diff --git a/cmd/tsdb/main.go b/cmd/tsdb/main.go
index 491c2ac35c..960c9c05e3 100644
--- a/cmd/tsdb/main.go
+++ b/cmd/tsdb/main.go
@@ -139,7 +139,7 @@ func (b *writeBenchmark) run(cmd *cobra.Command, args []string) {
 
 	dur := measureTime("ingestScrapes", func() {
 		b.startProfiling()
-		total, err = b.ingestScrapes(metrics, 2000)
+		total, err = b.ingestScrapes(metrics, 15000)
 		if err != nil {
 			exitWithError(err)
 		}
@@ -147,6 +147,7 @@ func (b *writeBenchmark) run(cmd *cobra.Command, args []string) {
 
 	fmt.Println(" > total samples:", total)
 	fmt.Println(" > samples/sec:", float64(total)/dur.Seconds())
+	select {}
 
 	measureTime("stopStorage", func() {
 		if err := b.storage.Close(); err != nil {
diff --git a/compact.go b/compact.go
index 7d8174f0d3..a3bc7d17a3 100644
--- a/compact.go
+++ b/compact.go
@@ -17,7 +17,6 @@ import (
 	"math/rand"
 	"os"
 	"path/filepath"
-	"runtime"
 	"sort"
 	"time"
 
@@ -365,10 +364,6 @@ func (c *LeveledCompactor) write(dest string, meta *BlockMeta, blocks ...BlockRe
 		}
 		c.metrics.ran.Inc()
 		c.metrics.duration.Observe(time.Since(t).Seconds())
-
-		// We might have done quite a few allocs. Enforce a GC so they do not accumulate
-		// with subsequent compactions or head GCs.
-		runtime.GC()
 	}(time.Now())
 
 	dir := filepath.Join(dest, meta.ULID.String())
@@ -570,14 +565,6 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 			return errors.Wrap(err, "write postings")
 		}
 	}
-	// Write a postings list containing all series.
-	all := make([]uint64, i)
-	for i := range all {
-		all[i] = uint64(i)
-	}
-	if err := indexw.WritePostings("", "", newListPostings(all)); err != nil {
-		return errors.Wrap(err, "write 'all' postings")
-	}
 
 	return nil
 }
diff --git a/db.go b/db.go
index ad034d8b02..c9745cfc6f 100644
--- a/db.go
+++ b/db.go
@@ -21,6 +21,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strconv"
 	"sync"
@@ -349,9 +350,12 @@ func (db *DB) compact() (changes bool, err error) {
 		}
 		changes = true
 
+		runtime.GC()
+
 		if err := db.reload(); err != nil {
 			return changes, errors.Wrap(err, "reload blocks")
 		}
+		runtime.GC()
 	}
 
 	// Check for compactions of multiple blocks.
@@ -380,10 +384,12 @@ func (db *DB) compact() (changes bool, err error) {
 				return changes, errors.Wrap(err, "delete compacted block")
 			}
 		}
+		runtime.GC()
 
 		if err := db.reload(); err != nil {
 			return changes, errors.Wrap(err, "reload blocks")
 		}
+		runtime.GC()
 	}
 
 	return changes, nil
diff --git a/encoding_helpers.go b/encoding_helpers.go
index 9aa4ba4097..17c3ff0811 100644
--- a/encoding_helpers.go
+++ b/encoding_helpers.go
@@ -77,6 +77,22 @@ 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/head.go b/head.go
index a74552bcaf..ea7b63f8a6 100644
--- a/head.go
+++ b/head.go
@@ -15,7 +15,6 @@ package tsdb
 
