Chapter 7. Ceph Monitor and OSD interaction configuration

After you have completed your initial Ceph configuration, you can deploy and run Ceph. When you execute a command such as ceph health or ceph -s, the Ceph Monitor reports on the current state of the Ceph storage cluster. The Ceph Monitor knows about the Ceph storage cluster by requiring reports from each Ceph OSD daemon, and by receiving reports from Ceph OSD daemons about the status of their neighboring Ceph OSD daemons. If the Ceph Monitor does not receive reports, or if it receives reports of changes in the Ceph storage cluster, the Ceph Monitor updates the status of the Ceph cluster map.

Ceph provides reasonable default settings for Ceph Monitor and OSD interaction. However, you can override the defaults. The following sections describe how Ceph Monitors and Ceph OSD daemons interact for the purposes of monitoring the Ceph storage cluster.

Each Ceph OSD daemon checks the heartbeat of other Ceph OSD daemons every 6 seconds. To change the heartbeat interval, add the osd heartbeat interval setting under the [osd] section of the Ceph configuration file, or change its value at runtime.

If a neighboring Ceph OSD daemon does not send heartbeat packets within a 20 second grace period, the Ceph OSD daemon might consider the neighboring Ceph OSD daemon down. It can report it back to a Ceph Monitor, which will update the Ceph cluster map. To change this grace period, add the osd heartbeat grace setting under the [osd] section of the Ceph configuration file, or set its value at runtime.

By default, two Ceph OSD Daemons from different hosts must report to the Ceph Monitors that another Ceph OSD Daemon is down before the Ceph Monitors acknowledge that the reported Ceph OSD Daemon is down.

However, there is chance that all the OSDs reporting the failure are in different hosts in a rack with a bad switch that causes connection problems between OSDs.

To avoid a "false alarm," Ceph considers the peers reporting the failure as a proxy for a "subcluster" that is similarly laggy. While this is not always the case, it may help administrators localize the grace correction to a subset of the system that is performing poorly.

Ceph uses the mon_osd_reporter_subtree_level setting to group the peers into the "subcluster" by their common ancestor type in the CRUSH map.

By default, only two reports from a different subtree are required to report another Ceph OSD Daemon down. Administrators can change the number of reporters from unique subtrees and the common ancestor type required to report a Ceph OSD Daemon down to a Ceph Monitor by adding the mon_osd_min_down_reporters and mon_osd_reporter_subtree_level settings under the [mon] section of the Ceph configuration file, or by setting the value at runtime.

If a Ceph OSD daemon cannot peer with any of the Ceph OSD daemons defined in its Ceph configuration file or the cluster map, it will ping a Ceph Monitor for the most recent copy of the cluster map every 30 seconds. You can change the Ceph Monitor heartbeat interval by adding the osd mon heartbeat interval setting under the [osd] section of the Ceph configuration file, or by setting the value at runtime.

If a Ceph OSD Daemon does not report to a Ceph Monitor, the Ceph Monitor will consider the Ceph OSD Daemon down after the mon osd report timeout elapses. A Ceph OSD Daemon sends a report to a Ceph Monitor when a reportable event such as a failure, a change in placement group stats, a change in up_thru or when it boots within 5 seconds. You can change the Ceph OSD Daemon minimum report interval by adding the osd mon report interval min setting under the [osd] section of the Ceph configuration file, or by setting the value at runtime.

A Ceph OSD Daemon sends a report to a Ceph Monitor every 120 seconds irrespective of whether any notable changes occur. You can change the Ceph Monitor report interval by adding the osd mon report interval max setting under the [osd] section of the Ceph configuration file, or by setting the value at runtime.