3. Guest Node Walk-through
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.
3.1. 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.
3.1.1. 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.
3.2. Configure the KVM guest
3.2.1. 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 9 mirrors list [https://mirrors.almalinux.org/isos.html] into a directory on your cluster node.
Run the following command, using your own path for the location flag:
[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 AlmaLinux 9. The above command will perform a text-based installation by default, but feel free to do a graphical installation, which exposes more options.
3.2.2. 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 the following ports, which can be used by various clustering components: TCP ports 2224, 3121, and 21064.
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
3.2.3. Configure /etc/hosts
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
3.2.4. 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
3.2.5. Configure pacemaker_remote on Guest Node
Install the pacemaker_remote daemon on the guest node. We’ll also install the
pacemaker package. It isn’t required for a guest node to run, but it
provides the crm_attribute tool, which many resource agents use.
[root@guest1 ~]# dnf config-manager --set-enabled highavailability
[root@guest1 ~]# dnf install -y pacemaker-remote resource-agents pcs \
pacemaker
3.3. Integrate Guest into Cluster
Now the fun part, integrating the virtual machine you’ve just created into the cluster. It is incredibly simple.
3.3.1. Start the Cluster
On the host, start Pacemaker if it’s not already running.
# pcs cluster start
3.3.2. Create a VirtualDomain Resource for the Guest VM
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
3.3.3. 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
3.3.4. 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)
3.3.5. 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
3.4. Starting Resources on KVM Guest
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
3.5. 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 pacemaker_remote daemon. 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
3.6. Accessing Cluster Tools from Guest Node
Besides allowing the cluster to manage resources on a guest node,
pacemaker_remote has one other trick. The pacemaker_remote daemon 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.
3.7. 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; on AlmaLinux 9
it’s 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.