A proper Ceph cluster comprises three or more Monitor daemons, ideally situated on distinct physical servers. If a cluster's MONs cannot form a quorum, which happens if not enough of the provisioned MONs are up, then new clients won't be able to connect to the system. Worse yet, existing clients will no longer be able to perform operations once their sessions expire. It is paramount for cluster health to have a majority of MONs up and able to fulfill their quorum duties.
We can retrieve basic information about all MONs in the cluster by running the ceph utility's mon stat subcommand. If our clusters contain MONs that are down or unavailable and thus have been temporarily kicked out of quorum, this command will alert us to the situation so that we can address it.
root@ceph-client0:~# ceph mon stat e1: 1 mons at {ceph-mon0=192.168.42.10:6789/0}, election epoch 5, quorum 0 ceph-mon0
If we need detailed information about the state of the entire cluster MON map, we...