diff --git a/db.go b/db.go
index eb640b103..b1a3a2421 100644
--- a/db.go
+++ b/db.go
@@ -87,17 +87,24 @@ const sep = '\xff'
 // DB handles reads and writes of time series falling into
 // a hashed partition of a seriedb.
 type DB struct {
-	dir     string
-	lockf   lockfile.Lockfile
+	dir   string
+	lockf lockfile.Lockfile
+
 	logger  log.Logger
 	metrics *dbMetrics
 	opts    *Options
 
+	// Mutex for that must be held when modifying the general
+	// block layout.
 	mtx       sync.RWMutex
 	persisted []*persistedBlock
-	heads     []*headBlock
 	seqBlocks map[int]Block
-	headGen   uint8
+
+	// Mutex that must be held when modifying just the head blocks
+	// or the general layout.
+	headmtx sync.RWMutex
+	heads   []*headBlock
+	headGen uint8
 
 	compactor Compactor
 
@@ -200,7 +207,15 @@ func (db *DB) run() {
 		case <-db.compactc:
 			db.metrics.compactionsTriggered.Inc()
 
-			if err := db.compact(); err != nil {
+			var merr MultiError
+
+			changes, err := db.compact()
+			merr.Add(err)
+
+			if changes {
+				merr.Add(db.reloadBlocks())
+			}
+			if err := merr.Err(); err != nil {
 				db.logger.Log("msg", "compaction failed", "err", err)
 			}
 
@@ -210,12 +225,17 @@ func (db *DB) run() {
 	}
 }
 
-func (db *DB) compact() error {
-	changes := false
+func (db *DB) compact() (changes bool, err error) {
 	// Check whether we have pending head blocks that are ready to be persisted.
 	// They have the highest priority.
-	db.mtx.RLock()
+	db.headmtx.RLock()
 
+	var singles []*headBlock
+
+	// Collect head blocks that are ready for compaction. Write them after
+	// returning the lock to not block Appenders.
+	// Selected blocks are semantically ensured to not be written to afterwards
+	// by appendable().
 	if len(db.heads) > db.opts.AppendableBlocks {
 		for _, h := range db.heads[:len(db.heads)-db.opts.AppendableBlocks] {
 			// Blocks that won't be appendable when instantiating a new appender
@@ -224,36 +244,38 @@ func (db *DB) compact() error {
 			if atomic.LoadUint64(&h.activeWriters) > 0 {
 				break
 			}
-
-			db.logger.Log("msg", "write head", "seq", h.Meta().Sequence)
-
-			select {
-			case <-db.stopc:
-				db.mtx.RUnlock()
-				return nil
-			default:
-			}
-
-			if err := db.compactor.Write(h.Dir(), h); err != nil {
-				db.mtx.RUnlock()
-				return errors.Wrap(err, "persist head block")
-			}
-			changes = true
+			singles = append(singles, h)
 		}
 	}
 
-	db.mtx.RUnlock()
+	db.headmtx.RUnlock()
+
+Loop:
+	for _, h := range singles {
+		db.logger.Log("msg", "write head", "seq", h.Meta().Sequence)
+
+		select {
+		case <-db.stopc:
+			break Loop
+		default:
+		}
+
+		if err = db.compactor.Write(h.Dir(), h); err != nil {
+			return changes, errors.Wrap(err, "persist head block")
+		}
+		changes = true
+	}
 
 	// Check for compactions of multiple blocks.
 	for {
 		plans, err := db.compactor.Plan(db.dir)
 		if err != nil {
-			return errors.Wrap(err, "plan compaction")
+			return changes, errors.Wrap(err, "plan compaction")
 		}
 
 		select {
 		case <-db.stopc:
-			return nil
+			return false, nil
 		default:
 		}
 		// We just execute compactions sequentially to not cause too extreme
@@ -264,7 +286,7 @@ func (db *DB) compact() error {
 			db.logger.Log("msg", "compact blocks", "seq", fmt.Sprintf("%v", p))
 
 			if err := db.compactor.Compact(p...); err != nil {
-				return errors.Wrapf(err, "compact %s", p)
+				return changes, errors.Wrapf(err, "compact %s", p)
 			}
 			changes = true
 		}
@@ -274,10 +296,7 @@ func (db *DB) compact() error {
 		}
 	}
 
-	if changes {
-		return errors.Wrap(db.reloadBlocks(), "reload blocks")
-	}
-	return nil
+	return changes, nil
 }
 
 // func (db *DB) retentionCutoff() error {
@@ -381,10 +400,11 @@ func (db *DB) Close() error {
 	close(db.stopc)
 	<-db.donec
 
