2. USAGE CONSIDERATIONS › 2.3 Use Within the Information Structure › 2.3.4 Installation Environment Analysis › 2.3.4.2 Environment: Capacity Management › 2.3.4.2.4 Capacity Analysis

2.3.4.2.4 Capacity Analysis

 
Capacity management is the process of apportioning limited
resources among different workloads based on the time
allotted to process each workload.  In most systems, the
instantaneous arrival rate of work exceeds the instantaneous
processing capacity at certain times, thus some work must
wait while other work is performed.
 
The longer the acceptable window of processing (response
time, in communication terms) for each transaction relative
to the actual time required to process, the fewer processing
resources are required to support the workload.  Conversely,
the shorter the processing window, the greater the resource
requirement.  In other words, workloads with infinitely long
processing times can be satisfied by systems with very small
processing capabilities per unit time (on a mainframe these
are background tasks).  However, workloads with infinitely
short processing times require an infinite amount of
resources to satisfy (these are all the applications for
which you are responsible).
 
In a network, available resources include:
 
o Processor power -- both mainframe and communications
  control units
 
o Storage -- both mainframe and CCU memory and mainframe
  peripheral storage (databases)
 
o Link bandwidth -- most critical in the network backbone
 
It is the interrelationship of these resources that makes the
projection of network capacity difficult.  Some of these
interrelations are the following:
 
o Insufficient processor power increases queuing, thus
  increasing the demand for storage and reducing the
  ability to utilize link bandwidth.
 
o Insufficient storage in one node increases its
  processor requirements and increases queuing in
  adjacent nodes, increasing their storage requirements.
  It also reduces the ability of all nodes to support
  increased link bandwidth.
 
o  Insufficient link bandwidth increases storage
   requirements through increased queuing.
 
The network analyst must remember that a change to any node
or link, either in capacity or workload, affects all adjacent
nodes and the links attached to them.