18.18 Highly Available Storage (HAST)

18.18.3.1 Synchronization and Replication Modes

HAST tries to provide fast failure recovery. For this reason, it is very important to reduce synchronization time after a node's outage. To provide fast synchronization, HAST manages an on-disk bitmap of dirty extents and only synchronizes those during a regular synchronization (with an exception of the initial sync).

There are many ways to handle synchronization. HAST implements several replication modes to handle different synchronization methods:

• memsync: report write operation as completed when the local write operation is finished and when the remote node acknowledges data arrival, but before actually storing the data. The data on the remote node will be stored directly after sending the acknowledgement. This mode is intended to reduce latency, but still provides very good reliability. The memsync replication mode is currently not implemented.

• fullsync: report write operation as completed when local write completes and when remote write completes. This is the safest and the slowest replication mode. This mode is the default.

• async: report write operation as completed when local write completes. This is the fastest and the most dangerous replication mode. It should be used when replicating to a distant node where latency is too high for other modes. The async replication mode is currently not implemented.

18.18.4.1 Failover Configuration

The goal of this example is to build a robust storage system which is resistant from the failures of any given node. The key task here is to remedy a scenario when a primary node of the cluster fails. Should it happen, 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.

In order to accomplish this task, it will be required to utilize another feature available under FreeBSD which provides for automatic failover on the IP layer -- CARP. CARP stands for Common Address Redundancy Protocol and 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 31.13. After completing this task, each node should have its own carp0 interface with a shared IP address 172.16.0.254. Obviously, the primary HAST node of the cluster has to 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, Samba etc, 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, which makes it possible to watch for the 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. In such a case, it is possible to run a particular script which will automatically handle the failover.

To be able to catch the state changes on the CARP interfaces, the following configuration has to be added to the /etc/devd.conf file 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";
};

To put the new configuration into effect, run the following command on both nodes:

# /etc/rc.d/devd restart

In the event that the carp0 interface goes up or down (i.e. the interface state changes), the system generates a notification, allowing the devd(8) subsystem to run an arbitrary script, in this case /usr/local/sbin/carp-hast-switch. This is the script which will handle the automatic failover. For further clarification about the above devd(8) configuration, please consult the devd.conf(5) manual page.

An example of such a script could be following:

#!/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 does the following when a node becomes master / primary:

• Promotes the HAST pools as primary on a given node.

• Checks the file system under the HAST pool.

• Mounts the pools at appropriate place.

When a node becomes backup / secondary:

• Unmounts the HAST pools.

• Degrades the HAST pools to secondary.

More detailed information with additional examples can be found in the HAST Wiki page.

18.18.5.1 General Troubleshooting Tips

HAST should be generally working without any issues, however as with any other software product, there may be times when it does not work as supposed. The sources of the problems may be different, but the rule of thumb is to ensure that the time is synchronized between all nodes of the cluster.

The debugging level of the hastd(8) should be increased when troubleshooting HAST problems. This can be accomplished by starting the hastd(8) daemon with the -d argument. Note, that this argument may be specified multiple times to further increase the debugging level. A lot of useful information may be obtained this way. It should be also considered to use -F argument, which will start the hastd(8) daemon in foreground.

18.18.5.2 Recovering from the Split-brain Condition

The consequence of a situation when both nodes of the cluster are not able to communicate with each other and both are configured as primary nodes is called split-brain. This is a dangerous condition because it allows both nodes to make incompatible changes to the data. This situation has to be handled by the system administrator manually.

In order to fix this situation the administrator has to decide which node has more important changes (or merge them manually) and let the HAST perform the full synchronization of the node which has the broken data. To do this, issue the following commands on the node which needs to be resynchronized:

# hastctl role init <resource>
# hastctl create <resource>
# hastctl role secondary <resource>