This document is for a development version of Ceph.

Configuring Monitor Election Strategies

By default, the monitors are in classic mode. We recommend staying in this mode unless you have a very specific reason.

If you want to switch modes BEFORE constructing the cluster, change the mon election default strategy option. This option takes an integer value:

  • 1 for classic

  • 2 for disallow

  • 3 for connectivity

After your cluster has started running, you can change strategies by running a command of the following form:

$ ceph mon set election_strategy {classic|disallow|connectivity}

Choosing a mode

The modes other than classic provide specific features. We recommend staying in classic mode if you don’t need these extra features because it is the simplest mode.

Disallow Mode

The disallow mode allows you to mark monitors as disallowed. Disallowed monitors participate in the quorum and serve clients, but cannot be elected leader. You might want to use this mode for monitors that are far away from clients.

To disallow a monitor from being elected leader, run a command of the following form:

ceph mon add disallowed_leader {name}

To remove a monitor from the disallowed list and allow it to be elected leader, run a command of the following form:

ceph mon rm disallowed_leader {name}

To see the list of disallowed leaders, examine the output of the following command:

ceph mon dump

Connectivity Mode

The connectivity mode evaluates connection scores that are provided by each monitor for its peers and elects the monitor with the highest score. This mode is designed to handle network partitioning (also called net-splits): network partitioning might occur if your cluster is stretched across multiple data centers or otherwise has a non-uniform or unbalanced network topology.

The connectivity mode also supports disallowing monitors from being elected leader by using the same commands that were presented in Disallow Mode.

Examining connectivity scores

The monitors maintain connection scores even if they aren’t in connectivity mode. To examine a specific monitor’s connection scores, run a command of the following form:

ceph daemon mon.{name} connection scores dump

Scores for an individual connection range from 0 to 1 inclusive and include whether the connection is considered alive or dead (as determined by whether it returned its latest ping before timeout).

Connectivity scores are expected to remain valid. However, if during troubleshooting you determine that these scores have for some reason become invalid, drop the history and reset the scores by running a command of the following form:

ceph daemon mon.{name} connection scores reset

Resetting connectivity scores carries little risk: monitors will still quickly determine whether a connection is alive or dead and trend back to the previous scores if those scores were accurate. Nevertheless, resetting scores ought to be unnecessary and it is not recommended unless advised by your support team or by a developer.

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