The Red Hat Cluster Suite is a framework to bind two or more machines together to jointly handle one task. The following article gives an introduction to RHCS in terms of service failover.

Linux is used daily in mission-critical environments all over the world. It follows that Linux can be required to fulfil a range of needs with relation to availability and stability.  The Red Hat Cluster Suite (RHCS) is designed with these needs in mind; it enables the admin to set up a cluster of machines which all handle the same task or provide the same service. If the machine providing the service goes down, another machine then steps in and takes over.

Core elements of RHCS

RHCS consists of four core components:

• cluster infrastructure
• high availability service management
• tools for the cluster administration
• Linux virtual server routing

The cluster infrastructure includes all the core components necessary for the set up and running of a cluster of several nodes. These components manage the integration of nodes, shutting them down where problems occur (fencing), replicating the configuration and so on.

After the cluster has been set up the next step is to define the high availability service management. This is a service running on one node with other nodes configured for failover. The HA service management includes defining the service, start/stop scripts, ports, storage places and other resources as well as the priority of the different failover nodes.

The next core component is not so much a necessary key element but more a set of helpful tools: the cluster administration tools. In theory they are not critical to the running of the RHCS, although in practise it would be stupid to run the RHCS without them. They incorporate GUI tools, web pages for accessing cluster data and tools for status queries, among other things.

The situation is similar for the Linux virtual server routing; although RHCS documentation lists Linux virtual server routing as a core component, this functionality is not always needed as it "only" provides load balancing functions on IP level and re-routes the traffic when a node brakes down.Besides these official core components of RHCS the system can incorporate other services when they are available: GFS (Global File System) and Cluster Logical Volume Manager. They help with mounting network block devices, making storage management much easier.

Structure of a RHCS Cluster

To create an initial RHCS cluster a substantial set of machines is needed:

1. Shared storage like iSCSI or Fibre Channel.
2. For each node a method to detach it from the cluster (fencing), either by network or by a controllable power switch.
3. At least two nodes with a network connection.
4. A switch.

It is important that the shared storage is not running on one of the nodes itself - that would render the idea of fencing useless. Also keep in mind that the machines listed here only describe the minimum hardware configuration - a larger cluster would of course require many more nodes.

Closing words

RHCS offers a well thought out framework for managing a cluster, especially when it comes to service failover. Using RHCS makes securing your mission-critical systems easy, and makes them highly available with standard hardware.

The R in RHCS implies that this method only runs on RHEL machines - but this is not the case, as we will demonstrate in one of our upcoming articles.