Aaahhh full disk this morning. Sometimes the logs can go crazy, and the files can quickly reach several gigabytes.

Show debug option (on host) :

# Look at log file
tail -n 1000 /var/log/ceph/ceph-osd.33.log

# Check debug levels
ceph daemon osd.33 config show | grep '"debug_'
    "debug_none": "0\/5",
    "debug_lockdep": "0\/1",
    "debug_context": "0\/1",
    "debug_crush": "1\/1",
    "debug_mds": "1\/5",
    ...
    "debug_filestore": "1\/5",
    ...

In my case it was about filestore, so "ceph tell" is my friend to apply the new value to the whole cluster (on admin host) :

ceph tell osd.* injectargs --debug-filestore 0/5

Now you can remove the log file on reopen it :

rm /var/log/ceph/ceph-osd.33.log

ceph daemon osd.33 log reopen

Then it will remain to be added in the ceph.conf file (on each osd hosts) :

[osd]
        debug filestore = 0/5

For more information from ceph documentation : http://docs.ceph.com/docs/master/rados/troubleshooting/log-and-debug/