Fixing a Ceph Mon map after disaster!

Cephs weakest leak is configuration.. once a cluster is deployed is incredibly durable and will survive most mistakes without punishment. However adding a monitor that is unreachable via all machines can yield a very broken cluster that cannot be managed.

For example, if you add a new monitor and the automatically detected ip (ansible or kolla) isn’t correct, possibly a loopback or other assigned ip, you will loose the ability to use the ceph tools on the cluster because of a broken monitor map config.

So heres what you need to know in a nut shell to fix it.

  1. Stop your monitors
  2. Export a monitor map from the last known good monitor
  3. Edit the monitor map to fix the broken entry
  4. Repeat this for all the monitors that were “working”.
  5. Inject the monitor maps on those monitors
  6. Start the monitors and check for them to forum a quorum.
ceph-mon -c /etc/ceph/cluster-name-ceph.conf -i MONITOR_NAME  --extract-monmap /tmp/monmap
monmaptool --print /tmp/monmap
monmaptool --rm bad-host-entry /tmp/monmap
monmaptool --print /tmp/monmap
ceph-mon --c /etc/ceph/cluster-name-ceph.conf -i MONITOR_NAME --inject-monmap /tmp/monmap
chown ceph:ceph -R /var/lib/ceph/mon/cluster-monitor-name/
systemctl start