diff --git a/lib/db/blockmap.go b/lib/db/blockmap.go
index 41354d348..3e572e227 100644
--- a/lib/db/blockmap.go
+++ b/lib/db/blockmap.go
@@ -42,6 +42,7 @@ func NewBlockMap(db *leveldb.DB, folder string) *BlockMap {
 func (m *BlockMap) Add(files []protocol.FileInfo) error {
 	batch := new(leveldb.Batch)
 	buf := make([]byte, 4)
+	var key []byte
 	for _, file := range files {
 		if file.IsDirectory() || file.IsDeleted() || file.IsInvalid() {
 			continue
@@ -49,7 +50,8 @@ func (m *BlockMap) Add(files []protocol.FileInfo) error {
 
 		for i, block := range file.Blocks {
 			binary.BigEndian.PutUint32(buf, uint32(i))
-			batch.Put(m.blockKey(block.Hash, file.Name), buf)
+			key = m.blockKeyInto(key, block.Hash, file.Name)
+			batch.Put(key, buf)
 		}
 	}
 	return m.db.Write(batch, nil)
@@ -59,6 +61,7 @@ func (m *BlockMap) Add(files []protocol.FileInfo) error {
 func (m *BlockMap) Update(files []protocol.FileInfo) error {
 	batch := new(leveldb.Batch)
 	buf := make([]byte, 4)
+	var key []byte
 	for _, file := range files {
 		if file.IsDirectory() {
 			continue
@@ -66,14 +69,16 @@ func (m *BlockMap) Update(files []protocol.FileInfo) error {
 
 		if file.IsDeleted() || file.IsInvalid() {
 			for _, block := range file.Blocks {
-				batch.Delete(m.blockKey(block.Hash, file.Name))
+				key = m.blockKeyInto(key, block.Hash, file.Name)
+				batch.Delete(key)
 			}
 			continue
 		}
 
 		for i, block := range file.Blocks {
 			binary.BigEndian.PutUint32(buf, uint32(i))
-			batch.Put(m.blockKey(block.Hash, file.Name), buf)
+			key = m.blockKeyInto(key, block.Hash, file.Name)
+			batch.Put(key, buf)
 		}
 	}
 	return m.db.Write(batch, nil)
@@ -82,9 +87,11 @@ func (m *BlockMap) Update(files []protocol.FileInfo) error {
 // Discard block map state, removing the given files
 func (m *BlockMap) Discard(files []protocol.FileInfo) error {
 	batch := new(leveldb.Batch)
+	var key []byte
 	for _, file := range files {
 		for _, block := range file.Blocks {
-			batch.Delete(m.blockKey(block.Hash, file.Name))
+			key = m.blockKeyInto(key, block.Hash, file.Name)
+			batch.Delete(key)
 		}
 	}
 	return m.db.Write(batch, nil)
@@ -93,7 +100,7 @@ func (m *BlockMap) Discard(files []protocol.FileInfo) error {
 // Drop block map, removing all entries related to this block map from the db.
 func (m *BlockMap) Drop() error {
 	batch := new(leveldb.Batch)
-	iter := m.db.NewIterator(util.BytesPrefix(m.blockKey(nil, "")[:1+64]), nil)
+	iter := m.db.NewIterator(util.BytesPrefix(m.blockKeyInto(nil, nil, "")[:1+64]), nil)
 	defer iter.Release()
 	for iter.Next() {
 		batch.Delete(iter.Key())
@@ -104,8 +111,8 @@ func (m *BlockMap) Drop() error {
 	return m.db.Write(batch, nil)
 }
 
-func (m *BlockMap) blockKey(hash []byte, file string) []byte {
-	return toBlockKey(hash, m.folder, file)
+func (m *BlockMap) blockKeyInto(o, hash []byte, file string) []byte {
+	return blockKeyInto(o, hash, m.folder, file)
 }
 
 type BlockFinder struct {
@@ -134,8 +141,9 @@ func (f *BlockFinder) String() string {
 // reason. The iterator finally returns the result, whether or not a
 // satisfying block was eventually found.
 func (f *BlockFinder) Iterate(folders []string, hash []byte, iterFn func(string, string, int32) bool) bool {
+	var key []byte
 	for _, folder := range folders {
-		key := toBlockKey(hash, folder, "")
+		key = blockKeyInto(key, hash, folder, "")
 		iter := f.db.NewIterator(util.BytesPrefix(key), nil)
 		defer iter.Release()
 
@@ -157,8 +165,8 @@ func (f *BlockFinder) Fix(folder, file string, index int32, oldHash, newHash []b
 	binary.BigEndian.PutUint32(buf, uint32(index))
 
 	batch := new(leveldb.Batch)
-	batch.Delete(toBlockKey(oldHash, folder, file))
-	batch.Put(toBlockKey(newHash, folder, file), buf)
+	batch.Delete(blockKeyInto(nil, oldHash, folder, file))
+	batch.Put(blockKeyInto(nil, newHash, folder, file), buf)
 	return f.db.Write(batch, nil)
 }
 
@@ -167,8 +175,13 @@ func (f *BlockFinder) Fix(folder, file string, index int32, oldHash, newHash []b
 //	   folder (64 bytes)
 //	   block hash (32 bytes)
 //	   file name (variable size)
-func toBlockKey(hash []byte, folder, file string) []byte {
-	o := make([]byte, 1+64+32+len(file))
+func blockKeyInto(o, hash []byte, folder, file string) []byte {
+	reqLen := 1 + 64 + 32 + len(file)
+	if cap(o) < reqLen {
+		o = make([]byte, reqLen)
+	} else {
+		o = o[:reqLen]
+	}
 	o[0] = KeyTypeBlock
 	copy(o[1:], []byte(folder))
 	copy(o[1+64:], []byte(hash))