I always forget how to mount the subfolder of a CephFS filesystem
sudo ceph-fuse -m 192.168.2.9:6789 -r /subfolder /folder/on/my/machine
Note: The proceeding / after the -r is important
Some great info here on restricting access to specific folders and more notes on mounting – https://manjusri.ucsc.edu/2017/09/25/ceph-fuse/