High Availability Add-On

The High Availability Add-On for Red Hat Enterprise Linux provides continuous availability of services by eliminating single points of failure. By offering failover services between nodes within a cluster, the High Availability Add-On supports high availability for up to 16 nodes. (Currently this capability is limited to a single LAN or datacenter located within one physical site.)

The High Availability Add-On also enables failover for off-the-shelf applications such as Apache, MySQL, and PostgreSQL, any of which can be coupled with resources like IP address and single-node file systems to form highly available services. The High Availability Add-On can also be easily extended to any user-specified application that is controlled by an init script per UNIX System V (SysV) standards.

When using the High Availability Add-On, a highly available service can fail over from one node to another with no apparent interruption to cluster clients. The High Availability Add-On also ensures absolute data integrity when one cluster node takes over control of a service from another cluster node. It achieves this by promptly evicting nodes from the cluster that are deemed to be faulty using a method called "fencing" that prevents data corruption. The High Availability Add-On supports several types of fencing, including both power- and storage area network (SAN)-based fencing.

High Availability Add-On Features and Benefits

Features	Benefits
Clustering	Red Hat's High Availability Add-On enables applications to be highly available by reducing downtime and ensuring that there is no single point of failure in a cluster. It also isolates unresponsive applications and nodes so they can't corrupt critical enterprise data
Conga	The Conga application of Red Hat Enterprise Linux provides centralized configuration and management for the High Availability Add-On
Corosync	Corosync is a cluster executive within the High Availability Add-On that implements the Totem Single Ring Ordering and Membership Protocol, delivering an extremely mature, secure, high-performing, and lightweight high-availability solution
Integrated virtualization	Virtualization is pervasive throughout today's enterprise datacenters. Not only is Red Hat Enterprise Linux designed to be a superior guest on any of the major hypervisors, but it can also be a virtualization host. Virtualization is integrated directly into the Red Hat Enterprise Linux kernel using kernel-based virtual machine (KVM) technology. As part of the kernel, your administrators get the complete breadth of Red Hat Enterprise Linux system management and security tools and certifications
Fencing & unfencing	Fencing is removing access to resources from a cluster node that has lost contact with the cluster, thereby protecting resources such as shared storage from uncoordinated modification. Red Hat has made extensive improvements in the SCSI-3 PR reservations-based fencing. By enabling manual specification of keys and devices for registration and reservation, cluster administrators can bypass clvm and improve configuration and system flexibility. After fencing, the unconnected cluster node would ordinarily need to be rebooted to safely rejoin the cluster. However, unfencing allows a node to re-enable access when starting up without administrative intervention.
Improved cluster configuration system	The cluster configuration system now supports load options other than XML, including the Lightweight Directory Access Protocol (LDAP). Configuration reload is validated and easily synchronized across the cluster for better usability and manageability.
Virtualization integration	You can now run virtualized KVM guests as managed services
Rich graphical user interface (GUI)-based cluster management and administration	In Red Hat Enterprise Linux 6, the Web interface to luci has been redesigned and runs on TurboGears2
Unified logging and debugging	System administrators can now enable, capture, and read cluster system logs via a single cluster configuration command

courtesy: Redhat.com