Yulia Saponova, 123RF.com

High availability clustering on a budget with KVM

Virtual Cluster

Article from ADMIN 00/2010

By Thorsten Scherf

High-availability clusters have become an important part of the system administration landscape. Tools like KVM bring the benefits of virtualization to HA clustering environments.

Cluster systems serve many different roles. Load-balancing clusters help distribute the load evenly among the various systems. High-performance clusters (HPCs) focus on maximizing the computer performance. The clustering alternative known as a High-Availability (HA) cluster attempts to maximize availability. If one system fails, the service running on it simply migrates to another cluster node, and the users will not even notice the failure.

With the rise of virtualization, virtual machines are increasingly deployed in clustering configurations – including in HA environments. This article describes how to set up an HA cluster, which could serve as a fail-safe web or mail server system. Instead of making individual services highly available, cluster managers create and monitor the complete virtual machine instances that provide high availability.

The virtual clustering configuration looks as if the physical host systems have a collection of virtual machines. All of the virtual machines run in a shared storage area accessible by all host systems. This storage area could consist of iSCSI or Fibre Channel (FC) storage, but an NFS export will do for test purposes (although the performance with NFS leaves much to be desired).

The cluster manager checks at regular intervals to ensure that individual host systems are available, using a variety of methods. For example, the cluster nodes might be expected to send a heartbeat across the network at regular intervals. Alternatively, some implementations store status messages or run heuristics, such as pinging a central router.

If the vital signs from a cluster node are missing, the cluster manager will take remedial action, such as restarting a downed system. This procedure is known as fencing (formerly referred to as STONITH – Shoot The Other Node In The Head). Fencing is necessary to avoid a cluster node trying to access what used to be its resources after another node has taken over

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