Cephmon

Cephmon, the Ceph Monitor daemon (ceph-mon), is a core component of the Ceph distributed storage system. It maintains the cluster's authoritative state, including the cluster map, authentication data, and health information. Monitors run on multiple nodes and use a consensus protocol to present a consistent view of the cluster to clients and peers.

Monitors store the cluster map, the monmap, and related configuration in a local data directory. The monmap lists active monitors and their locations; maps for OSDs and PGs are derived from it. Changes are committed by a majority quorum, with leader election and propagation of updates. Clients query monitors for the current map and cluster status.

For high availability, Ceph clusters deploy an odd number of monitors (typically 3 or 5) across failure domains. With quorum, the cluster continues to operate; loss of a majority can suspend changes until quorum is restored. Monitors coordinate with OSDs and MDSs but do not store data themselves; they provide the authoritative state and access control.

Ceph-mon runs as a service and is typically managed by systemd or orchestration tooling, invoked as ceph-mon -i <id> -n mon. Maintenance tasks include checking status (ceph mon stat, ceph status), inspecting the monmap (ceph mon dump), and replacing failed monitors to restore quorum. Cephx authentication keys are stored in mon data to secure communication.