Monitoring Ceph MONs
A proper Ceph cluster comprises three or more Monitor daemons, ideally situated on distinct physical servers. As discussed in Chapter 4, Planning your Deployment, multiple MONs are recommended to ensure fault tolerance and maintainability. 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.
MON status
We can retrieve basic information about all MONs in the cluster by running the ceph utility's mon stat subcommand. If our clusters contains 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...