WebSphere Application Server V7.0: Concepts ... - IBM Redbooks

In addition, be aware that the law of diminishing returns plays a role when using
either vertical or horizontal scaling technique. The law of diminishing returns is
an economics principle that states that if one factor of production is increased
while all others remain constant, the overall returns will reach a peak and then
begin to decrease. This law can be applied to scaling in computer systems as
well. This law means that adding two additional processors will not necessarily
grant you twice the processing capacity. Nor will adding two additional horizontal
servers in the application server tier necessarily grant you twice the request
serving capacity. Additional processing cycles are required to manage those
additional resources. Although the degradation might be small, it is not a direct
linear function of change in processing power to say that adding n additional
machines will result in n times the throughput.
Note: The benefit you get by adding resources depends on the hardware and
operating system platform you are using.
7.2.2 Scaling the system
Examine the entire application environment to identify potential bottlenecks.
Throwing additional resources into the environment without fully understanding
what those resources are addressing can potentially worsen performance,
maintainability and scalability. It is necessary to know the application
environment, end-to-end, to identify the bottlenecks and ensure the appropriate
response is timely.
Network
When scaling at the network layer, such as with firewalls or switches, the most
common solution is vertical scaling. Network devices have processing capacity
and use memory much like any other hardware resource. Adding additional
hardware resources to a network device will increase the throughput of that
device, which positively impacts the scaling of that device. For example, moving
from 100 MBit to 1 GBit connections or even higher can significantly increase
performance at the network layer.
HTTP server
When scaling at the Web server layer, the most typical solution is horizontal
scaling. This means adding additional Web servers to the environment. To do so,
load balancers must be used. Be careful when adding servers and make sure
that the load balancer has adequate capacity, or adding the new Web servers will
simply shift the bottleneck from the Web tier to the load balancing tier. Vertical
scaling can be performed as well by adjusting HTTP tuning parameters, or by
increasing memory or processors.
Chapter 7. Performance, scalability, and high availability 233