Requesting OSD settings

requesting-osd-settings.txt

A ceph client may require that ceph is configured with particular settings. In
the case of the ceph iscsi gateway the osd timeout value needs to be lowered
for detecting down OSDs to reduce the possibility of initiator timeouts. To
facilitate this the existing ceph-client and osd relations could be extended
to pass on the request from a client.

1) The client charm sets a new 'osd-settings' option. The value is a json
   encoded dictionary of settings e.g.

   {"osd-settings": {"osd heartbeat grace": 20, "osd heartbeat interval": 5}}

2) The ceph-mon charm looks for this setting across all clients. Any
   contradictions and it goes into a blocked state.

3) ceph-mon passes the consolidated requests to the ceph-osd charm via
   the mon/osd relation.

4) If an update is needed the ceph-osd charm does an online update of these
   settings and also renders a new ceph.conf with the settings in. Note that
   the ceph-osd charm grows osd-heartbeat-grace and osd-heartbeat-interval
   settings. These default to unset but if they are set then they override any
   values sent down the relation.