ceph启动失败：must specify '--mon-data=foo' data path

root@mon:/var/lib/ceph# service ceph start
=== mon.a ===
Starting Ceph mon.a on mon...
must specify '--mon-data=foo' data path
usage: ceph-mon -i monid [--mon-data=pathtodata] [flags]
  --debug_mon n
        debug monitor level (e.g. 10)
  --mkfs
        build fresh monitor fs
--conf/-c        Read configuration from the given configuration file
-d               Run in foreground, log to stderr.
-f               Run in foreground, log to usual location.
--id/-i          set ID portion of my name
--name/-n        set name (TYPE.ID)
--version        show version and quit


   --debug_ms N
        set message debug level (e.g. 1)
failed: ' /usr/bin/ceph-mon -i a -c /etc/ceph/ceph.conf '