In information technology, scalability (frequently spelled scaleability) seems to have two usages:

1) It is the ability of a computer application or product (hardware or software) to continue to function well when it (or its context) is changed in size or volume in order to meet a user need. Typically, the rescaling is to a larger size or volume. The rescaling can be of the product itself (for example, a line of computer systems of different sizes in terms of storage, RAM, and so forth) or in the scalable object's movement to a new context (for example, a new operating system).

An example: John Young in his book Exploring IBM's New-Age Mainframes describes the RS/6000 SP operating system as one that delivers scalability ("the ability to retain performance levels when adding additional processors"). Another example: In printing, scalable fonts are fonts that can be resized smaller or larger using software without losing quality.

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2) It is the ability not only to function well in the rescaled situation, but to actually take full advantage of it. For example, an application program would be scalable if it could be moved from a smaller to a larger operating system and take full advantage of the larger operating system in terms of performance (user response time and so forth) and the larger number of users that could be handled.

It is usually easier to have scalability upward rather than downward since developers often must make full use of a system's resources (for example, the amount of disk storage available) when an application is initially coded. Scaling a product downward may mean trying to achieve the same results in a more constrained environment.

This was last updated in April 2006

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How has rescaling / descaling affected your hardware or software performance?
I could not have actual image that how could be cloud working with there three IAAS,PAAS and SAAS is any video available or blog you recommend to me where the whole picture of cloud vivid for those who not good this technology like me. 

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