the image used size becomes 0 after export/import with snapshot

Hi, I have an image with a snapshot and some changes after snapshot. ``` $ rbd du backup/f0408e1e-06b6-437b-a2b5-70e3751d0a26 NAME PROVISIONED USED f0408e1e-06b6-437b-a2b5-70e3751d0a26@snapshot-eb085877-7557-4620-9c01-c5587b857029 10 GiB 2.4 GiB f0408e1e-06b6-437b-a2b5-70e3751d0a26 10 GiB 2.4 GiB <TOTAL> 10 GiB 4.8 GiB ``` If there is no changes after snapshot, the image line will show 0 used. I did export and import. ``` $ rbd export --export-format 2 backup/f0408e1e-06b6-437b-a2b5-70e3751d0a26 - | rbd import --export-format 2 - backup/test Exporting image: 100% complete...done. Importing image: 100% complete...done. ``` When check the imported image, the image line shows 0 used. ``` $ rbd du backup/test NAME PROVISIONED USED test@snapshot-eb085877-7557-4620-9c01-c5587b857029 10 GiB 2.4 GiB test 10 GiB 0 B <TOTAL> 10 GiB 2.4 GiB ``` Any clues how that happened? I'd expect the same du as the source. I tried another quick test. It works fine. ``` $ rbd create backup/test-src --size 10G $ sudo rbd map backup/test-src /dev/rbd0 $ echo "hello" | sudo tee /dev/rbd0 hello $ rbd du backup/test-src NAME PROVISIONED USED test-src 10 GiB 4 MiB $ rbd snap create backup/test-src@snap-1 Creating snap: 100% complete...done. $ rbd du backup/test-src NAME PROVISIONED USED test-src@snap-1 10 GiB 4 MiB test-src 10 GiB 0 B <TOTAL> 10 GiB 4 MiB $ echo "world" | sudo tee /dev/rbd0 world $ rbd du backup/test-src NAME PROVISIONED USED test-src@snap-1 10 GiB 4 MiB test-src 10 GiB 4 MiB <TOTAL> 10 GiB 8 MiB $ rbd export --export-format 2 backup/test-src - | rbd import --export-format 2 - backup/test-dst Exporting image: 100% complete...done. Importing image: 100% complete...done. $ rbd du backup/test-dst NAME PROVISIONED USED test-dst@snap-1 10 GiB 4 MiB test-dst 10 GiB 4 MiB <TOTAL> 10 GiB 8 MiB ``` Thanks! Tony