diff --git a/db.go b/db.go
index ac57bc710..6c0382ece 100644
--- a/db.go
+++ b/db.go
@@ -18,7 +18,6 @@ import (
 	"golang.org/x/sync/errgroup"
 
 	"github.com/coreos/etcd/pkg/fileutil"
-	"github.com/fabxc/tsdb/chunks"
 	"github.com/fabxc/tsdb/labels"
 	"github.com/go-kit/kit/log"
 	"github.com/pkg/errors"
@@ -523,32 +522,6 @@ func (db *DB) nextBlockDir() (string, error) {
 	return filepath.Join(db.dir, fmt.Sprintf("b-%0.6d", i+1)), nil
 }
 
-// chunkDesc wraps a plain data chunk and provides cached meta data about it.
-type chunkDesc struct {
-	ref   uint32
-	lset  labels.Labels
-	chunk chunks.Chunk
-
-	// Caching fielddb.
-	firstTimestamp int64
-	lastTimestamp  int64
-	lastValue      float64
-	numSamples     int
-
-	app chunks.Appender // Current appender for the chunkdb.
-}
-
-func (cd *chunkDesc) append(ts int64, v float64) {
-	if cd.numSamples == 0 {
-		cd.firstTimestamp = ts
-	}
-	cd.app.Append(ts, v)
-
-	cd.lastTimestamp = ts
-	cd.lastValue = v
-	cd.numSamples++
-}
-
 // PartitionedDB is a time series storage.
 type PartitionedDB struct {
 	logger log.Logger
diff --git a/head.go b/head.go
index 5685dba0b..a56f3ad2f 100644
--- a/head.go
+++ b/head.go
@@ -116,12 +116,55 @@ func (h *HeadBlock) Chunk(ref uint32) (chunks.Chunk, error) {
 	return h.descs[int(ref)].chunk, nil
 }
 
+type headSeriesReader struct {
+	h *HeadBlock
+}
+
+func (h *headSeriesReader) Chunk(ref uint32) (chunks.Chunk, error) {
+	h.h.mtx.RLock()
+	defer h.h.mtx.RUnlock()
+
+	if int(ref) >= len(h.h.descs) {
+		return nil, errNotFound
+	}
+	return &safeChunk{
+		cd: h.h.descs[int(ref)],
+	}, nil
+}
+
+type safeChunk struct {
+	cd *chunkDesc
+}
+
+func (c *safeChunk) Iterator() chunks.Iterator {
+	c.cd.mtx.Lock()
+	defer c.cd.mtx.Unlock()
+
+	return c.cd.iterator()
+}
+
+func (c *safeChunk) Appender() (chunks.Appender, error) {
+	panic("illegal")
+}
+
+func (c *safeChunk) Bytes() []byte {
+	panic("illegal")
+}
+
+func (c *safeChunk) Encoding() chunks.Encoding {
+	panic("illegal")
+}
+
 func (h *HeadBlock) interval() (int64, int64) {
+	h.bstats.mtx.RLock()
+	defer h.bstats.mtx.RUnlock()
 	return h.bstats.MinTime, h.bstats.MaxTime
 }
 
 // Stats returns statisitics about the indexed data.
 func (h *HeadBlock) Stats() (BlockStats, error) {
+	h.bstats.mtx.RLock()
+	defer h.bstats.mtx.RUnlock()
 	return *h.bstats, nil
 }
 
@@ -161,11 +204,13 @@ func (h *HeadBlock) Series(ref uint32) (labels.Labels, []ChunkMeta, error) {
 	}
 	cd := h.descs[ref]
 
+	cd.mtx.RLock()
 	meta := ChunkMeta{
 		MinTime: cd.firstTimestamp,
 		MaxTime: cd.lastTimestamp,
 		Ref:     ref,
 	}
+	cd.mtx.RUnlock()
 	return cd.lset, []ChunkMeta{meta}, nil
 }
 