-	var merr MultiError
-
+	// Lock mutex and leave it locked so we panic if there's a bug causing
+	// the block to be used afterwards.
 	db.mtx.Lock()
-	defer db.mtx.Unlock()
+
+	var merr MultiError
 
 	for _, pb := range db.persisted {
 		merr.Add(pb.Close())
@@ -403,9 +423,14 @@ func (db *DB) Appender() Appender {
 	db.mtx.RLock()
 	a := &dbAppender{db: db}
 
+	db.headmtx.RLock()
+
 	for _, b := range db.appendable() {
 		a.heads = append(a.heads, b.Appender().(*headAppender))
 	}
+
+	db.headmtx.RUnlock()
+
 	return a
 }
 
@@ -468,15 +493,12 @@ func (a *dbAppender) AddFast(ref uint64, t int64, v float64) error {
 func (a *dbAppender) appenderFor(t int64) (*headAppender, error) {
 	// If there's no fitting head block for t, ensure it gets created.
 	if len(a.heads) == 0 || t >= a.heads[len(a.heads)-1].meta.MaxTime {
-		a.db.mtx.RUnlock()
+		a.db.headmtx.Lock()
 
 		if err := a.db.ensureHead(t); err != nil {
-			a.db.mtx.RLock()
+			a.db.headmtx.Unlock()
 			return nil, err
 		}
-
-		a.db.mtx.RLock()
-
 		if len(a.heads) == 0 {
 			for _, b := range a.db.appendable() {
 				a.heads = append(a.heads, b.Appender().(*headAppender))
@@ -489,6 +511,8 @@ func (a *dbAppender) appenderFor(t int64) (*headAppender, error) {
 				}
 			}
 		}
+
+		a.db.headmtx.Unlock()
 	}
 	for i := len(a.heads) - 1; i >= 0; i-- {
 		if h := a.heads[i]; t >= h.meta.MinTime {
@@ -500,8 +524,8 @@ func (a *dbAppender) appenderFor(t int64) (*headAppender, error) {
 }
 
 func (db *DB) ensureHead(t int64) error {
-	db.mtx.Lock()
-	defer db.mtx.Unlock()
+	// db.mtx.Lock()
+	// defer db.mtx.Unlock()
 
 	// Initial case for a new database: we must create the first
 	// AppendableBlocks-1 front padding heads.
@@ -557,31 +581,6 @@ func (db *DB) appendable() []*headBlock {
 	return db.heads[len(db.heads)-db.opts.AppendableBlocks:]
 }
 
-func (db *DB) compactable() []Block {
-	db.mtx.RLock()
-	defer db.mtx.RUnlock()
-
-	var blocks []Block
-	for _, pb := range db.persisted {
-		blocks = append(blocks, pb)
-	}
-
-	if len(db.heads) <= db.opts.AppendableBlocks {
-		return blocks
-	}
-
-	for _, h := range db.heads[:len(db.heads)-db.opts.AppendableBlocks] {
-		// Blocks that won't be appendable when instantiating a new appender
-		// might still have active appenders on them.
-		// Abort at the first one we encounter.
-		if atomic.LoadUint64(&h.activeWriters) > 0 {
-			break
-		}
-		blocks = append(blocks, h)
-	}
-	return blocks
-}
-
 func intervalOverlap(amin, amax, bmin, bmax int64) bool {
 	if bmin >= amin && bmin <= amax {
 		return true
diff --git a/head.go b/head.go
index 969bf3416..058c8fe79 100644
--- a/head.go
+++ b/head.go
@@ -145,6 +145,10 @@ func (h *headBlock) inBounds(t int64) bool {
 
 // Close syncs all data and closes underlying resources of the head block.
 func (h *headBlock) Close() error {
+	// Lock mutex and leave it locked so we panic if there's a bug causing
+	// the block to be used afterwards.
+	h.mtx.Lock()
+
 	if err := h.wal.Close(); err != nil {
 		return err
 	}
diff --git a/querier.go b/querier.go
index 5f02c77f7..7783ef312 100644
--- a/querier.go
+++ b/querier.go
@@ -47,7 +47,9 @@ type querier struct {
 func (s *DB) Querier(mint, maxt int64) Querier {
 	s.mtx.RLock()
 
+	s.headmtx.RLock()
 	blocks := s.blocksForInterval(mint, maxt)
+	s.headmtx.RUnlock()
 
 	sq := &querier{
 		blocks: make([]Querier, 0, len(blocks)),
diff --git a/wal.go b/wal.go
index 5962711f9..8b88d110d 100644
--- a/wal.go
+++ b/wal.go
@@ -265,8 +265,9 @@ func (w *WAL) Close() error {
 	close(w.stopc)
 	<-w.donec
 
+	// Lock mutex and leave it locked so we panic if there's a bug causing
+	// the block to be used afterwards.
 	w.mtx.Lock()
-	defer w.mtx.Unlock()
 
 	if err := w.sync(); err != nil {
 		return err