An exemple for migrating journal from file on the default location to a dedicated partition on /dev/sde1.
$ apt-get install hdparm
### Stop osd
$ service ceph stop osd.0
### Flush Journal
$ ceph-osd --flush-journal -i 0
### Create symlink to partition
$ rm /var/lib/ceph/osd/ceph-0/journal
$ ln -s /dev/sde1 /var/lib/ceph/osd/ceph-0/journal
### Create new journal
$ ceph-osd --mkjournal -i 0
$ service ceph start osd.0