*** Note: this may be partially obsolete
Install MDS on Proxmox:
apt-get install ceph-mds
mkdir -p /var/lib/ceph/mds/ceph-mds3
chown -R ceph:ceph /var/lib/ceph/mds/ceph-mds3
ceph --cluster ceph --name client.bootstrap-mds --keyring /var/lib/ceph/bootstrap-mds/ceph.keyring auth get-or-create mds.mds3 osd 'allow rwx' mds 'allow' mon 'allow profile mds' -o /var/lib/ceph/mds/ceph-mds3/keyring
systemctl start ceph-mds@ceph3
systemctl enable ceph-mds@ceph3