diff --git a/weed/topology/topology.go b/weed/topology/topology.go
index 902e89bd1..e463aae29 100644
--- a/weed/topology/topology.go
+++ b/weed/topology/topology.go
@@ -166,6 +166,7 @@ func (t *Topology) GetOrCreateDataCenter(dcName string) *DataCenter {
 }
 
 func (t *Topology) SyncDataNodeRegistration(volumes []*master_pb.VolumeInformationMessage, dn *DataNode) (newVolumes, deletedVolumes []storage.VolumeInfo) {
+	// convert into in memory struct storage.VolumeInfo
 	var volumeInfos []storage.VolumeInfo
 	for _, v := range volumes {
 		if vi, err := storage.NewVolumeInfo(v); err == nil {
@@ -174,6 +175,7 @@ func (t *Topology) SyncDataNodeRegistration(volumes []*master_pb.VolumeInformati
 			glog.V(0).Infof("Fail to convert joined volume information: %v", err)
 		}
 	}
+	// find out the delta volumes
 	newVolumes, deletedVolumes = dn.UpdateVolumes(volumeInfos)
 	for _, v := range newVolumes {
 		t.RegisterVolumeLayout(v, dn)