diff --git a/cmd/tsdb/main.go b/cmd/tsdb/main.go
index 01b843653..b5e0984c1 100644
--- a/cmd/tsdb/main.go
+++ b/cmd/tsdb/main.go
@@ -134,7 +134,7 @@ func (b *writeBenchmark) run(cmd *cobra.Command, args []string) {
 
 	measureTime("ingestScrapes", func() {
 		b.startProfiling()
-		if err := b.ingestScrapes(metrics, 1000); err != nil {
+		if err := b.ingestScrapes(metrics, 3000); err != nil {
 			exitWithError(err)
 		}
 	})
diff --git a/db.go b/db.go
index c27e491ba..54e4632bd 100644
--- a/db.go
+++ b/db.go
@@ -282,8 +282,8 @@ func (s *SeriesShard) persist() error {
 	defer sw.Close()
 	defer iw.Close()
 
-	for _, cd := range head.index.forward {
-		if err := sw.WriteSeries(cd.lset, []*chunkDesc{cd}); err != nil {
+	for ref, cd := range head.index.forward {
+		if err := sw.WriteSeries(ref, cd.lset, []*chunkDesc{cd}); err != nil {
 			return err
 		}
 	}
@@ -297,8 +297,15 @@ func (s *SeriesShard) persist() error {
 			s = append(s, x)
 		}
 
-		iw.WriteLabelIndex([]string{n}, s)
+		if err := iw.WriteLabelIndex([]string{n}, s); err != nil {
+			return err
+		}
+	}
 
+	for t := range head.index.postings.m {
+		if err := iw.WritePostings(t.name, t.value, head.index.postings.get(t)); err != nil {
+			return err
+		}
 	}
 
 	sz := fmt.Sprintf("%fMiB", float64(sw.Size()+iw.Size())/1024/1024)
diff --git a/index.go b/index.go
index f236410a5..7faf45af6 100644
--- a/index.go
+++ b/index.go
@@ -24,7 +24,7 @@ func newMemIndex() *memIndex {
 		lastID:   0,
 		forward:  make(map[uint32]*chunkDesc),
 		values:   make(map[string]stringset),
-		postings: &memPostings{m: make(map[string][]uint32)},
+		postings: &memPostings{m: make(map[term][]uint32)},
 	}
 }
 
@@ -32,22 +32,20 @@ func (ix *memIndex) numSeries() int {
 	return len(ix.forward)
 }
 
-func (ix *memIndex) Postings(s string) Iterator {
-	return ix.postings.get(s)
+func (ix *memIndex) Postings(t term) Iterator {
+	return ix.postings.get(t)
+}
+
+type term struct {
+	name, value string
 }
 
 func (ix *memIndex) add(chkd *chunkDesc) {
 	// Add each label pair as a term to the inverted index.
-	terms := make([]string, 0, len(chkd.lset))
-	b := make([]byte, 0, 64)
+	terms := make([]term, 0, len(chkd.lset))
 
 	for _, l := range chkd.lset {
-		b = append(b, l.Name...)
-		b = append(b, sep)
-		b = append(b, l.Value...)
-
-		terms = append(terms, string(b))
-		b = b[:0]
+		terms = append(terms, term{name: l.Name, value: l.Value})
 
 		// Add to label name to values index.
 		valset, ok := ix.values[l.Name]
@@ -67,17 +65,17 @@ func (ix *memIndex) add(chkd *chunkDesc) {
 }
 
 type memPostings struct {
-	m map[string][]uint32
+	m map[term][]uint32
 }
 
 // Postings returns an iterator over the postings list for s.
-func (p *memPostings) get(s string) Iterator {
-	return &listIterator{list: p.m[s], idx: -1}
+func (p *memPostings) get(t term) Iterator {
+	return &listIterator{list: p.m[t], idx: -1}
 }
 
 // add adds a document to the index. The caller has to ensure that no
 // term argument appears twice.
-func (p *memPostings) add(id uint32, terms ...string) {
+func (p *memPostings) add(id uint32, terms ...term) {
 	for _, t := range terms {
 		p.m[t] = append(p.m[t], id)
 	}
diff --git a/writer.go b/writer.go
index 252cd84b3..d1a773f28 100644
--- a/writer.go
+++ b/writer.go
@@ -21,7 +21,9 @@ const (
 // SeriesWriter serializes a time block of chunked series data.
 type SeriesWriter interface {
 	// WriteSeries writes the time series data chunks for a single series.
-	WriteSeries(Labels, []*chunkDesc) error
+	// The reference is used to resolve the correct series in the written index.
+	// It only has to be valid for the duration of the write.
+	WriteSeries(ref uint32, l Labels, cds []*chunkDesc) error
 
 	// Size returns the size of the data written so far.
 	Size() int64
@@ -64,7 +66,7 @@ func (w *seriesWriter) writeMeta() error {
 	return w.write(w.w, metab)
 }
 
-func (w *seriesWriter) WriteSeries(lset Labels, chks []*chunkDesc) error {
+func (w *seriesWriter) WriteSeries(ref uint32, lset Labels, chks []*chunkDesc) error {
 	// Initialize with meta data.
 	if w.n == 0 {
 		if err := w.writeMeta(); err != nil {
@@ -109,7 +111,7 @@ func (w *seriesWriter) WriteSeries(lset Labels, chks []*chunkDesc) error {
 	}
 
 	if w.index != nil {
-		w.index.AddOffsets(lset, offsets...)
+		w.index.AddSeries(ref, lset, offsets...)
 	}
 	return nil
 }
@@ -142,9 +144,11 @@ type BlockStats struct {
 // IndexWriter serialized the index for a block of series data.
 // The methods must generally be called in order they are specified.
 type IndexWriter interface {
-	// AddOffsets populates the index writer with offsets of chunks
-	// for a series that the index can reference.
-	AddOffsets(Labels, ...ChunkOffset)
+	// AddSeries populates the index writer witha series and its offsets
+	// of chunks that the index can reference.
+	// The reference number is used to resolve a series against the postings
+	// list iterator. It only has to be available during the write processing.
+	AddSeries(ref uint32, l Labels, o ...ChunkOffset)
 
