14.2.3. Using Heartbeat with DRBD and dopd

As a further extension to using DRBD and Heartbeat together, you can enable dopd. The dopd daemon handles the situation where a DRBD node is out of date compared to the master and prevents the slave from being promoted to master in the event of a failure. This stops a situation where you have two machines that have been masters ending up different data on the underlying device.

For example, imagine that you have a two server DRBD setup, master and slave. If the DRBD connectivity between master and slave fails, then the slave is out of the sync with the master. If Heartbeat identifies a connectivity issue for master and then switches over to the slave, the slave DRBD device is promoted to the primary device, even though the data on the slave and the master is not in synchronization.

In this situation, with dopd enabled, the connectivity failure between the master and slave would be identified and the metadata on the slave would be set to Outdated. Heartbeat refuses to switch over to the slave even if the master failed. In a dual-host solution this would effectively render the cluster out of action, as there is no additional fail over server. In an HA cluster with three or more servers, control would be passed to the slave that has an up to date version of the DRBD device data.

To enable dopd, you need to modify the Heartbeat configuration and specify dopd as part of the commands executed during the monitoring process. Add the following lines to your ha.cf file:

respawn hacluster /usr/lib/heartbeat/dopd
apiauth dopd gid=haclient uid=hacluster

Make sure you make the same modification on both your primary and secondary nodes.

Reload the Heartbeat configuration:

root-shell> /etc/init.d/heartbeat reload

Modify your DRBD configuration by configuration the outdate-peer option. Add the configuration line into the common section of /etc/drbd.conf on both hosts. An example of the full block is shown below:

common {
  handlers {
    outdate-peer "/usr/lib/heartbeat/drbd-peer-outdater";
  }
}

Finally, set the fencing option on your DRBD configured resources:

resource my-resource {
  disk {
    fencing    resource-only;
  }
}

Now reload your DRBD configuration:

root-shell> drbdadmin adjust all

You can test the system by unplugging your DRBD link and monitoring the output from /proc/drbd.

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