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 The dopd daemon handles the situation where a DRBD node is out of date compared to the master and prevents the slave from being promoted to master in the event of a failure.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 would be 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 will be 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Outdated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 will then refuse 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.
You will need to reload the Heartbeat configuration:
root-shell> /etc/init.d/heartbeat reload
You will also need to modify your DRBD configuration by
configuration the outdate-peer
option. You will
need to 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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