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Guest Node Walk-through
Updated 69 Days AgoPublic
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What this tutorial is: An in-depth walk-through of how to get Pacemaker to manage a KVM guest instance and integrate that guest into the cluster as a guest node.

What this tutorial is not: A realistic deployment scenario. The steps shown here are meant to get users familiar with the concept of guest nodes as quickly as possible.

This uses a RHEL-like OS as an example, using pcs as a high-level tool. The concepts apply to other OSes and tools, but the exact syntax may differ.

Configure cluster nodes

This walk-through assumes you already have a Pacemaker cluster configured. For examples, we will use a cluster with two cluster nodes named pcmk-1 and pcmk-2. You can substitute whatever your node names are, for however many nodes you have. If you are not familiar with setting up basic Pacemaker clusters, follow the walk-through in the Clusters From Scratch document before attempting this one.

Install virtualization software

On each node within your cluster, install virt-install, libvirt, and qemu-kvm. Start and enable virtnetworkd.

# dnf install -y virt-install libvirt qemu-kvm
# systemctl start virtnetworkd
# systemctl enable virtnetworkd

Reboot the host.

NOTE: While KVM is used in this example, any virtualization platform with a Pacemaker resource agent can be used to create a guest node. The resource agent needs only to support usual commands (start, stop, etc.); Pacemaker implements the remote-node meta-attribute, independent of the agent.

Configure the guest

Create guest

Create a KVM guest to use as a guest node. Be sure to configure the guest with a hostname and a static IP address (as an example here, we will use guest1 and 192.168.122.10). Here's an example way to create a guest:

  • Download an .iso file from the AlmaLinux mirrors list into a directory on your cluster node.
  • Run the following command, using your own path for the --location option:
[root@pcmk-1 ~]# virt-install \
  --name vm-guest1 \
  --memory 1536 \
  --disk path=/var/lib/libvirt/images/vm-guest1.qcow2,size=4 \
  --vcpus 2 \
  --os-variant almalinux9 \
  --network bridge=virbr0 \
  --graphics none \
  --console pty,target_type=serial \
  --location /tmp/AlmaLinux-9-latest-x86_64-dvd.iso \
  --extra-args 'console=ttyS0,115200n8'
NOTE: See the Clusters from Scratch document for more details about installing the OS. The above command will perform a text-based installation by default, but feel free to do a graphical installation, which exposes more options.

Configure firewall on guest

On each guest, allow cluster-related services through the local firewall. If you're using firewalld, run the following commands.

[root@guest1 ~]# firewall-cmd --permanent --add-service=high-availability
success
[root@guest1 ~]# firewall-cmd --reload
success
NOTE: If you are using some other firewall solution besides firewalld, simply open TCP ports 2224, 3121, and 21064, which can be used by various clustering components. If you run into any problems during testing, you might want to disable the firewall and SELinux entirely until you have everything working. This may create significant security issues and should not be performed on machines that will be exposed to the outside world, but may be appropriate during development and testing on a protected host. To disable security measures:
[root@guest1 ~]# setenforce 0
[root@guest1 ~]# sed -i.bak "s/SELINUX=enforcing/SELINUX=permissive/g" \
    /etc/selinux/config
[root@guest1 ~]# systemctl mask firewalld.service
[root@guest1 ~]# systemctl stop firewalld.service

Configure hostname resolution

You will need to add the remote node's hostname (we're using guest1 in this tutorial) to the cluster nodes' /etc/hosts files if you haven't already. This is required unless you have DNS set up in a way where guest1's address can be discovered.

For each guest, execute the following on each cluster node and on the guests, replacing the IP address with the actual IP address of the guest node.

# cat << END >> /etc/hosts
192.168.122.10  guest1
END

Also add entries for each cluster node to the /etc/hosts file on each guest. For example:

# cat << END >> /etc/hosts
192.168.122.101  pcmk-1
192.168.122.102  pcmk-2
END

Verify connectivity

At this point, you should be able to ping and ssh into guests from hosts, and vice versa.

Depending on your installation method, you may have to perform an additional step to make SSH work. The simplest approach is to open the /etc/ssh/sshd_config file and set PermitRootLogin yes. Then to make the change take effect, run the following command.

[root@guest1 ~]# systemctl restart sshd

Configure Pacemaker Remote on guest

Install Pacemaker Remote on the guest.

[root@guest1 ~]# dnf config-manager --set-enabled highavailability
[root@guest1 ~]# dnf install -y pacemaker-remote resource-agents pcs

Integrate guest into cluster

Now the fun part, integrating the virtual machine you've just created into the cluster. It is incredibly simple.

Start the cluster

On the host, start Pacemaker if it's not already running.

