very large size and very slow speed of upload image file in raw format with rbd backend

when uploading a image file in raw format to ceph backend, the size of rbd device in ceph is very large and upload speed is very slow.

Why not use 'rbd import' command in 'add' function in glance_store/_drivers/rbd.py file?
Now the upload of image file in raw format (sparse file) is block by block, the rbd device in images pool is very large, and its size is equal to 'virtual size' of image file in raw format.
But if we use 'rbd import' for image file in raw format, the rbd device in imgaes pool is very small, and its size is equal to 'disk size' of image file, and the upload speed is fast.