Managing Serviceguard Extension for SAP, December 2007

Understanding Serviceguard Extension for SAP
Combining SGeSAP with Complementary Solutions
Chapter 1 47
All host systems in a Metrocluster environment must be members of a
single Serviceguard cluster. Either a single Data Center architecture
without an arbitrator or a Three Data Center architecture with one or
two arbitrator systems can be implemented. The arbitrator(s) are not
physically connected to the storage units. In a three data center
architecture, no cluster lock device is used since it does not work across
the ESCON link. Three hosts are allowed for each data center. In a
campus environment, the same number of systems must be present in
each of the two data centers whose systems are connected to the storage
units.
A Metrocluster environment may increase package startup time
significantly. Packages with many disk devices will take longer to start
up than those with fewer devices due to the time needed to get device
status from the XP / Symmetrix array. Clusters with multiple packages
that use devices on the XP / Symmetrix will cause package startup time
to increase when more than one package is starting at the same time.
You have to add the time needed to restart all attached Dialog Instances
to get an estimation of the total package startup time. It is strongly
recommended to disable the timeout mechanism of Serviceguard that
monitors the switchover.
If you are planning to control additional SAP application servers with
SGeSAP, an analysis of the physical machine environment for the Dialog
Instances is required. In disaster tolerant configurations it is required to
run the complete SAP production setup, including all the SAP
Application Servers in the primary DC and fail it over completely to the
secondary DC in case of a disaster. This requires changes in the Dialog
Instance configuration within the SGeSAP environment. You need a
specific SGeSAP configuration file sap.config for the primary DC and a
different sap.config file for the secondary DC.
Refer to the HP whitepaper Designing Disaster Tolerant HA Clusters for
more details on Metrocluster.