 
 type uint32slice []uint32