Ceph

In my opinion, copy an image without any data to the same pool should be a easy thing.
But in my ceph cluster,rbd cp rbd/3T_img rbd/3T_img_cp takes 99 minutes or even longer.
Is that normal?
And in 0.93 or later version,this copy operation takes 73 minuters because of the objecte map?
Thx for any help.