The High Availability Add-On is a clustered system that provides reliability, scalability, and availability to critical production services. The following sections provide a high-level description of the components and functions of the High Availability Add-On:
A cluster is two or more computers (called nodes or members) that work together to perform a task. There are four major types of clusters:
Storage clusters provide a consistent file system image across servers in a cluster, allowing the servers to simultaneously read and write to a single shared file system. A storage cluster simplifies storage administration by limiting the installation and patching of applications to one file system. Also, with a cluster-wide file system, a storage cluster eliminates the need for redundant copies of application data and simplifies backup and disaster recovery. The High Availability Add-On provides storage clustering in conjunction with Red Hat GFS2 (part of the Resilient Storage Add-On).
High availability clusters provide highly available services by eliminating single points of failure and by failing over services from one cluster node to another in case a node becomes inoperative. Typically, services in a high availability cluster read and write data (by means of read-write mounted file systems). Therefore, a high availability cluster must maintain data integrity as one cluster node takes over control of a service from another cluster node. Node failures in a high availability cluster are not visible from clients outside the cluster. (High availability clusters are sometimes referred to as failover clusters.) The High Availability Add-On provides high availability clustering through its High Availability Service Management component,
Load-balancing clusters dispatch network service requests to multiple cluster nodes to balance the request load among the cluster nodes. Load balancing provides cost-effective scalability because you can match the number of nodes according to load requirements. If a node in a load-balancing cluster becomes inoperative, the load-balancing software detects the failure and redirects requests to other cluster nodes. Node failures in a load-balancing cluster are not visible from clients outside the cluster. Load balancing is available with the Load Balancer Add-On.
High-performance clusters use cluster nodes to perform concurrent calculations. A high-performance cluster allows applications to work in parallel, therefore enhancing the performance of the applications. (High performance clusters are also referred to as computational clusters or grid computing.)
The cluster types summarized in the preceding text reflect basic configurations; your needs might require a combination of the clusters described.
Additionally, the Red Hat Enterprise Linux High Availability Add-On contains support for configuring and managing high availability servers only. It does not support high-performance clusters.