thank you for your reply.
I agree maybe a system bottleneck on primary or secondary .
The service of my netapp has expired , so unable to open a case.
below is the more description:
1.the status of busy is not stop ,unless make a manual sync or automatic sync backup
2. then the busy one will transfer and finished quickly.
3. the new sync will again go to a busy status.
it's like a cycle...
I try to delete the snapshot ,but it is failed to delete. due to the snap in use.