btw, I also tried putting
[client]
rbd cache = false
in the /etc/ceph/ceph.conf file on the main node, then doing
systemctl stop ceph.target
systemctl status ceph.target
on the main node.
but after restart, it tells me rbd cache is still enabled
# ceph --admin-daemon /var/run/ceph/7994e544-3a5f-11eb-8a7b-9cb65496ebd0/ceph-osd.0.asok
config show|grep rbd_cache
"rbd_cache": "true",
....