 import (
 	"math"
-	"runtime"
 	"sort"
 	"sync"
 	"sync/atomic"
@@ -402,10 +401,13 @@ func (a *headAppender) AddFast(ref uint64, t int64, v float64) error {
 	if s == nil {
 		return errors.Wrap(ErrNotFound, "unknown series")
 	}
-	if err := s.appendable(t, v); err != nil {
+	s.Lock()
+	err := s.appendable(t, v)
+	s.Unlock()
+
+	if err != nil {
 		return err
 	}
-
 	if t < a.mint {
 		return ErrOutOfBounds
 	}
@@ -435,7 +437,10 @@ func (a *headAppender) Commit() error {
 	total := len(a.samples)
 
 	for _, s := range a.samples {
+		s.series.Lock()
 		ok, chunkCreated := s.series.append(s.T, s.V)
+		s.series.Unlock()
+
 		if !ok {
 			total--
 		}
@@ -509,8 +514,6 @@ Outer:
 
 // gc removes data before the minimum timestmap from the head.
 func (h *Head) gc() {
-	defer runtime.GC()
-
 	// Only data strictly lower than this timestamp must be deleted.
 	mint := h.MinTime()
 
@@ -672,9 +675,9 @@ func (h *headChunkReader) Chunk(ref uint64) (chunks.Chunk, error) {
 
 	s := h.head.series.getByID(sid)
 
-	s.mtx.RLock()
+	s.Lock()
 	c := s.chunk(int(cid))
-	s.mtx.RUnlock()
+	s.Unlock()
 
 	// Do not expose chunks that are outside of the specified range.
 	if c == nil || !intervalOverlap(c.minTime, c.maxTime, h.mint, h.maxt) {
@@ -694,9 +697,10 @@ type safeChunk struct {
 }
 
 func (c *safeChunk) Iterator() chunks.Iterator {
-	c.s.mtx.RLock()
-	defer c.s.mtx.RUnlock()
-	return c.s.iterator(c.cid)
+	c.s.Lock()
+	it := c.s.iterator(c.cid)
+	c.s.Unlock()
+	return it
 }
 
 // func (c *safeChunk) Appender() (chunks.Appender, error) { panic("illegal") }
@@ -803,8 +807,8 @@ func (h *headIndexReader) Series(ref uint64, lbls *labels.Labels, chks *[]ChunkM
 	}
 	*lbls = append((*lbls)[:0], s.lset...)
 
-	s.mtx.RLock()
-	defer s.mtx.RUnlock()
+	s.Lock()
+	defer s.Unlock()
 
 	*chks = (*chks)[:0]
 
@@ -956,11 +960,11 @@ func (s *stripeSeries) gc(mint int64) (map[uint64]struct{}, int) {
 
 		for hash, all := range s.hashes[i] {
 			for _, series := range all {
-				series.mtx.Lock()
+				series.Lock()
 				rmChunks += series.truncateChunksBefore(mint)
 
 				if len(series.chunks) > 0 {
-					series.mtx.Unlock()
+					series.Unlock()
 					continue
 				}
 
@@ -983,7 +987,7 @@ func (s *stripeSeries) gc(mint int64) (map[uint64]struct{}, int) {
 					s.locks[j].Unlock()
 				}
 
-				series.mtx.Unlock()
+				series.Unlock()
 			}
 		}
 
@@ -1040,8 +1044,10 @@ type sample struct {
 	v float64
 }
 
+// memSeries is the in-memory representation of a series. None of its methods
+// are goroutine safe and its the callers responsibility to lock it.
 type memSeries struct {
-	mtx sync.RWMutex
+	sync.Mutex
 
 	ref          uint64
 	lset         labels.Labels
@@ -1143,8 +1149,6 @@ func (s *memSeries) truncateChunksBefore(mint int64) (removed int) {
 func (s *memSeries) append(t int64, v float64) (success, chunkCreated bool) {
 	const samplesPerChunk = 120
 
