The goal of this example is to build a robust storage system which is resistant to the failure of any given node. If the primary node fails, the secondary node is there to take over seamlessly, check and mount the file system, and continue to work without missing a single bit of data.

To accomplish this task, the Common Address Redundancy Protocol (CARP) is used to provide for automatic failover at the IP layer. CARP allows multiple hosts on the same network segment to share an IP address. Set up CARP on both nodes of the cluster according to the documentation available in Section 31.10, “Common Address Redundancy Protocol (CARP)”. In this example, each node will have its own management IP address and a shared IP address of 172.16.0.254. The primary HAST node of the cluster must be the master CARP node.

The HAST pool created in the previous section is now ready to be exported to the other hosts on the network. This can be accomplished by exporting it through NFS or Samba, using the shared IP address 172.16.0.254. The only problem which remains unresolved is an automatic failover should the primary node fail.

In the event of CARP interfaces going up or down, the FreeBSD operating system generates a devd(8) event, making it possible to watch for state changes on the CARP interfaces. A state change on the CARP interface is an indication that one of the nodes failed or came back online. These state change events make it possible to run a script which will automatically handle the HAST failover.

To catch state changes on the CARP interfaces, add this configuration to /etc/devd.conf on each node:

notify 30 {
	match "system" "IFNET";
	match "subsystem" "carp0";
	match "type" "LINK_UP";
	action "/usr/local/sbin/carp-hast-switch master";
};

notify 30 {
	match "system" "IFNET";
	match "subsystem" "carp0";
	match "type" "LINK_DOWN";
	action "/usr/local/sbin/carp-hast-switch slave";
};

Restart devd(8) on both nodes to put the new configuration into effect:

# service devd restart

When the specified interface state changes by going up or down , the system generates a notification, allowing the devd(8) subsystem to run the specified automatic failover script, /usr/local/sbin/carp-hast-switch. For further clarification about this configuration, refer to devd.conf(5).

Here is an example of an automated failover script:

#!/bin/sh

# Original script by Freddie Cash <fjwcash@gmail.com>
# Modified by Michael W. Lucas <mwlucas@BlackHelicopters.org>
# and Viktor Petersson <vpetersson@wireload.net>

# The names of the HAST resources, as listed in /etc/hast.conf
resources="test"

# delay in mounting HAST resource after becoming master
# make your best guess
delay=3

# logging
log="local0.debug"
name="carp-hast"

# end of user configurable stuff

case "$1" in
	master)
		logger -p $log -t $name "Switching to primary provider for ${resources}."
		sleep ${delay}

		# Wait for any "hastd secondary" processes to stop
		for disk in ${resources}; do
			while $( pgrep -lf "hastd: ${disk} \(secondary\)" > /dev/null 2>&1 ); do
				sleep 1
			done

			# Switch role for each disk
			hastctl role primary ${disk}
			if [ $? -ne 0 ]; then
				logger -p $log -t $name "Unable to change role to primary for resource ${disk}."
				exit 1
			fi
		done

		# Wait for the /dev/hast/* devices to appear
		for disk in ${resources}; do
			for I in $( jot 60 ); do
				[ -c "/dev/hast/${disk}" ] && break
				sleep 0.5
			done

			if [ ! -c "/dev/hast/${disk}" ]; then
				logger -p $log -t $name "GEOM provider /dev/hast/${disk} did not appear."
				exit 1
			fi
		done

		logger -p $log -t $name "Role for HAST resources ${resources} switched to primary."


		logger -p $log -t $name "Mounting disks."
		for disk in ${resources}; do
			mkdir -p /hast/${disk}
			fsck -p -y -t ufs /dev/hast/${disk}
			mount /dev/hast/${disk} /hast/${disk}
		done

	;;

	slave)
		logger -p $log -t $name "Switching to secondary provider for ${resources}."

		# Switch roles for the HAST resources
		for disk in ${resources}; do
			if ! mount | grep -q "^/dev/hast/${disk} on "
			then
			else
				umount -f /hast/${disk}
			fi
			sleep $delay
			hastctl role secondary ${disk} 2>&1
			if [ $? -ne 0 ]; then
				logger -p $log -t $name "Unable to switch role to secondary for resource ${disk}."
				exit 1
			fi
			logger -p $log -t $name "Role switched to secondary for resource ${disk}."
		done
	;;
esac

In a nutshell, the script takes these actions when a node becomes master:

When a node becomes secondary:

More detailed information with additional examples can be found at http://wiki.FreeBSD.org/HAST.

Split-brain occurs when the nodes of the cluster are unable to communicate with each other, and both are configured as primary. This is a dangerous condition because it allows both nodes to make incompatible changes to the data. This problem must be corrected manually by the system administrator.

The administrator must either decide which node has more important changes, or perform the merge manually. Then, let HAST perform full synchronization of the node which has the broken data. To do this, issue these commands on the node which needs to be resynchronized:

# hastctl role init test
# hastctl create test
# hastctl role secondary test