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 can use the mon dump subcommand.

root@ceph-client0:~# ceph mon dump
dumped monmap epoch 1
epoch 1
fsid e6d4e4ab-f59f-470d-bb76-511deebc8de3
last_changed 2017-09-10 20:20:16.458985
created 2017-09-10 20:20:16.458985
0: 192.168.42.10:6789/0 mon.ceph-mon0

Most of this information is also present in the Ceph health status summary, but it is sometimes useful to detect issues when we are adding new MONs to an existing cluster if they are not making themselves a part of the quorum. Examining the MON map dumps on the new MONs and comparing them with those of the existing MONs might help point a finger towards the problem.