-	s.mtx.Lock()
-
 	c := s.head()
 
 	if c == nil {
@@ -1152,7 +1156,6 @@ func (s *memSeries) append(t int64, v float64) (success, chunkCreated bool) {
 		chunkCreated = true
 	}
 	if c.maxTime >= t {
-		s.mtx.Unlock()
 		return false, chunkCreated
 	}
 	if c.chunk.NumSamples() > samplesPerChunk/4 && t >= s.nextAt {
@@ -1175,8 +1178,6 @@ func (s *memSeries) append(t int64, v float64) (success, chunkCreated bool) {
 	s.sampleBuf[2] = s.sampleBuf[3]
 	s.sampleBuf[3] = sample{t: t, v: v}
 
-	s.mtx.Unlock()
-
 	return true, chunkCreated
 }
 
diff --git a/index.go b/index.go
index 06f347aaa1..fd9b251623 100644
--- a/index.go
+++ b/index.go
@@ -347,10 +347,6 @@ func (w *indexWriter) AddSymbols(sym map[string]struct{}) error {
 
 	for _, s := range symbols {
 		w.symbols[s] = uint32(w.pos) + headerSize + uint32(w.buf2.len())
-
-		// NOTE: len(s) gives the number of runes, not the number of bytes.
-		// Therefore the read-back length for strings with unicode characters will
-		// be off when not using putUvarintStr.
 		w.buf2.putUvarintStr(s)
 	}
 
@@ -648,7 +644,7 @@ 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.uvarintStr())
+			keys = append(keys, d2.uvarintTempStr())
 		}
 		res[strings.Join(keys, sep)] = uint32(d2.uvarint())
 
@@ -685,7 +681,7 @@ func (r *indexReader) section(o uint32) (byte, []byte, error) {
 func (r *indexReader) lookupSymbol(o uint32) (string, error) {
 	d := r.decbufAt(int(o))
 
-	s := d.uvarintStr()
+	s := d.uvarintTempStr()
 	if d.err() != nil {
 		return "", errors.Wrapf(d.err(), "read symbol at %d", o)
 	}
@@ -700,7 +696,7 @@ func (r *indexReader) Symbols() (map[string]struct{}, error) {
 	sym := make(map[string]struct{}, count)
 
 	for ; count > 0; count-- {
-		s := d2.uvarintStr()
+		s := d2.uvarintTempStr()
 		sym[s] = struct{}{}
 	}
 
diff --git a/wal.go b/wal.go
index 68c48838cc..9af9a18536 100644
--- a/wal.go
+++ b/wal.go
@@ -398,6 +398,10 @@ func (w *SegmentWAL) LogSeries(series []RefSeries) error {
 	buf := w.getBuffer()
 
 	flag := w.encodeSeries(buf, series)
+
+	w.mtx.Lock()
+	defer w.mtx.Unlock()
+
 	err := w.write(WALEntrySeries, flag, buf.get())
 
 	w.putBuffer(buf)
@@ -413,10 +417,6 @@ func (w *SegmentWAL) LogSeries(series []RefSeries) error {
 			tf.minSeries = s.Ref
 		}
 	}
-
-	if w.flushInterval <= 0 {
-		return errors.Wrap(w.Sync(), "sync")
-	}
 	return nil
 }
 
@@ -425,6 +425,10 @@ func (w *SegmentWAL) LogSamples(samples []RefSample) error {
 	buf := w.getBuffer()
 
 	flag := w.encodeSamples(buf, samples)
+
+	w.mtx.Lock()
+	defer w.mtx.Unlock()
+
 	err := w.write(WALEntrySamples, flag, buf.get())
 
 	w.putBuffer(buf)
@@ -439,10 +443,6 @@ func (w *SegmentWAL) LogSamples(samples []RefSample) error {
 			tf.maxTime = s.T
 		}
 	}
-
-	if w.flushInterval <= 0 {
-		return errors.Wrap(w.Sync(), "sync")
-	}
 	return nil
 }
 
