Building a High-Availability KVM Cluster with Pacemaker on Fedora
In modern IT, downtime is not just an inconvenience; it’s a direct threat to business operations. While KVM virtualization provides incredible flexibility, the physical hardware it runs on remains a single point of failure. When a host server goes down, all virtual machines (VMs) on it go down too. The solution is to build a high-availability (HA) cluster that can automatically detect failures and migrate VMs to a healthy host, ensuring near-continuous service.
This guide explores how to create a robust KVM high-availability environment on Fedora using the powerful combination of Pacemaker and Corosync.
Why High Availability for KVM is Essential
A high-availability cluster eliminates the risks associated with a single physical server. If one of your cluster nodes fails due to a hardware issue, power outage, or network problem, an HA setup automatically initiates a failover. The virtual machines running on the failed node are gracefully restarted on another available node in the cluster, drastically minimizing downtime and maintaining business continuity.
This automated process is crucial for critical applications like databases, web servers, and enterprise software that require constant uptime.
The Core Components of an HA Cluster
To build a successful KVM failover cluster, you need to understand the key software components that make it work.
Pacemaker: This is the heart of the cluster, often called the Cluster Resource Manager (CRM). Pacemaker is responsible for monitoring the health of cluster resources (like your KVM virtual machines) and nodes. It makes the intelligent decisions about starting, stopping, and migrating services to ensure they remain available.
Corosync: This is the cluster communication layer. Corosync provides reliable messaging between all nodes, handling membership and ensuring that every node knows the status of the others. It’s the nervous system that informs Pacemaker when a node has joined or left the cluster.
pcs: This is the unified command-line interface used to configure and manage both Pacemaker and Corosync. It simplifies the entire process of setting up resources, configuring properties, and monitoring cluster status.Fencing/STONITH: This is arguably the most critical component for data integrity. STONITH stands for “Shoot The Other Node In The Head.” It is the mechanism that prevents a “split-brain” scenario, where a communication failure might lead two nodes to believe they are both the primary master, causing data corruption. Fencing ensures that a failed or unresponsive node is definitively powered off before its resources are started elsewhere.
Prerequisites for Your HA Cluster
Before you begin, ensure your environment is prepared. A successful HA deployment depends on a solid foundation.
- Multiple Server Nodes: You will need at least two (preferably three or more) servers running a recent version of Fedora.
- Shared Storage: This is non-negotiable. For a VM to failover from one host to another, its virtual disk image must be accessible by all nodes. Common shared storage solutions include NFS, iSCSI, or a Fibre Channel SAN.
- Reliable Networking: All cluster nodes must be able to communicate with each other over a reliable, low-latency network. It is a best practice to use a dedicated, private network for cluster communication (Corosync traffic).
- Time Synchronization: All nodes must have their clocks synchronized using NTP (Network Time Protocol). Time discrepancies can cause severe issues with cluster communication and consensus.
Configuring Your KVM Failover Cluster: The High-Level Steps
Setting up the cluster involves installing the necessary software, establishing communication between nodes, and defining your KVM virtual machine as a managed resource.
1. Install the Essential Software
On all your Fedora nodes, you will need to install the core packages for the cluster stack. This typically includes pacemaker, corosync, pcs, and the fencing agents.
2. Authenticate Cluster Nodes
The pcs command needs to securely communicate between all nodes. This is achieved by setting up an administrative user (typically hacluster) and authenticating each node to the cluster.
3. Create and Start the Cluster
Using the pcs tool, you will define the nodes that belong to the cluster and start the services. Once started, you can use pcs status to verify that all nodes are online and part of the quorum.
4. Configure Fencing (STONITH)
This step is absolutely mandatory for any production cluster. You must configure a fencing device that Pacemaker can use to power off a faulty node. For physical hardware, this is often done via IPMI, iDRAC, or iLO interfaces. If you skip this step, you risk catastrophic data corruption.
While it is common to temporarily disable STONITH during initial setup (pcs property set stonith-enabled=false), you must configure a valid fencing agent and re-enable it before putting any critical services under cluster management.
5. Define Your KVM Virtual Machine as a Resource
Finally, you will tell Pacemaker about the VM you want to make highly available. This is done by creating a cluster resource using the VirtualDomain resource agent. You will need to provide Pacemaker with the path to the VM’s XML configuration file, which is typically stored in /etc/libvirt/qemu/.
# Example command to create a VM resource
pcs resource create MyCriticalVM VirtualDomain config=/etc/libvirt/qemu/my-critical-vm.xml
Once the resource is created, Pacemaker will automatically start the VM on one of the cluster nodes.
Testing Your Failover and Best Practices
A cluster is only as good as its last successful test. You should regularly simulate node failures to ensure the failover mechanism works as expected.
- Simulate a Failure: You can test the failover by cleanly stopping the cluster services on the active node (
pcs cluster stop <node_name>) or by forcefully powering it off. - Monitor the Cluster: Use
pcs statusto watch as Pacemaker detects the node failure, fences the downed node (if configured), and migrates theMyCriticalVMresource to a surviving node.
Security and Reliability Tips:
- Use a Dedicated Network Ring: Isolate Corosync communication to a dedicated, redundant network to prevent cluster communication from being impacted by public network traffic.
- Monitor Cluster Health: Use monitoring tools to keep an eye on the output of
pcs statusand cluster logs to proactively identify issues. - Keep Documentation: Document your cluster configuration, fencing methods, and recovery procedures.
By properly implementing a KVM high-availability cluster with Pacemaker, you can transform your virtualization infrastructure from a collection of failure-prone servers into a resilient, self-healing platform that ensures your critical services are always online.
Source: https://infotechys.com/kvm-high-availability-with-fedora-and-pacemaker/
