The capacity of any computer system is its maximum performance or output. Functionality is the ability of the system to perform correctly. Usually, there is a strong concern about the functionality of a computer system, and very little concern about its performance.
Generally performance becomes an issue only when it is perceived as being poor by the users. Rarely do computer professionals worry about planning the capacity of the systems they are designing and managing.
In todays world of computers, this aspect is not to be neglected, keeping in view the amount of resources utilised or served by the computers. Therefore it is necessary that capacity planning and performance modelling be done for all computer systems in a scientific manner.