Ubuntu HA - Pacemaker Fence Agents

From the ClusterLabs definition:

A fence agent (or fencing agent) is a stonith-class resource agent.

The fence agent standard provides commands (such as off and reboot) that the cluster can use to fence nodes. As with other resource agent classes, this allows a layer of abstraction so that Pacemaker doesn’t need any knowledge about specific fencing technologies — that knowledge is isolated in the agent.

At the moment, the fence-agents binary package has been split into two: fence-agents-base and fence-agents-extra. The fence-agents-base binary package contains a set of curated agents which the Ubuntu Server team continuously run tests to make sure everything is working as expected. All the other agents previously in the fence-agents binary package are now moved to the fence-agents-extra.

The fence-agents-base binary package contains the following agents in the latest Ubuntu release:

fence_ipmilan
- fence_idrac
- fence_ilo3
- fence_ilo4
- fence_ilo5
- fence_imm
- fence_ipmilanplus
fence_mpath
fence_sbd
fence_scsi
fence_virsh

All those agents are in main and are fully supported. The remaining agents in fence-agents-extra are supported by upstream but they are not curated by the Ubuntu Server team.

For the fence agents provided by fence-agents-base, we will be briefly describing how to use them.

Note:
There are two well known tools used to manage fence agents, they are crmsh and pcs. Here we will present examples with both, since crmsh is the recommended and supported tool until Ubuntu 22.10 Kinetic Kudu, and pcs is the recommended and supported tool from Ubuntu 23.04 Lunar Lobster onwards. For more information on how to migrate from crmsh to pcs please refer to this migration guide.

fence_ipmilan

The content of this section is also applicable to the following fence agents: fence_idrac, fence_ilo3, fence_ilo4, fence_ilo5, fence_imm, and fence_ipmilanplus. All of them are symlinks to fence_ipmilan.

From its manpage:

fence_ipmilan is an I/O Fencing agentwhich can be used with machines controlled by IPMI.This agent calls support software ipmitool. WARNING! This fence agent might report success before the node is powered off. You should use -m/method onoff if your fence device works correctly with that option.

In a system which supports IPMI and with ipmitool installed, one could configure a fence_ipmilan resource with the following command:

$ crm configure primitive $RESOURCE_NAME stonith:fence_ipmilan \
            ip=$IP \
            ipport=$PORT \
            username=$USER \
            password=$PASSWD \
            lanplus=1 \
            action=$ACTION

One can do the same using pcs via the following command:

$ pcs stonith create $RESOURCE_NAME fence_ipmilan \
            ip=$IP \
            ipport=$PORT \
            username=$USER \
            password=$PASSWD \
            lanplus=1 \
            action=$ACTION

Where $IP is the IP address or hostname of fencing device, $PORT is the TCP/UDP port to use for connection, $USER is the login name and $PASSWD its password, and $ACTION is the fencing actions which by default is reboot.

This is one way to set up fence_ipmilan, for more information please check its manpage.

fence_mpath

From its manpage:

fence_mpath is an I/O fencing agent that uses SCSI-3 persistent reservations to control access multipath devices. Underlying devices must support SCSI-3 persistent reservations (SPC-3 or greater) as well as the “preempt-and-abort” subcommand. The fence_mpath agent works by having a unique key for each node that has to be set in /etc/multipath.conf. Once registered, a single node will become the reservation holder by creating a “write exclusive, registrants only” reservation on the device(s). The result is that only registered nodes may write to the device(s). When a node failure occurs, the fence_mpath agent will remove the key belonging to the failed node from the device(s). The failed node will no longer be able to write to the device(s). A manual reboot is required.