# pcs cluster start

Create a VirtualDomain resource for the guest

For this simple walk-through, we have created the VM and made its disk available only on node pcmk-1, so that's the only node where the VM is capable of running. In a more realistic scenario, you'll probably want to have multiple nodes that are capable of running the VM.

Next we'll assign an attribute to node 1 that denotes its eligibility to host vm-guest1. If other nodes are capable of hosting your guest VM, then add the attribute to each of those nodes as well.

[root@pcmk-1 ~]# pcs node attribute pcmk-1 can-host-vm-guest1=1

Then we'll create a VirtualDomain resource so that Pacemaker can manage vm-guest1. Be sure to replace the XML file path below with your own if it differs. We'll also create a rule to prevent Pacemaker from trying to start the resource or probe its status on any node that isn't capable of running the VM. We'll save the CIB to a file, make both of these edits, and push them simultaneously.

[root@pcmk-1 ~]# pcs cluster cib vm_cfg
[root@pcmk-1 ~]# pcs -f vm_cfg resource create vm-guest1 VirtualDomain \
    hypervisor="qemu:///system" config="/etc/libvirt/qemu/vm-guest1.xml"
Assumed agent name 'ocf:heartbeat:VirtualDomain' (deduced from 'VirtualDomain')
[root@pcmk-1 ~]# pcs -f vm_cfg constraint location vm-guest1 rule \
    resource-discovery=never score=-INFINITY can-host-vm-guest1 ne 1
[root@pcmk-1 ~]# pcs cluster cib-push --config vm_cfg --wait
NOTE: If all nodes in your cluster are capable of hosting the VM that you've created, then you can skip the pcs node attribute and pcs constraint location commands.
NOTE: The ID of the resource managing the virtual machine (vm-guest1 in the above example) must be different from the virtual machine's node name (guest1 in the above example). Pacemaker will create an implicit internal resource for the Pacemaker Remote connection to the guest. This implicit resource will be named with the value of the VirtualDomain resource's remote-node meta attribute, which will be set by pcs to the guest node's node name. Therefore, that value cannot be used as the name of any other resource.

Now we can confirm that the VirtualDomain resource is running on pcmk-1.

[root@pcmk-1 ~]# pcs resource status
  * vm-guest1	(ocf:heartbeat:VirtualDomain):	 Started pcmk-1

Prepare pcsd

Now we need to prepare pcsd on the guest so that we can use pcs commands to communicate with it.

Start and enable the pcsd daemon on the guest.

[root@guest1 ~]# systemctl start pcsd
[root@guest1 ~]# systemctl enable pcsd
Created symlink /etc/systemd/system/multi-user.target.wants/pcsd.service → /usr/lib/systemd/system/pcsd.service.

Next, set a password for the hacluster user on the guest.

[root@guest1 ~]# echo MyPassword | passwd --stdin hacluster
Changing password for user hacluster.
passwd: all authentication tokens updated successfully.

Now authenticate the existing cluster nodes to pcsd on the guest. The below command only needs to be run from one cluster node.

[root@pcmk-1 ~]# pcs host auth guest1 -u hacluster
Password: 
guest1: Authorized

Integrate guest node into cluster

We're finally ready to integrate the VM into the cluster as a guest node. Run the following command, which will create a guest node from the VirtualDomain resource and take care of all the remaining steps. Note that the format is pcs cluster node add-guest <guest_name> <vm_resource_name>.

[root@pcmk-1 ~]# pcs cluster node add-guest guest1 vm-guest1
No addresses specified for host 'guest1', using 'guest1'
Sending 'pacemaker authkey' to 'guest1'
guest1: successful distribution of the file 'pacemaker authkey'
Requesting 'pacemaker_remote enable', 'pacemaker_remote start' on 'guest1'
guest1: successful run of 'pacemaker_remote enable'
guest1: successful run of 'pacemaker_remote start'

You should soon see guest1 appear in the pcs status output as a node. The output should look something like this:

[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
  * Stack: corosync
  * Current DC: pcmk-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
  * Last updated: Wed Aug 10 00:08:58 2022
  * Last change:  Wed Aug 10 00:02:37 2022 by root via cibadmin on pcmk-1
  * 3 nodes configured
  * 3 resource instances configured

Node List:
  * Online: [ pcmk-1 pcmk-2 ]
  * GuestOnline: [ guest1@pcmk-1 ]

Full List of Resources:
  * xvm	(stonith:fence_xvm):	 Started pcmk-1
  * vm-guest1	(ocf:heartbeat:VirtualDomain):	 Started pcmk-1

Daemon Status:
  corosync: active/disabled
  pacemaker: active/disabled
  pcsd: active/enabled

The resulting configuration should look something like the following:

