1.2 Overview of UNIX Cluster Types

There are three basic types of UNIX clusters: the Failover Cluster, the Single System Image (SSI) Application Cluster, and the Single Systems Image (SSI) Systems Cluster. There would be four different types of UNIX clusters if you count Linux or Beowulf Clustering, but the discussion in this book is limited to the first three types listed above.

1.2.1 Failover Cluster

The Failover Cluster is currently the most common form of UNIX clustering. In its many incarnations and flavors, it is available from most major computer hardware and system software vendors. While the main purpose of the failover cluster is high availability of applications, it is generally considered the most difficult to configure and manage due to the customizations required of application failover scripts.

In looking at a failover cluster from a hardware perspective, it usually has some kind of interconnect between cluster nodes, access to a common disk or storage subsystem from each node, and a network failover capability. Failover of applications is accomplished through scripts that start and stop the applications during cluster node failure and recovery.

Even though each node of a failover cluster is closely coupled through hardware, for the most part it is considered "shared-nothing" from a systems standpoint. Each cluster node must have its own copy of the operating system, and there can be no simultaneous access of disks or of memory between cluster nodes.

The difficulty in developing good, robust application failover scripts usually has to do with the timing and the synchronization for startup or failover of applications versus the common disk subsystem accessibility.

1.2.2 Single System Image (SSI) Application Cluster

The principal difference between a SSI application cluster and a failover cluster is the application software. The application software must not only be "cluster-aware" but also "parallelized" to operate on each node of the cluster at the same time. These multiple components of the application are presented as one application to the users and the application administrator. The most well-known application, providing a single view of the application and its data, is Oracle Parallel Server (OPS) from the Oracle Corporation.

For the most part, a SSI application cluster usually consists of the application, like OPS, and a fully configured failover cluster. All nodes of the failover cluster would run the application software, and failover scripts would control the actual failover of the application software.

1.2.3 Single System Image (SSI) Systems Cluster

It is generally accepted that adding more Single System Image features increases availability, performance and scalability, and manageability to a cluster. However, there has been a great deal of disagreement, among computer hardware and systems manufacturers, regarding which SSI features constitute a full SSI systems cluster. Not surprisingly, each manufacturer believes they are correct in how they define what constitutes a SSI systems cluster, regardless of what the customers think.

From a hardware perspective, again, there is little or no difference between a SSI systems cluster and a failover cluster. The real differences come in the software.

Features that are essential to any SSI systems cluster include the following:

SSI device access - a common view and access to all storage devices.
A cluster file system - a common view and access to the entire file system hierarchy.
A cluster alias or cluster Internet Protocol (IP) addressing - clients view the cluster as one system.
SSI systems management - the cluster is managed like a single system.

As these features provide the core functionality of any SSI cluster, they must be available prior to the addition of any other SSI related feature(s).

Additional SSI systems cluster features are:

Batch-load leveling - allows for the running of certain processes on the least loaded cluster node.
SSI interprocess communications (IPC) - allows for a single name space and the sharing of standard IPC capabilities like pipes, semaphores, and shared memory.
SSI process management - allows for a single namespace for processes.
Dynamic load balancing - along with SSI process management, allows for process relocation between cluster nodes thereby dynamically balancing the load on the cluster as a whole.

Inclusion of one or all of these additional SSI systems cluster features may or may not determine whether it is a full or partial SSI systems cluster. Rather than have a computer manufacturer's marketing department dictate what is a full or partial SSI systems cluster, we believe that the definition depends on whether the cluster's functionality meets the requirements that you, the user, have for a full or partial SSI cluster.

The three themes to consider for any type of cluster solution are high availability, performance and scalability, and manageability.