mvcc: allow large concurrent reads under light write workload

internal/mvcc/backend/backend.go

@@ -310,11 +310,7 @@ func (b *backend) defrag() error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
-	// block concurrent read requests while resetting tx
-	b.readTx.mu.Lock()
-	defer b.readTx.mu.Unlock()
-
-	b.batchTx.unsafeCommit(true)
+	b.batchTx.commit(true)
 	b.batchTx.tx = nil
 
 	tmpdb, err := bolt.Open(b.db.Path()+".tmp", 0600, boltOpenOptions)

internal/mvcc/backend/batch_tx.go

@@ -98,6 +98,7 @@ func unsafeRange(c *bolt.Cursor, key, endKey []byte, limit int64) (keys [][]byte
 		isMatch = func(b []byte) bool { return bytes.Equal(b, key) }
 		limit = 1
 	}
+
 	for ck, cv := c.Seek(key); ck != nil && isMatch(ck); ck, cv = c.Next() {
 		vs = append(vs, cv)
 		keys = append(keys, ck)
@@ -231,6 +232,13 @@ func (t *batchTxBuffered) CommitAndStop() {
 }
 
 func (t *batchTxBuffered) commit(stop bool) {
+	// no need to reset read tx if there is no write.
+	// call commit to update stats.
+	if t.batchTx.pending == 0 && !stop {
+		t.batchTx.commit(stop)
+		return
+	}
+
 	// all read txs must be closed to acquire boltdb commit rwlock
 	t.backend.readTx.mu.Lock()
 	defer t.backend.readTx.mu.Unlock()

internal/mvcc/kvstore_test.go

@@ -638,6 +638,8 @@ func TestTxnBlockBackendForceCommit(t *testing.T) {
 	s := NewStore(b, &lease.FakeLessor{}, nil)
 	defer os.Remove(tmpPath)
 
+	// Put a key into the store so that force commit can take effect.
+	s.Put([]byte("foo"), []byte("bar"), lease.NoLease)
 	txn := s.Read()
 
 	done := make(chan struct{})