[root@pcmk-1 ~]# pcs resource config
 Resource: vm-guest1 (class=ocf provider=heartbeat type=VirtualDomain)
  Attributes: config=/etc/libvirt/qemu/vm-guest1.xml hypervisor=qemu:///system
  Meta Attrs: remote-addr=guest1 remote-node=guest1
  Operations: migrate_from interval=0s timeout=60s (vm-guest1-migrate_from-interval-0s)
              migrate_to interval=0s timeout=120s (vm-guest1-migrate_to-interval-0s)
              monitor interval=10s timeout=30s (vm-guest1-monitor-interval-10s)
              start interval=0s timeout=90s (vm-guest1-start-interval-0s)
              stop interval=0s timeout=90s (vm-guest1-stop-interval-0s)

How pcs configures the guest

Let's take a closer look at what the pcs cluster node add-guest command is doing. There is no need to run any of the commands in this section.

First, pcs copies the Pacemaker authkey file to the VM that will become the guest. If an authkey is not already present on the cluster nodes, this command creates one and distributes it to the existing nodes and to the guest.

If you want to do this manually, you can run a command like the following to generate an authkey in /etc/pacemaker/authkey, and then distribute the key to the rest of the nodes and to the new guest.

[root@pcmk-1 ~]# dd if=/dev/urandom of=/etc/pacemaker/authkey bs=4096 count=1

Then pcs starts and enables the pacemaker_remote service on the guest. If you want to do this manually, run the following commands.

[root@guest1 ~]# systemctl start pacemaker_remote
[root@guest1 ~]# systemctl enable pacemaker_remote

Finally, pcs creates a guest node from the VirtualDomain resource by adding remote-addr and remote-node meta attributes to the resource. If you want to do this manually, you can run the following command if you're using pcs. Alternativately, run an equivalent command if you're using another cluster shell, or edit the CIB manually.

[root@pcmk-1 ~]# pcs resource update vm-guest1 meta remote-addr='guest1' \
    remote-node='guest1' --force

Starting resources on a guest node

The following example demonstrates that resources can be run on the guest node in the exact same way as on the cluster nodes.

Create a few Dummy resources. A Dummy resource is a real resource that actually executes operations on its assigned node. However, these operations are trivial (creating, deleting, or checking the existence of an empty or small file), so Dummy resources are ideal for testing purposes. Dummy resources use the ocf:heartbeat:Dummy or ocf:pacemaker:Dummy resource agent.

# for i in {1..5}; do pcs resource create FAKE${i} ocf:heartbeat:Dummy; done

Now run pcs resource status. You should see something like the following, where some of the resources are started on the cluster nodes, and some are started on the guest node.

[root@pcmk-1 ~]# pcs resource status
  * vm-guest1	(ocf:heartbeat:VirtualDomain):	 Started pcmk-1
  * FAKE1	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE2	(ocf:heartbeat:Dummy):	 Started pcmk-2
  * FAKE3	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE4	(ocf:heartbeat:Dummy):	 Started pcmk-2
  * FAKE5	(ocf:heartbeat:Dummy):	 Started guest1

The guest node, guest1, behaves just like any other node in the cluster with respect to resources. For example, choose a resource that is running on one of your cluster nodes. We'll choose FAKE2 from the output above. It's currently running on pcmk-2. We can force FAKE2 to run on guest1 in the exact same way as we could force it to run on any particular cluster node. We do this by creating a location constraint:

# pcs constraint location FAKE2 prefers guest1

Now the pcs resource status output shows that FAKE2 is on guest1.

[root@pcmk-1 ~]# pcs resource status
  * vm-guest1	(ocf:heartbeat:VirtualDomain):	 Started pcmk-1
  * FAKE1	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE2	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE3	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE4	(ocf:heartbeat:Dummy):	 Started pcmk-2
  * FAKE5	(ocf:heartbeat:Dummy):	 Started guest1

Testing recovery and fencing

Pacemaker's scheduler is smart enough to know fencing guest nodes associated with a virtual machine means shutting off/rebooting the virtual machine. No special configuration is necessary to make this happen. If you are interested in testing this functionality out, trying stopping the guest's remote executor. This would be equivalent of abruptly terminating a cluster node's corosync membership without properly shutting it down.

ssh into the guest and run this command.

[root@guest1 ~]# kill -9 $(pidof pacemaker-remoted)

Within a few seconds, your pcs status output will show a monitor failure, and the guest1 node will not be shown while it is being recovered.