@@ -273,6 +318,7 @@ func (h *HeadBlock) appendBatch(samples []hashedSample) error {
 		// We don't want to reserve a new space for each.
 		if ref, ok := uniqueHashes[s.hash]; ok {
 			s.ref = ref
+			newSamples = append(newSamples, s)
 			continue
 		}
 		s.ref = uint32(len(newSeries))
@@ -315,12 +361,14 @@ func (h *HeadBlock) appendBatch(samples []hashedSample) error {
 	)
 	for _, s := range samples {
 		cd := h.descs[s.ref]
+		cd.mtx.Lock()
 		// Skip duplicate samples.
 		if cd.lastTimestamp == s.t && cd.lastValue != s.v {
 			total--
 			continue
 		}
 		cd.append(s.t, s.v)
+		cd.mtx.Unlock()
 
 		if mint > s.t {
 			mint = s.t
@@ -394,6 +442,78 @@ func (h *HeadBlock) remapPostings(p Postings) Postings {
 	return newListPostings(list)
 }
 
+// chunkDesc wraps a plain data chunk and provides cached meta data about it.
+type chunkDesc struct {
+	mtx sync.RWMutex
+
+	ref   uint32
+	lset  labels.Labels
+	chunk chunks.Chunk
+
+	// Caching fielddb.
+	firstTimestamp int64
+	lastTimestamp  int64
+	lastValue      float64
+	numSamples     int
+
+	sampleBuf [4]sample
+
+	app chunks.Appender // Current appender for the chunkdb.
+}
+
+func (cd *chunkDesc) append(ts int64, v float64) {
+	if cd.numSamples == 0 {
+		cd.firstTimestamp = ts
+	}
+	cd.app.Append(ts, v)
+
+	cd.lastTimestamp = ts
+	cd.lastValue = v
+	cd.numSamples++
+
+	cd.sampleBuf[0] = cd.sampleBuf[1]
+	cd.sampleBuf[1] = cd.sampleBuf[2]
+	cd.sampleBuf[2] = cd.sampleBuf[3]
+	cd.sampleBuf[3] = sample{t: ts, v: v}
+}
+
+func (cd *chunkDesc) iterator() chunks.Iterator {
+	it := &memSafeIterator{
+		Iterator: cd.chunk.Iterator(),
+		i:        -1,
+		total:    cd.numSamples,
+		buf:      cd.sampleBuf,
+	}
+	return it
+}
+
+type memSafeIterator struct {
+	chunks.Iterator
+
+	i     int
+	total int
+	buf   [4]sample
+}
+
+func (it *memSafeIterator) Next() bool {
+	if it.i+1 >= it.total {
+		return false
+	}
+	it.i++
+	if it.total-it.i > 4 {
+		return it.Iterator.Next()
+	}
+	return true
+}
+
+func (it *memSafeIterator) At() (int64, float64) {
+	if it.total-it.i > 4 {
+		return it.Iterator.At()
+	}
+	s := it.buf[4-(it.total-it.i)]
+	return s.t, s.v
+}
+
 // positionMapper stores a position mapping from unsorted to
 // sorted indices of a sortable collection.
 type positionMapper struct {
diff --git a/querier.go b/querier.go
index 3080a29d1..73158b6ec 100644
--- a/querier.go
+++ b/querier.go
@@ -61,13 +61,17 @@ func (s *DB) Querier(mint, maxt int64) Querier {
 			index:  b.index(),
 			series: b.series(),
 		}
-		sq.blocks = append(sq.blocks, q)
 
 		// TODO(fabxc): find nicer solution.
 		if hb, ok := b.(*HeadBlock); ok {
 			hb.updateMapping()
 			q.postingsMapper = hb.remapPostings
+			q.series = &headSeriesReader{
+				h: hb,
+			}
 		}
+
+		sq.blocks = append(sq.blocks, q)
 	}
 
 	return sq