 	// WriteStats writes final stats for the indexed block.
 	WriteStats(*BlockStats) error
@@ -164,18 +168,23 @@ type IndexWriter interface {
 	Close() error
 }
 
+type indexWriterSeries struct {
+	labels Labels
+	chunks []ChunkOffset // series file offset of chunks
+	offset uint32        // index file offset of series reference
+}
+
 // indexWriter implements the IndexWriter interface for the standard
 // serialization format.
 type indexWriter struct {
 	w io.Writer
 	n int64
 
-	series        []Labels        // series in data section
-	offsets       [][]ChunkOffset // chunk offsets per series
-	seriesOffsets []uint32        // offset of series references
+	series map[uint32]*indexWriterSeries
 
 	symbols      map[string]uint32 // symbol offsets
 	labelIndexes []hashEntry       // label index offsets
+	postings     []hashEntry       // postings lists offsets
 }
 
 func newIndexWriter(w io.Writer) *indexWriter {
@@ -183,6 +192,7 @@ func newIndexWriter(w io.Writer) *indexWriter {
 		w:            w,
 		n:            0,
 		symbols:      make(map[string]uint32, 4096),
+		series:       make(map[uint32]*indexWriterSeries, 4096),
 		labelIndexes: make([]hashEntry, 10),
 	}
 }
@@ -218,15 +228,17 @@ func (w *indexWriter) writeMeta() error {
 	return w.write(w.w, metab)
 }
 
-func (w *indexWriter) AddOffsets(lset Labels, offsets ...ChunkOffset) {
-	w.series = append(w.series, lset)
-	w.offsets = append(w.offsets, offsets)
-
+func (w *indexWriter) AddSeries(ref uint32, lset Labels, offsets ...ChunkOffset) {
 	// Populate the symbol table from all label sets we have to reference.
 	for _, l := range lset {
 		w.symbols[l.Name] = 0
 		w.symbols[l.Value] = 0
 	}
+
+	w.series[ref] = &indexWriterSeries{
+		labels: lset,
+		chunks: offsets,
+	}
 }
 
 func (w *indexWriter) WriteStats(*BlockStats) error {
@@ -274,11 +286,13 @@ func (w *indexWriter) writeSeries() error {
 	b := make([]byte, 0, 4096)
 	buf := [binary.MaxVarintLen32]byte{}
 
-	for _, lset := range w.series {
-		for _, l := range lset {
-			n := binary.PutUvarint(buf[:], uint64(len(lset)))
-			b = append(b, buf[:n]...)
+	for _, s := range w.series {
+		s.offset = uint32(w.n) + uint32(len(b))
+
+		n := binary.PutUvarint(buf[:], uint64(len(s.labels)))
+		b = append(b, buf[:n]...)
 
+		for _, l := range s.labels {
 			n = binary.PutUvarint(buf[:], uint64(w.symbols[l.Name]))
 			b = append(b, buf[:n]...)
 			n = binary.PutUvarint(buf[:], uint64(w.symbols[l.Value]))
@@ -320,7 +334,23 @@ func (w *indexWriter) WriteLabelIndex(names []string, values []string) error {
 }
 
 func (w *indexWriter) WritePostings(name, value string, it Iterator) error {
-	return nil
+	key := name + string(sep) + value
+
+	w.postings = append(w.postings, hashEntry{
+		name:   key,
+		offset: uint32(w.n),
+	})
+
+	b := make([]byte, 0, 4096)
+
+	for it.Next() {
+		v := w.series[it.Value()].offset
+		b = append(b, ((*[4]byte)(unsafe.Pointer(&v)))[:]...)
+	}
+
+	return w.section(uint32(len(b)), flagStd, func(wr io.Writer) error {
+		return w.write(wr, b)
+	})
 }
 
 func (w *indexWriter) Size() int64 {
@@ -337,7 +367,7 @@ const hashEntrySize = uint32(unsafe.Sizeof(hashEntry{}))
 func (w *indexWriter) finalize() error {
 	l := 1 + uint32(len(w.labelIndexes))*hashEntrySize
 
-	return w.section(l, flagStd, func(wr io.Writer) error {
+	err := w.section(l, flagStd, func(wr io.Writer) error {
 		for _, e := range w.labelIndexes {
 			b := ((*[hashEntrySize]byte)(unsafe.Pointer(&e)))[:]
 
@@ -347,6 +377,24 @@ func (w *indexWriter) finalize() error {
 		}
 		return nil
 	})
+
+	if err != nil {
+		return err
+	}
+
+	err = w.section(l, flagStd, func(wr io.Writer) error {
+		for _, e := range w.postings {
+			b := ((*[hashEntrySize]byte)(unsafe.Pointer(&e)))[:]
+
+			if err := w.write(w.w, b); err != nil {
+				return nil
+			}
+		}
+		return nil
+	})
+	// TODO(fabxc): write hashmap offsets.
+
+	return err
 }
 
 func (w *indexWriter) Close() error {