什么是系统的可扩展性？
到底什么是可扩展性？这年头，作为软件设计架构师如果系统没有可扩展性对外交流时都不好意思。但是如何选择可扩展性方案？水平扩展还是垂直扩展？是不是很矛盾呢，本文为你分析可扩展性的真实含义和实际项目中的取舍。
When asked what they mean by scalability, a lot of people talk about improving performance, about implementing HA, or even talk about a particular technology or protocol. Unfortunately, scalability is none of that. Don’t get me wrong. You still need to know all about speed, performance, HA technology, application platform, network, etc. But that is not the definition of scalability.
每每和别人提及可扩展性的含义时，很多人开始讨论提高性能，实施高可用性，甚至谈论特定的技术或协议。显然这些并不是可扩展性。不要误会，您当然需要了解关于速度，性能，可用性，应用平台，网络等相关的一切，但这并非可扩展性的定义。
Scalability, simply, is about doing what you do in a bigger way. Scaling a web application is all about allowing more people to use your application. If you can’t figure out how to improve performance while scaling out, its okay. And as long as you can scale to handle larger number of users its ok to have multiple single points of failures as well.
简单地说，可扩展性就是关于如何处理更大规模的业务。比如，Web应用程序就是允许更多的人使用你的服务。如果你不能弄清楚如何提高性能的同时
向外扩展，没关系。只要你能处理更大规模的用户，即使是存在多个单点故障也没有问题。
There are two key primary ways of scaling web applications which is in practice today.
在今天实践中有两个关键的缩放Web应用程序的方式：垂直扩展和水平扩展。
“Vertical Scalability” – Adding resource within the same logical unit to increase capacity. An example of this would be to add CPUs to an existing server, or expanding storage by adding hard drive on an existing RAID/SAN storage.
“垂直扩展“ -在同一个逻辑单位添加资源以增加容量。这样的例子比比皆是，比如升级服务器的CPU，比如在RAID/ SAN存储设备上增加硬盘。
“Horizontal Scalability” – Adding multiple logical units of resources and making them work as a single unit. Most clustering solutions, distributed file systems, load-balancers help you with horizontal scalability.
“横向扩展“- 增加多个逻辑单元资源并且使他们作为一个整体在工作。大多数的集群解决方案，比如分布式文件系统，负载均衡都是通过横向扩展技术来进行的。
Every component, whether its processors, servers, storage drives or load-balancers have some kind of management/operational overhead. When you try to scale that,
its important to understand what percentage of the resource is actually usable. This measurement is called “scalability factor“. If you loose 5% of a processor power every time you add a CPU to your system, then your “scalability factor” is 0.95. A scalability factor of 0.9 means you will only be able to use 90% of the resource.
每一个部件，无论它是处理器，服务器，存储驱动器或负载均衡有一定的管理上或者操作上的开销。当您尝试进行扩展时，很重要的一点是要了解实际的资源利用率，这种测量方法被称为“可扩展性因子“法。如果每添加一个CPU到系统，都会失去5％的处理器功率，那么您的可扩展性系数为0.95。当可扩展系数为0.9时，意味着你将只能使用90％的资源。
Scalability can be further sub-classified based on the “scalability factor”.
以“可扩展性因子“为基础，可扩展性可以进一步细化分类。
If the scalability factor stays constant as you scale. This is called “linear scalability“.
如果你的可扩展性系数保持不变，这就是所谓的“线性的可扩展性“。
But chances are that some components may not scale as well as others. A scalability factor below 1.0 is calleub-linear scalability“.
但某些组件有可能无法像别人一样可以扩展，该系数低于1.0的就是所谓“分线性可扩展性。“
Though rare, its possible to get better performance (scalability factor) just by adding more components (i/o across multiple disk spindles in a RAID gets better with more spindles). This is called “supra-linear scalability“.
虽然数量很少，但是加入更多的组件确实可能会获得更好的性能（或者可伸缩性因子）（跨越多个磁盘的RAID I / O性能确实会更好）。这就是所谓的“超线性的可扩展性。“
If the application is not designed for scalability, its possible that things can actually get worse as it scales. This is called “negative scalability“.
如果应用程序没有可扩展性，当对其进行扩展时，得到了更加糟糕的结果。这就是所谓的“负可扩展性“。
If you need scalability, urgently, going vertical is probably going to be the easiest (provided you have the bank balance to go with it). In most cases, without a line of code change, you might be able to drop in your application on a super-expensive 64 CPU server from Sun or HP and storage from EMC, Hitachi or Netapp and everything will be fine. For a while at least. Unfortunately Vertical scaling, gets more and more expensive as you grow.
如果您迫切需要可扩展性，垂直扩展可能是最容易的（只要你有足够的银行存款余额）。在大多数情况下，只要能砸一个超级昂贵的64个来自Sun或HP CPU服务器和来自EMC，NetApp和日立的存储设备，不用改变一行代码，
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