Capacity Planning and Architecture - Expert Oracle RAC Performance Diagnostics and Tuning - page 22

Here lambda (l) is the average customer arrival rate and T is the average service time for a customer. Applying
the preceding formula to the our restaurant situation and relating the same to a computer system model illustrated in
Figure 2-1 , the queuing will depend on

• How many customers arrive at the restaurant? Customers can arrive one by one or can arrive

in batches. In information technology, it could be related to the number of requests received
and getting added to the queue.

• How much time do customers spend in the restaurant? Customers are willing to wait or

customers could be in hurry. In information technology, it could be related to the time
required to process a request.

• How many tables does the restaurant have to service the customers? This also depends on the

discipline followed in the restaurant, for example, FIFO, random order, and priorities based on
age (senior citizens). In information technology, it could be related to the number of servers
available to process the request.

Queuing is an indication of delayed processing and increased service or response times. In the Oracle database,
this analogy can be related to contention for resources due to concurrency, lack of system resources, lack of CPU
processing power, slow network, network bandwidth, and so forth. Making system selections and the various
resources that the system will contain should take into consideration the amount of processing, number of users
accessing the system, and usage patterns.
Servers have a fixed amount of resources. Businesses are always on the positive note when gaining an increased
user base. It becomes a need of utmost importance that focus and attention be given to determine the capacity of the
servers and plan for these increases in workload to provided consistent response time for users.

Capacity Planning
A simple direct question probably arises as to why we should do capacity planning. Servers will let us know when
they are out of resources, and user volumes are unpredictable. If we assume certain things, such as expected number
of users, and we don't get the increased number of users, all of the investment could be wasted. On the contrary, if
we did not plan, we would have surprises with overloaded servers and poor response times to users, thus affecting
performance. Support for increased business is only one of the many benefits of capacity planning for the IT
infrastructure. Other benefits include the following:

• Cost avoidance, cost savings, and competitive advantage. By predicting business growth

through informed sources, organizations and management make informed decisions. This
can be a considerable cost savings and advantage in the field. Because at the end of the day,
slow systems and poor responses will drive customers/users to other similar businesses.

• Greater visibility into current and potential performance, and availability constraints that relate to

• System and application resource constraints

• Helping to understand design flaws. When applications cannot scale to increased

workload, it indicates flaws in the overall architecture of the system. Stress testing and
workload testing of the application would help determine such flaws.

• Ability to track and predict capacity consumption and related costs helps toward realistic

future planning.

Similar to scalability, which is tomorrow's need (when the business grows and more users access the system),
capacity planning is also for a future period; it is planning in infrastructure and resources required for the future. It
involves estimating the space, computer hardware, technical expertise, and infrastructure resources that are required
for a future period of time.