First, rotate the OSD log, or just do "cd /var/log/ceph ; mv ceph-osd.0.log ceph-osd.0.log-foo"
Then, edit /etc/ceph/ceph.conf to add the following lines to the
[osd] debug osd = 20 debug filestore = 20 debug ms = 1
Make sure you know the "number" (id) of the OSD. You can find this out by looking at, e.g.
ceph osd tree
Then start the OSD manually, by running the following command as root:
# /usr/bin/ceph-osd -f --cluster ceph --id 0 --setuser ceph --setgroup ceph
Harvest the log - it is in
/etc/ceph/ceph.confback the way it was! (Otherwise, the increased debug levels cause OSDs to emit huge quantities of log messages which could easily fill up your root filesystem.)