Managing Serviceguard Extension for SAP, December 2007

Understanding Serviceguard Extension for SAP
Dialog Instance Clusters as Simple Tool for Adaptive Enterprises
Chapter 1 27
Dialog Instance packages allow an uncomplicated approach to achieve
abstraction from the hardware layer. It is possible to shift around Dialog
Instance packages between servers at any given time. This might be
desirable if the CPU resource consumption is eventually balanced poorly
due to changed usage patterns. Dialog Instances can then be moved
between the different hosts to address this. A Dialog Instance can also be
moved to a standby host to allow planned hardware maintenance for the
node it was running on.
One can simulate this flexibility by installing Dialog Instances on every
host and activating them if required. This might be a feasible approach
for many purposes and saves the need to maintain virtual IP addresses
for each Dialog Instance. But there are ways that SAP users
unintentionally create additional short-term SPOFs during operation if
they reference a specific instance via its hostname. This could e.g. be
done during batch scheduling. With Dialog Instance packages, the
system becomes invulnerable against this type of user error.
Dialog Instance virtualization packages still provide high availability.
The system becomes more robust and flexible using Dialog Instance
packages.