diff --git a/supernode/daemon/mgr/cdn/path_util.go b/supernode/daemon/mgr/cdn/path_util.go
index c9f5def0b..ab850e7db 100644
--- a/supernode/daemon/mgr/cdn/path_util.go
+++ b/supernode/daemon/mgr/cdn/path_util.go
@@ -25,14 +25,9 @@ import (
 	"github.com/dragonflyoss/Dragonfly/supernode/store"
 )
 
-var getDownloadKeyFunc = getDownloadKey
-var getMetaDataKeyFunc = getMetaDataKey
-var getMd5DataKeyFunc = getMd5DataKey
-var getUploadKeyFunc = getUploadKey
 var getDownloadRawFunc = getDownloadRaw
 var getMetaDataRawFunc = getMetaDataRaw
 var getMd5DataRawFunc = getMd5DataRaw
-var getUploadRawFunc = getUploadRaw
 
 func getDownloadKey(taskID string) string {
 	return path.Join(cutil.SubString(taskID, 0, 3), taskID)
diff --git a/supernode/daemon/mgr/cdn/super_writer_test.go b/supernode/daemon/mgr/cdn/super_writer_test.go
index 77fa40c3a..3d8edf2f5 100644
--- a/supernode/daemon/mgr/cdn/super_writer_test.go
+++ b/supernode/daemon/mgr/cdn/super_writer_test.go
@@ -29,14 +29,12 @@ import (
 	"github.com/dragonflyoss/Dragonfly/supernode/store"
 
 	"github.com/go-check/check"
-	"github.com/prashantv/gostub"
 )
 
 type SuperWriterTestSuite struct {
-	workHome         string
-	config           string
-	writer           *superWriter
-	downloadPathStub *gostub.Stubs
+	workHome string
+	config   string
+	writer   *superWriter
 }
 
 func init() {
@@ -49,13 +47,9 @@ func (s *SuperWriterTestSuite) SetUpSuite(c *check.C) {
 	fileStore, err := store.NewStore(store.LocalStorageDriver, store.NewLocalStorage, s.config)
 	c.Check(err, check.IsNil)
 	s.writer = newSuperWriter(fileStore, nil)
-
-	s.downloadPathStub = gostub.Stub(&getDownloadKeyFunc, getDownloadKeyFuncMock)
 }
 
 func (s *SuperWriterTestSuite) TearDownSuite(c *check.C) {
-	s.downloadPathStub.Reset()
-
 	if s.workHome != "" {
 		if err := os.RemoveAll(s.workHome); err != nil {
 			fmt.Printf("remove path: %s error", s.workHome)
@@ -107,12 +101,8 @@ func (s *SuperWriterTestSuite) TestWriteToFile(c *check.C) {
 func checkFileSize(cdnStore *store.Store, taskID string, expectedSize int64, c *check.C) {
 	storageInfo, err := cdnStore.Stat(context.TODO(), &store.Raw{
 		Bucket: config.DownloadHome,
-		Key:    getDownloadKeyFuncMock(taskID),
+		Key:    getDownloadKey(taskID),
 	})
 	c.Check(err, check.IsNil)
 	c.Check(storageInfo.Size, check.Equals, expectedSize)
 }
-
-func getDownloadKeyFuncMock(taskID string) string {
-	return taskID
-}
diff --git a/supernode/daemon/mgr/cdn/super_writer_util.go b/supernode/daemon/mgr/cdn/super_writer_util.go
index 337163576..204b1bd5e 100644
--- a/supernode/daemon/mgr/cdn/super_writer_util.go
+++ b/supernode/daemon/mgr/cdn/super_writer_util.go
@@ -103,7 +103,7 @@ func (cw *superWriter) writeToFile(ctx context.Context, bytesBuffer *bytes.Buffe
 	// write to the storage
 	return cw.cdnStore.Put(ctx, &store.Raw{
 		Bucket: config.DownloadHome,
-		Key:    getDownloadKeyFunc(taskID),
+		Key:    getDownloadKey(taskID),
 		Offset: int64(pieceNum) * int64(PieceSize),
 	}, resultBuf)
 }
diff --git a/supernode/daemon/mgr/cdn_mgr.go b/supernode/daemon/mgr/cdn_mgr.go
index deff23ea0..6e385e34d 100644
--- a/supernode/daemon/mgr/cdn_mgr.go
+++ b/supernode/daemon/mgr/cdn_mgr.go
@@ -10,10 +10,13 @@ import (
 // operates on the underlying files stored on the local disk, etc.
 type CDNMgr interface {
 	// TriggerCDN will trigger CDN to download the file from sourceUrl.
-	// In common, it will including the following steps:
+	// It will including the following steps:
 	// 1). download the source file
-	// 2). update the taskInfo
-	// 3). write the file to disk
+	// 2). write the file to disk
+	//
+	// In fact, it's a very time consuming operation.
+	// So if not necessary, it should usually be executed concurrently.
+	// In addition, it's not thread-safe.
 	TriggerCDN(ctx context.Context, taskInfo *types.TaskInfo) (*types.TaskInfo, error)
 
 	// GetStatus get the status of the file.