Use batches in blockmap, speeds up and reduces memory usage on large Replace and Update ops

benchmark                         old ns/op      new ns/op      delta
BenchmarkReplaceAll-8             2880834572     1868198122     -35.15%
BenchmarkUpdateOneChanged-8       236596         231852         -2.01%
BenchmarkUpdateOneUnchanged-8     227326         230624         +1.45%
BenchmarkNeedHalf-8               105151538      104601744      -0.52%
BenchmarkHave-8                   28827492       29102480       +0.95%
BenchmarkGlobal-8                 150768724      150547687      -0.15%
BenchmarkNeedHalfTruncated-8      104434216      102471355      -1.88%
BenchmarkHaveTruncated-8          27860093       28758368       +3.22%
BenchmarkGlobalTruncated-8        149972888      151192913      +0.81%

benchmark                         old allocs     new allocs     delta
BenchmarkReplaceAll-8             555451         555577         +0.02%
BenchmarkUpdateOneChanged-8       1135           1135           +0.00%
BenchmarkUpdateOneUnchanged-8     1135           1135           +0.00%
BenchmarkNeedHalf-8               374779         374780         +0.00%
BenchmarkHave-8                   151996         151992         -0.00%
BenchmarkGlobal-8                 530066         530033         -0.01%
BenchmarkNeedHalfTruncated-8      374702         374699         -0.00%
BenchmarkHaveTruncated-8          151834         151834         +0.00%
BenchmarkGlobalTruncated-8        530049         530037         -0.00%

benchmark                         old bytes      new bytes      delta
BenchmarkReplaceAll-8             5018351912     1765116216     -64.83%
BenchmarkUpdateOneChanged-8       135085         135085         +0.00%
BenchmarkUpdateOneUnchanged-8     134976         134976         +0.00%
BenchmarkNeedHalf-8               44769400       44758752       -0.02%
BenchmarkHave-8                   11930612       11845052       -0.72%
BenchmarkGlobal-8                 81523668       80431136       -1.34%
BenchmarkNeedHalfTruncated-8      46692342       46526459       -0.36%
BenchmarkHaveTruncated-8          11348357       11348357       +0.00%
BenchmarkGlobalTruncated-8        81843956       80977672       -1.06%

lib/db/blockmap.go

@@ -26,6 +26,8 @@ import (
 
 var blockFinder *BlockFinder
 
+const maxBatchSize = 256 << 10
+
 type BlockMap struct {
 	db     *leveldb.DB
 	folder string
@@ -44,6 +46,13 @@ func (m *BlockMap) Add(files []protocol.FileInfo) error {
 	buf := make([]byte, 4)
 	var key []byte
 	for _, file := range files {
+		if batch.Len() > maxBatchSize {
+			if err := m.db.Write(batch, nil); err != nil {
+				return err
+			}
+			batch.Reset()
+		}
+
 		if file.IsDirectory() || file.IsDeleted() || file.IsInvalid() {
 			continue
 		}
@@ -63,6 +72,13 @@ func (m *BlockMap) Update(files []protocol.FileInfo) error {
 	buf := make([]byte, 4)
 	var key []byte
 	for _, file := range files {
+		if batch.Len() > maxBatchSize {
+			if err := m.db.Write(batch, nil); err != nil {
+				return err
+			}
+			batch.Reset()
+		}
+
 		if file.IsDirectory() {
 			continue
 		}
@@ -89,6 +105,13 @@ func (m *BlockMap) Discard(files []protocol.FileInfo) error {
 	batch := new(leveldb.Batch)
 	var key []byte
 	for _, file := range files {
+		if batch.Len() > maxBatchSize {
+			if err := m.db.Write(batch, nil); err != nil {
+				return err
+			}
+			batch.Reset()
+		}
+
 		for _, block := range file.Blocks {
 			key = m.blockKeyInto(key, block.Hash, file.Name)
 			batch.Delete(key)
@@ -103,6 +126,13 @@ func (m *BlockMap) Drop() error {
 	iter := m.db.NewIterator(util.BytesPrefix(m.blockKeyInto(nil, nil, "")[:1+64]), nil)
 	defer iter.Release()
 	for iter.Next() {
+		if batch.Len() > maxBatchSize {
+			if err := m.db.Write(batch, nil); err != nil {
+				return err
+			}
+			batch.Reset()
+		}
+
 		batch.Delete(iter.Key())
 	}
 	if iter.Error() != nil {