Data Availability

High Availability

Clustering is the ability to combine multiple systems in such a way that they provide services a single system could not. Clustering is used to achieve higher availability, scalability and easier management. Higher availability can be achieved by the use of "failover" clusters, in which resources can automatically move between two or more nodes in the event of a failure. Scalability can be achieved by balancing the load of an application across several computer systems.

High availability clustering joins together two or more PowerEdge servers equipped with PERCs to help ensure against system failures including planned shutdowns (maintenance, backups) and unplanned outages (system failure, software failure, operator errors). The group of connected systems is known as a cluster.

How A Cluster Works

Servers that are clustered together share a hard disk array and are interconnected via a private LAN. The cluster utilizes this LAN for a heartbeat mechanism to determine the health of each system. When a failure occurs, the failed system's applications and application data is migrated to the healthy system. Ideally, when an application switches from one node to another, the user experiences no disruption at all and isn't even aware that the job has been switched to a different server.

This image presents an overview of a clustered environment

Realistically, the disruption the user experiences can range from a slight delay to an extensive application restart. By transferring the users to a backup system, high availability clustering minimizes the amount of downtime

Active vs. Passive Clustering

Clustered servers can be implemented in one of two basic configurations. An active/active cluster is one in which both servers are actively processing specifically-assigned applications simultaneously. If one of the servers fails, its applications are transferred to the remaining server.

In an active/passive cluster, all of the applications are handled by one server. The second server is there strictly as a failover device. Dell supports the active/passive cluster configuration.