Cluster Technology

Use

This section discusses the software and hardware aspects of cluster architectures relevant for SAP systems. A cluster consists of a small number of host machines, known as cluster nodes. The nodes can be single-processor machines or multi-processor machines, for example, Symmetrical Multi-Processor (SMP). Cluster technology is only used for SAP system application hosts or database hosts.

The following aspects of cluster technology are relevant to SAP systems:

·        High Availability

·        Scalability

This section focuses on high availability.

Features

In general, cluster hardware for SAP systems can be divided into the following categories:

Cluster Hardware for the SAP System

The physically or virtual shared disk systems are relevant for current SAP systems.

The new architectures using shared memory clusters – for example, “Scalable Coherent Interconnect Technology” (SCSI) – where several cluster nodes share memory by means of a software or hardware layer, might be used in the future with SAP systems.

Shared Disk Systems

·        Physically-shared disk system

In this type of system, the SCSI disk controller is directly connected to two (or more) computers, as shown below:

Physically-Shared Disk System

·        Virtual-shared disk systems

In the case of virtual-shared disk systems a software layer (the virtual disk layer) enables access to disks connected locally on another host machine.

Some MPP (Massively Parallel Processing) systems, the main characteristic of which is a large number of processors, use a virtual disk layer. The virtual disk layer in an MPP system theoretically allows the sharing of disks between a large number of cluster nodes, as shown in the following diagram:

Virtual-Shared Disk System

Concurrent and Non-Concurrent Disk Access

The access to the shared disks can be concurrent or non-concurrent. If concurrent access is allowed, this is handled by distributed lock manager software, which is usually tightly integrated in the operating system.

An example of non-concurrent access is the use of switchover software. Refer to Switchover Software for High Availability. Examples of concurrent access are Oracle Parallel Server and Sysplex failover support for DB2 for OS/390. Refer to Replicated Database Servers.

Cluster Communications

From the high availability viewpoint, a high-speed communication network between the cluster nodes – faster in terms of bandwidth and latency than FDDI networks – is not necessary for the server traffic in an SAP system. For more information, see Network System High Availability.

High-speed communication networks are needed for low-latency communications between the cluster nodes (for example, distributed lock manager communications).

Clusters or SMP Machines?

From a high availability viewpoint the question of the advantages and disadvantages of cluster systems versus SMP systems can be simply answered. SAP recommends you to run the SAP system on a cluster of SMP machines. The use of a cluster of SMP machines (that is, the cluster nodes are SMP host machines) for an SAP system increases both availability and performance.

Consider using switchover software with a cluster

The use of switchover software increases the availability of an SAP system running on a cluster. For more information, see Switchover Software for High Availability.