ceph 启动失败： unable to read magic from mon data.. did you run mkcephfs?

Question

ceph 0.72 搭建两个osd，一个 mds+mon

初始化成功

mkcephfs -a -c /etc/ceph/ceph.conf -k /etc/ceph/ceph.keyring

启动失败：

root@mon :/var/lib/ceph/mon/ceph-a# service ceph start

=== mon.a ===
Starting Ceph mon.a on mon...
unable to read magic from mon data.. did you run mkcephfs?
failed: ' /usr/bin/ceph-mon -i a -c /etc/ceph/ceph.conf '

再求大神指导！