NonStop Systems Introduction

The NonStop Kernel
NonStop Systems Introduction—527825-001
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Support for System Expandability
Support for System Expandability
In addition to promoting fault tolerance, the message system supports the expansion of
a NonStop system as it grows to match business growth.
When a successful user application incorporates more functions or adds new users, it
might require additional processing power. NonStop systems are designed to
accommodate this growth by expanding from a minimum of two processors to a
maximum of sixteen processors in a single system. You just connect more client
systems and disk drives to the added processors and run additional copies of the
application in the new processors.
But won’t this require substantial changes to the application programs? After all, in
conventional systems, you must rework your code to identify new processors and
devices that your application will use.
The message-based operating system solves this potential problem. NonStop
applications do not need to be reprogrammed when processors and devices are
added. This is because application processes send and receive messages on the
basis of process names, not process locations. The operating system uses its tables
to map process names to physical locations.
Thus, if you add two or three new processors and some workstations to an overloaded
system, run copies of some applications in the new processors, and move some
applications from existing processors to the new processors to balance the overall
workload, you do not need to change any of the application programs. They will run
just as they did on the old configuration, except that throughput and response times will
be better.
Adding more processors can also provide a substantial performance increase because
of the ability of the NonStop system to divide a workload and share it among many
processors simultaneously. For example, a large database search might take a single
processor hours to perform. However, on a NonStop system, the search could be
subdivided into smaller tasks and shared among several processors for a significant
increase in performance. This load balancing is performed automatically and does not
require any changes to your applications.
Role of Expand Software
You have seen that the message-based operating system provides distribution of
control, fault tolerance, and system expandability within the local environment of a
NonStop system. Expand network software extends the functions of the operating
system to the network environment.
When an interprocess message is sent between two processes in the same system,
the message system can handle the message delivery by itself. But when an
interprocess message is sent between two processes in different systems, the
message system uses the Expand software to route the message over physical