@@ -451,6 +451,10 @@ func (w *SegmentWAL) LogDeletes(stones []Stone) error {
 	buf := w.getBuffer()
 
 	flag := w.encodeDeletes(buf, stones)
+
+	w.mtx.Lock()
+	defer w.mtx.Unlock()
+
 	err := w.write(WALEntryDeletes, flag, buf.get())
 
 	w.putBuffer(buf)
@@ -467,10 +471,6 @@ func (w *SegmentWAL) LogDeletes(stones []Stone) error {
 			}
 		}
 	}
-
-	if w.flushInterval <= 0 {
-		return errors.Wrap(w.Sync(), "sync")
-	}
 	return nil
 }
 
@@ -525,19 +525,26 @@ func (w *SegmentWAL) createSegmentFile(name string) (*os.File, error) {
 func (w *SegmentWAL) cut() error {
 	// Sync current head to disk and close.
 	if hf := w.head(); hf != nil {
-		if err := w.sync(); err != nil {
-			return err
-		}
-		off, err := hf.Seek(0, os.SEEK_CUR)
-		if err != nil {
-			return err
-		}
-		if err := hf.Truncate(off); err != nil {
-			return err
-		}
-		if err := hf.Close(); err != nil {
+		if err := w.flush(); err != nil {
 			return err
 		}
+		// Finish last segment asynchronously to not block the WAL moving along
+		// in the new segment.
+		go func() {
+			off, err := hf.Seek(0, os.SEEK_CUR)
+			if err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+			if err := hf.Truncate(off); err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+			if err := hf.Sync(); err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+			if err := hf.Close(); err != nil {
+				w.logger.Log("msg", "finish old segment", "segment", hf.Name(), "err", err)
+			}
+		}()
 	}
 
 	p, _, err := nextSequenceFile(w.dirFile.Name())
@@ -549,9 +556,11 @@ func (w *SegmentWAL) cut() error {
 		return err
 	}
 
-	if err = w.dirFile.Sync(); err != nil {
-		return err
-	}
+	go func() {
+		if err = w.dirFile.Sync(); err != nil {
+			w.logger.Log("msg", "sync WAL directory", "err", err)
+		}
+	}()
 
 	w.files = append(w.files, newSegmentFile(f))
 
@@ -661,8 +670,6 @@ const (
 )
 
 func (w *SegmentWAL) write(t WALEntryType, flag uint8, buf []byte) error {
-	w.mtx.Lock()
-	defer w.mtx.Unlock()
 	// Cut to the next segment if the entry exceeds the file size unless it would also
 	// exceed the size of a new segment.
 	// TODO(gouthamve): Add a test for this case where the commit is greater than segmentSize.
diff --git a/wal_test.go b/wal_test.go
index 45279b1b5f..f36b22e90e 100644
--- a/wal_test.go
+++ b/wal_test.go
@@ -91,9 +91,8 @@ func TestSegmentWAL_cut(t *testing.T) {
 
 	require.NoError(t, w.cut(), "cut failed")
 
-	// Cutting creates a new file and close the previous tail file.
+	// Cutting creates a new file.
 	require.Equal(t, 2, len(w.files))
-	require.Error(t, w.files[0].Close())
 
 	require.NoError(t, w.write(WALEntrySeries, 1, []byte("Hello World!!")))
 
@@ -383,7 +382,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 		t.Run(c.name, func(t *testing.T) {
 			// Generate testing data. It does not make semantical sense but
 			// for the purpose of this test.
-			dir, err := ioutil.TempDir("", "test_corrupted_checksum")
+			dir, err := ioutil.TempDir("", "test_corrupted")
 			require.NoError(t, err)
 			defer os.RemoveAll(dir)
 
@@ -400,6 +399,10 @@ func TestWALRestoreCorrupted(t *testing.T) {
 
 			require.NoError(t, w.Close())
 
+			// cut() truncates and fsyncs the first segment async. If it happens after
+			// the corruption we apply below, the corruption will be overwritten again.
+			// Fire and forget a sync to avoid flakyness.
+			w.files[0].Sync()
 			// Corrupt the second entry in the first file.
 			// After re-opening we must be able to read the first entry
 			// and the rest, including the second file, must be truncated for clean further