I lookup rbd disk space usage, but find different statistics from ceph and the host which mounts the disk.
$ rbd -p rbd du NAME PROVISIONED USED kubernetes-dynamic-pvc-13a2d932-6be0-11e9-b53a-0a580a800339 40GiB 37.8GiB
From the host which mounts the disk
$ df -h Filesystem Size Used Available Use% Mounted on /dev/rbd0 39.2G 26.6G 10.6G 72% /data
How could I explain the difference?