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How Many Users Can Exchange Support?
When a user initiates a request, the server uses one or more of its hardware resources to complete the task. The main resources are the CPU, memory, mass storage (disk drives), and the network card. Suppose that a request requires 1 second of CPU time and 2 seconds of disk time—and these cannot overlap. Assume also that no other process will interfere with this request's execution. The disk, therefore, is the "bottleneck" of the operation because it is the resource that expends the most time during a request's execution.
For example, two users issue the same request. Requests arrive at the server spaced 3 seconds apart. Each request will be serviced, and the users will not notice any unusual delays (other than the typical 3-second response time).
However, if the second request arrives 1 second earlier, a bottleneck momentarily forms as the server is placed under a slightly heavier load and begins to form a queue. Consequently, the second user notices a slightly longer (1-second) delay in response time.
As more users connect and requests begin to queue up, the bottleneck becomes more pronounced. The server will slow down, and there will be some unhappy users.
The point at which server load increases and response time becomes unacceptable is when you've reached the number of users that Exchange can support given the hardware capabilities.
Three variables affect response time:
Number of user requests
Number of users on one server
Capacity of server hardware
As the number of user requests increases, so does response time. The same relationship is true for users per server. Hardware capacity has the opposite relationship: As it increases, so does its capability to handle more users and requests, which results in decreased response time.
The next section shows you how to make your operation more efficient by tuning existing resources and planning for your users' demands.