Managing HP Serviceguard Extension for SAP for Linux, December 2013

Figure 2 Visualization of a one-package cluster concept in Serviceguard Manager
If the primary node fails, the database and the Central Instance failover and continue functioning
on an adoptive node. After failover, the system runs without any manual intervention. All redundant
Application Servers and Dialog Instances, even those that are not part of the cluster, can stay up
or can be restarted (triggered by a failover). A sample configuration in Figure 2 (page 13) shows
node1 with a failure, which causes the package containing the database and central instance to
failover to node2. A Quality Assurance System and additional Dialog Instances get shut down,
before the database and Central Instance are restarted.
2.4 Example 2: A mutual failover scenario
If you are planning a simple three-tier SAP layout in a two node cluster, use the SGeSAP mutual
failover model. This approach distinguishes two SGeSAP packages—one for the database SPOF
and the other for the SAP SPOFs as defined above. In small and medium size environments, the
database package gets combined with NFS toolkit server functionality to provide all filesystems
that are required by the software in both packages. During normal operation, the two packages
are running on different nodes of the cluster. The major advantage of this approach is that the
failed SAP package will never cause a costly failover of the underlying database because it is
separated in a different package.
The failover process results in downtime that typically lasts a few minutes, depending on the work
in progress when the failover takes place. A main portion of downtime is needed for the recovery
of a database. The total recovery time of a failed database cannot be predicted reliably.
By tuning the Serviceguard heartbeat on a dedicated heartbeat LAN, it is possible to achieve
failover times in the range of about a minute or two for a ci package that contains a lightweight
[A]SCS instance without database.
A cluster can be configured in a way that two nodes back up each other. The basic layout is
depicted in Figure 3 (page 14).
2.4 Example 2: A mutual failover scenario 13