[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
  * Stack: corosync
  * Current DC: pcmk-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
  * Last updated: Wed Aug 10 01:39:40 2022
  * Last change:  Wed Aug 10 01:34:55 2022 by root via cibadmin on pcmk-1
  * 3 nodes configured
  * 8 resource instances configured

Node List:
  * Online: [ pcmk-1 pcmk-2 ]

Full List of Resources:
  * xvm	(stonith:fence_xvm):	 Started pcmk-1
  * vm-guest1	(ocf:heartbeat:VirtualDomain):	 FAILED pcmk-1
  * FAKE1	(ocf:heartbeat:Dummy):	 FAILED guest1
  * FAKE2	(ocf:heartbeat:Dummy):	 FAILED guest1
  * FAKE3	(ocf:heartbeat:Dummy):	 FAILED guest1
  * FAKE4	(ocf:heartbeat:Dummy):	 Started pcmk-2
  * FAKE5	(ocf:heartbeat:Dummy):	 FAILED guest1

Failed Resource Actions:
  * guest1 30s-interval monitor on pcmk-1 could not be executed (Error) because 'Lost connection to remote executor' at Wed Aug 10 01:39:38 2022

Daemon Status:
  corosync: active/disabled
  pacemaker: active/disabled
  pcsd: active/enabled
NOTE: A guest node involves two resources: an explicitly configured resource that you create, which manages the virtual machine (the VirtualDomain resource in our example); and an implicit resource that Pacemaker creates, which manages the pacemaker-remoted connection to the guest. The implicit resource's name is the value of the explicit resource's remote-node meta attribute. When we killed pacemaker-remoted, the implicit resource is what failed. That's why the failed action starts with guest1 and not vm-guest1.

Once recovery of the guest is complete, you'll see it automatically get re-integrated into the cluster. The final pcs status output should look something like this.

[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
  * Stack: corosync
  * Current DC: pcmk-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
  * Last updated: Wed Aug 10 01:40:05 2022
  * Last change:  Wed Aug 10 01:34:55 2022 by root via cibadmin on pcmk-1
  * 3 nodes configured
  * 8 resource instances configured

Node List:
  * Online: [ pcmk-1 pcmk-2 ]
  * GuestOnline: [ guest1@pcmk-1 ]

Full List of Resources:
  * xvm	(stonith:fence_xvm):	 Started pcmk-1
  * vm-guest1	(ocf:heartbeat:VirtualDomain):	 Started pcmk-1
  * FAKE1	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE2	(ocf:heartbeat:Dummy):	 Started guest1
  * FAKE3	(ocf:heartbeat:Dummy):	 Started pcmk-2
  * FAKE4	(ocf:heartbeat:Dummy):	 Started pcmk-2
  * FAKE5	(ocf:heartbeat:Dummy):	 Started guest1

Failed Resource Actions:
  * guest1 30s-interval monitor on pcmk-1 could not be executed (Error) because 'Lost connection to remote executor' at Wed Aug 10 01:39:38 2022

Daemon Status:
  corosync: active/disabled
  pacemaker: active/disabled
  pcsd: active/enabled

Normally, once you've investigated and addressed a failed action, you can clear the failure. However Pacemaker does not yet support cleanup for the implicitly created connection resource while the explicit resource is active. If you want to clear the failed action from the status output, stop the guest resource before clearing it. For example:

# pcs resource disable vm-guest1 --wait
# pcs resource cleanup guest1
# pcs resource enable vm-guest1

Accessing cluster tools from guest node

Besides allowing the cluster to manage resources on a guest node, Pacemaker Remote has one other trick. It allows nearly all the pacemaker tools (crm_resource, crm_mon, crm_attribute, etc.) to work on guest nodes natively.

Try it: Run crm_mon on the guest after pacemaker has integrated the guest node into the cluster. These tools just work. This means resource agents such as promotable resources (which need access to tools like crm_attribute) work seamlessly on the guest nodes.

Higher-level command shells such as pcs may have partial support on guest nodes, but it is recommended to run them from a cluster node.

Troubleshooting a remote connection

If connectivity issues occur, it's worth verifying that the cluster nodes can communicate with the guest node on TCP port 3121. We can use the nc command to test the connection.

On the cluster nodes, install the package that provides the nc command. The package name may vary by distribution; for example, nmap-ncat.

Now connect using nc from each of the cluster nodes to the guest and run a /bin/true command that does nothing except return success. No output indicates that the cluster node is able to communicate with the guest on TCP port 3121. An error indicates that the connection failed. This could be due to a network issue or because pacemaker-remoted is not currently running on the guest node.

Example of success:

[root@pcmk-1 ~]# nc guest1 3121 --sh-exec /bin/true
[root@pcmk-1 ~]#

Examples of failure:

[root@pcmk-1 ~]# nc guest1 3121 --sh-exec /bin/true
Ncat: Connection refused.
[root@pcmk-1 ~]# nc guest1 3121 --sh-exec /bin/true
Ncat: No route to host.
Last Author
kgaillot
Last Edited
Sep 12 2024, 5:53 PM