Scalability

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Contents 1 Definition 2 Dimensions 2.1 Load scalability 2.2 Geographic scalability 2.3 Administrative scalability 3 Examples 4 Scale vertically vs. horizontally 4.1 Scale vertically 4.2 Scale horizontally 5 Design for scalability 6 Notes 7 See also

[edit] Definition

In telecommunications and software engineering, scalability is a desirable property of a system, a network, or a process, which indicates its ability to either handle growing amounts of work in a graceful manner, or to be readily enlarged.^[1] For example, it can refer to the capability of a system to increase total throughput under an increased load when resources (typically hardware) are added. An analogous meaning is implied when the word is used in a commercial context, where scalability of a company implies that the underlying business model offers the potential for economic growth within the company.

Scalability, as a property of systems, is generally difficult to define ^[2] and in any particular case it is necessary to define the specific requirements for scalability on those dimensions which are deemed important. It is a highly significant issue in electronics systems, database, routers, and networking. A system whose performance improves after adding hardware, proportionally to the capacity added, is said to be a scalable system.

[edit] Dimensions

Scalability can be measured in various dimensions, such as:

[edit] Load scalability

A distributed system should make it easy for us to expand and contract its resource pool to accommodate heavier or lighter loads.

[edit] Geographic scalability

A geographically scalable system is one that maintains its usefulness and usability, regardless of how far apart its users or resources.

[edit] Administrative scalability

No matter how many different organizations need to share a single distributed system, it should still be easy to use and manage.

[edit] Examples

For example, a scalable online transaction processing system or database management system is one that can be upgraded to process more transactions by adding new processors, devices and storage, and which can be upgraded easily and transparently without shutting it down.

A routing protocol is considered scalable with respect to network size, if the size of the necessary routing table on each node grows as O(log N), where N is the number of nodes in the network.

[edit] Scale vertically vs. horizontally

[edit] Scale vertically

To scale vertically (or scale up) means to add resources to a single node in a system, such as adding memory or a faster hard drive to a single computer.

[edit] Scale horizontally

To scale horizontally (or scale out) means to add more nodes to a system, such as adding a new computer to a distributed software application.

[edit] Design for scalability

It is often advised to focus system design on hardware scalability rather than on capacity. It is typically cheaper to add a new node to a system in order to achieve improved performance than to partake in performance tuning to improve the capacity that each node can handle. But this approach can have diminishing returns (as discussed in performance engineering). For example: suppose a portion of a program can be sped up by parallelizing it. Suppose we can improve 70% of a module by parallelizing it, and run on four CPUs instead of one. If α is the fraction of a calculation that is sequential, and 1 − α is the fraction that can be parallelized, then the maximum speedup that can be achieved by using P processors is given according to Amdahl's Law: . Substituting the values for this example, we get . If we double the compute power to 8 processors we get . Doubling the processing power has only improved the speedup by roughly one-fifth. If the whole problem was parallelizable, we would, of course, expect the speed up to double also. Therefore, throwing in more hardware is not necessarily the optimal approach.

[edit] Notes

[edit] See also

• Amdahl's law
• Asymptotic complexity
• Load balancing (computing)
• Lock (computer science)
• Parallelism in computing
• Performance Engineering
• Ilities
• Interconnect Driven Server