Note: the following does not currently work in Firefly because of http://tracker.ceph.com/issues/9677 . It has been backported to Firefly and will likely be in 0.80.8.
ceph tell osd.* injectargs '--osd_disk_thread_ioprio_priority 7' ceph tell osd.* injectargs '--osd_disk_thread_ioprio_class idle'
All other threads in the OSD will be be (best effort) with priority 4 which is the default for daemons. The disk thread will show as idle:
$ sudo iotop --batch --iter 1 | grep 'ceph-osd -i 0' | grep -v be/4 4156 idle loic 0.00 B/s 0.00 B/s 0.00 % 0.00 % ./ceph-osd -i 0 ..