One could configure a fence_mpath resource with the following command:

$ crm configure primitive $RESOURCE_NAME stonith:fence_mpath \
            pcmk_host_map="$NODE1:$NODE1_RES_KEY;$NODE2:$NODE2_RES_KEY;$NODE3:$NODE3_RES_KEY" \
            pcmk_host_argument=key \
            pcmk_monitor_action=metadata \
            pcmk_reboot_action=off \
            devices=$MPATH_DEVICE \
            meta provides=unfencing

One can do the same using pcs via the following command:

$ pcs stonith create $RESOURCE_NAME fence_mpath \
            pcmk_host_map="$NODE1:$NODE1_RES_KEY;$NODE2:$NODE2_RES_KEY;$NODE3:$NODE3_RES_KEY" \
            pcmk_host_argument=key \
            pcmk_monitor_action=metadata \
            pcmk_reboot_action=off \
            devices=$MPATH_DEVICE \
            meta provides=unfencing

The $NODE1_RES_KEY is the reservation key used by this node 1 (same for the others node with access to the multipath device), please make sure you have reservation_key <key> in the default section inside /etc/multipath.conf and the multipathd service was reloaded after it.

This is one way to set up fence_mpath, for more information please check its manpage.

fence_sbd

From its manpage:

fence_sbd is I/O Fencing agent which can be used in environments where sbd can be used (shared storage).

With SBD (STONITH Block Device) configured on a system, one could configure the fence_sbd resource with the following command:

$ crm configure primitive $RESOURCE_NAME stonith:fence_sbd devices=$DEVICE

One can do the same using pcs via the following command:

$ pcs stonith create $RESOURCE_NAME fence_sbd devices=$DEVICE

This is one way to set up fence_sbd, for more information please check its manpage.

fence_scsi

From its manpage:

fence_scsi is an I/O fencing agent that uses SCSI-3 persistent reservations to control access to shared storage devices. These devices must support SCSI-3 persistent reservations (SPC-3 or greater) as well as the “preempt-and-abort” subcommand. The fence_scsi agent works by having each node in the cluster register a unique key with the SCSI device(s). Reservation key is generated from “node id” (default) or from “node name hash” (RECOMMENDED) by adjusting “key_value” option. Using hash is recommended to prevent issues when removing nodes from cluster without full cluster restart. Once registered, a single node will become the reservation holder by creating a “write exclusive, registrants only” reservation on the device(s). The result is that only registered nodes may write to the device(s). When a node failure occurs, the fence_scsi agent will remove the key belonging to the failed node from the device(s). The failed node will no longer be able to write to the device(s). A manual reboot is required.

One could configure a fence_scsi resource with the following command:

$ crm configure primitive $RESOURCE_NAME stonith:fence_scsi \
            pcmk_host_list="$NODE1 $NODE2 $NODE3" \
            devices=$SCSI_DEVICE \
            meta provides=unfencing

One can do the same using pcs via the following command:

$ pcs stonith create $RESOURCE_NAME fence_scsi \
            pcmk_host_list="$NODE1 $NODE2 $NODE3" \
            devices=$SCSI_DEVICE \
            meta provides=unfencing

The pcmk_host_list parameter contains a list of cluster nodes that can access the managed SCSI device.

This is one way to set up fence_scsi, for more information please check its manpage.

fence_virsh

From its manpage:

fence_virsh is an I/O Fencing agent which can be used with the virtual machines managed by libvirt. It logs via ssh to a dom0 and there run virsh command, which does all work. By default, virsh needs root account to do properly work. So you must allow ssh login in your sshd_config.

One could configure a fence_virsh resource with the following command:

$ crm configure primitive $RESOURCE_NAME stonith:fence_virsh \
            ip=$HOST_IP_ADDRESS \
            login=$HOST_USER \
            identity_file=$SSH_KEY \
            plug=$NODE \
            ssh=true \
            use_sudo=true

One can do the same using pcs via the following command:

$ pcs stonith create $RESOURCE_NAME fence_virsh \
            ip=$HOST_IP_ADDRESS \
            login=$HOST_USER \
            identity_file=$SSH_KEY \
            plug=$NODE \
            ssh=true \
            use_sudo=true

This is one way to set up fence_virsh, for more information please check its manpage.

In order to avoid running the resource in the same node that should be fenced, we need to add a location restriction:

$ crm configure location fence-$NODE-location $RESOURCE_NAME -inf: $NODE

Using pcs:

$ pcs constraint location $RESOURCE_NAME avoids $NODE

References

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