What is a Cluster?

VERITAS Cluster Server (VCS) connects, or clusters, multiple, independent systems into a management framework for increased availability. Each system, or node, runs its own operating system and cooperates at the software level to form a cluster. VCS links commodity hardware with intelligent software to provide application failover and control. When a node or a monitored application fails, other nodes can take predefined actions to take over and bring up services elsewhere in the cluster.

Detecting Failure

VCS can detect application failure and node failure among cluster members.

Detecting Application Failure

VCS is typically deployed to keep business-critical applications online and available to users. VCS provides a mechanism to detect failure of an application by issuing specific commands, tests, or scripts that monitor the overall health of an application. VCS also determines the health of underlying resources supporting the application, such as file systems and network interfaces.

Detecting Node Failure

One of the most difficult tasks in clustering is correctly discriminating between loss of a system and loss of communication between systems. VCS uses a redundant network heartbeat along with SCSI III-based membership coordination and data protection for detecting failure on a node and on fencing. For more information on detecting node failure and how VCS protects data, see Cluster Control, Communications, and Membership.

Switchover and Failover

Failover and switchover are the processes of bringing up application services on a different node in a cluster. In both cases, an application and its network identity are brought up on a selected node. Client systems access a virtual IP address that moves with the service. Client systems are unaware of which server they are using.

A virtual IP address is an address brought up in addition to the base address of systems in the cluster. For example, in a 2-node cluster consisting of db-server1 and db-server2, a virtual address may be called db-server. Clients then access db-server and are unaware of which physical server actually hosts the db-server. Virtual IP addresses use a technology known as IP Aliasing.

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Switchover

A switchover is an orderly shutdown of an application and its supporting resources on one server and a controlled startup on another server. Typically this means unassigning the virtual IP, stopping the application, and deporting shared storage. On the other server, the process is reversed. Storage is imported, file systems are mounted, the application is started, and the virtual IP address is brought up

Failover

A failover is similar to a switchover, except the ordered shutdown of applications on the original node may not be possible, so the services are started on another node. The process of starting the application on the node is identical in a failover or switchover.