Switchover Products for the SAP System

Use

This section summarizes the available switchover products for the SAP system, looking at general features, product range, architecture, and functionality.

Switchover products protect system services by switching them over to standby resources in case a critical resource fails. These products address the single points of failure in hardware that can not be protected by standard technology (such as hot pluggable RAID, UPS, backup power supply, and so on). If you use switchover products in conjunction with the standard technologies discussed elsewhere in this documentation, you can substantially improve the availability of your SAP system by comprehensively covering its single points of failure.

Switchover products offer a certain level of automation in monitoring the health of system components as well as in the detection of and reaction to component failures. Switchover software clearly cannot guarantee “zero downtime.” However, switchover products can limit the impact of host machine failures to your SAP system and restrict its unplanned downtime to tolerable levels.

Switchover products allow the definition of highly available cluster systems, which are defined as a number of loosely coupled hosts with shared disks. For more information, see Cluster Technology. In general, switchover products are capable of monitoring and controlling different system resources such as host machines, network adapters so on. In the event of failure, the service offered by the resource is automatically taken over by a standby resource.

This section focuses on how to use switchover products to protect the SAP system against failures of host machines (such as power supply failure, CPU failure, board failure), which are of key importance to the availability of your SAP system.

Switchover products can be:

·        Part of the operating system

·        Closely attached to the operating system, but not actually part of it

Integration

For the Windows operating system, Microsoft offers the Microsoft Cluster Server (MSCS), an example of a switchover product that is part of the operating system. SAP supports this product as the standard switchover solution for Windows.

For more information, see the Windows installation guides on SAP Service Marketplace at:

service.sap.com/instguidesNW04 ® Installation

Examples of the second type of switchover product – that is, closely attached to the operating system but not actually part of it – include the following, available for UNIX, Windows, and IBM platforms (in alphabetical order):

·        EMC

·        HP TruCluster Available Server

·        HP MC/ServiceGuard

·        IBM AS/400 Cluster (from Version 4.R4)

·        IBM DB2 Tracker, DB2 Geoplex, DPropR for Replication

·        IBM DB2/390 Sysplex Failover

·        IBM HACMP for AIX

·        IBM Tivoli System Automation for z/OS

·        IBM Tivoli System Automation for Multiplatforms (for AIX and Linux)

·        NCR Lifekeeper Fault Resilient Systems

·        Fujitsu Siemens PRIMECLUSTER

·        SteelEye LifeKeepr (for Linux)

·        VERITAS Cluster Server

·        SUN Cluster

This is not a complete list. It does not intend to express the significance of the products mentioned, nor their tested compliance with the SAP system. If you have further questions on specific products please contact your hardware vendor for details, since this discussion is general (individual products are not described).

Activities

SAP does not certify switchover software for the compliance with SAP products. However, SAP provides both detailed technical guidelines and a list of tested switchover products for the SAP system to its customers.

You can find more information, including the documentation SAP Web Application Server in Switchover Environments: UNIX Platforms on SAP Service Marketplace at:

service.sap.com/ha ® Media Library ® Documentation ® Switchover

At the center of switchover products are one or more software components, usually called the “cluster manager.” The cluster manager establishes a “heartbeat” between the cluster nodes, which is used diagnostically to decide whether a network link or a node has gone down. The cluster manager might also monitor other local resources and take appropriate actions in the event of failure. However, this section focuses on host machine failures and corresponding switchover solutions.

In the SAP system, all data is stored on a central database. Disk sharing is therefore necessary to make switchover consistent and transparent for the end user. The normal implementation for this is a twin-tailed SCSI bus, where each connected node has its own SCSI interface (so using an additional SCSI-ID). Some products also allow the usage of a proprietary disk sub-system. Shared disk access is controlled by an additional component of the switchover software. Access is usually exclusive to one of the nodes, and accordingly has to be switched with any SAP service switchover.

Apart from the surveillance of network links by a heartbeat, the network adapters are also controlled. Switchover products enable configuration of an additional network adapter as a “standby” for the primary adapter. If the primary one fails, the standby adapter – which does not have an IP address – takes over the IP address of the primary. For the purpose of increasing the redundancy in the communication link between the cluster nodes, you can connect the two network cards in each node to separate physical network hardware.

The diagram below shows in simplified form the basic logical components of a switchover product, showing a single cluster node and the connections to and from it. Note that the diagram does not imply a single cohesive process. For example, disk control is usually part of the operating system (for example, a logical volume manager) and part of network control might be firmware in a physical device or might also be located in the operating system (for example, programming of MAC address).

Switchover Product Components

Apart from the physical configuration of the switchover environment, the most difficult part of the set-up is the definition of the actions that are necessary to properly switch an application over to the standby resource.

Some switchover products pre-define events (for example, node_down, network_down) as well as rules and actions for the cluster manager to react to these events. The actions are normally defined as a sequence of commands in a simple shell script. These actions include some “generic” system commands (activate shared disks, check and mount filesystems) and application-specific commands (start database, move some files).

With the Microsoft Cluster Server (MSCS), SAP delivers a pre-configured solution. Therefore, you do not need to define switchover events, rules, or actions.

All switchover products allow management of the cluster with system commands or with more user-friendly interfaces. There are basic commands for cluster management to perform the following tasks:

·        Configure the cluster and its nodes

·        Start and stop the cluster

·        Add nodes to the cluster

·        Switch applications manually to another node (for node maintenance)

·        Query the status of the cluster

Manual switchover is useful to make cluster nodes available for maintenance (that is, to allow planned downtime).

This documentation and the current solutions focus on node failures. For more information about the use of switchover products for the SAP system, see the SAP documentation listed below.

See also:

SAP Web Application Server in Switchover Environments: UNIX Platforms on SAP Service Marketplace at:

service.sap.com/ha ® Media Library ® Documentation ® Switchover