计算机专业时文选读之三

Virtual Servers(1)

Server virtualization is one of those rare technologies that sounds too good to be true, but it's real. Its earliest use was to consolidate underutilized server hardware onto a smaller number of machines. Since those early days, it has grown into a multipurpose solution that enables greater reliability, improved management, and other benefits that make it an all-but-indispensable tool for enterprise datacenter administrators.

To use an oversimplified definition, a virtual server mimics, using software alone, the behavior and capabilities of a stand-alone computer.

The nomenclature of virtualization is simple. The bottom of the software stack is occupied by a single instance of an ordinary operating system that's installed directly onto the server hardware. Above that, a virtualization layer handles the redirection and emulation that make up the virtual computer. The combination of these two lower layers is referred to as the host. The host provides the full workings of a familiar PC right down to its BIOS ROM, and it can spawn as many independent PCs— using varying user-defined configurations—as you choose.

As are physical servers, a virtual PC is useless until you install an operating system on it. The operating systems that you install on your virtual hosts are called guests. Installing a guest OS can be as easy as booting from the OS's installation CD. It's just like installing an OS on a PC, and in general, if you wonder how virtualization will behave, that's the answer: Just like a PC.

In fact, in an all-Windows environment, it's easy to lose your place: Are you looking at your Windows host OS or at one of the four Windows guest OSes you just installed? You might get confused, but your guest OSes and their applications never do. Each guest OS believes it has the whole machine to itself. And, in a sense, it does.

Software-based hardware

Operating systems and applications running on virtual servers don't have direct control over resources such as memory, hard drives, and network ports. Instead, the VM that sits beneath the OS and applications intercepts requests for interaction with hardware and handles them as it sees fit.

The real mindblower that turns this technology into something close to magic is that a world-class virtualization solution such as VMware ESX Server can synthesize an entire hardware configuration that has little resemblance to the underlying hardware. For example, the host might simulate the initialization process of a SCSI controller to the last detail, convincing the guest OS that this initialization is being performed even when no physical SCSI controller exists. It can make IDE drives look like SCSI drives, convert network shares into locally attached storage, turn one Ethernet adapter into several, and create gateways between older operating systems and unsupported modern hardware such as Fibre Channel adapters. You build your own servers that precisely fit the needs of your applications, but you use a mouse instead of a screwdriver.

Installing the OS and software onto a physical PC server every time you need one can be tedious. Fortunately, with virtualization you don't have to. After you've tuned a virtualized hardware configuration precisely to your liking, you can save that server's disk image to a file and use it as a template for other guest systems. In practice, this is a delight. You can back up a virtual server by copying the file. You can create a new server by duplicating the file—copying Windows requires reactivation and an appropriate license—or move an existing server to different physical hardware. (To Be continued)

时文选读

虚拟服务器(1)

服务器虚拟化是少有的几种技术之一，它们听上去太好而好像不真实，但实际上是真的。其早期的用途是减少使用率不足的服务器硬件，(将处理任务)集中到数量更少的机器上。自这些早期应用以来，它已经发展成一种多目的的解决方案，实现更高的可靠性、更好的管理、以及成为企业数据中心管理员不可或缺的工具。

按照非常简化的定义，虚拟服务器是仅仅利用软件来模拟独立计算机的行为和能力。

虚拟化这个术语是很简单的。(服务器虚拟化)软件栈集的底部是直接安装在服务器硬件上的普通操作系统。其上是处理重新定向和仿真的虚拟层，而重新定向和仿真构成了虚拟计算机。这底部的两层合起来被叫做宿主机。宿主机提供大家熟悉的 PC机BIOS ROM的全部工作，按照用户的选择，利用改变用户定义的配制而生出很多独立的PC机。

作为物理的服务器，虚拟 PC机只有安装了操作系统才有用。安装在虚拟宿主机上的操作系统称做客OS。安装客操作系统(OS)非常简便，只需从OS的安装光盘上引导过来。这和PC机上安装OS没有什么两样。一般来说，如果你不知道虚拟化是如何运转的，这就是答案：就像一台PC机。

事实上，在全 Windows环境下，你很容易找不到北：是在看Windows宿主机OS，还是在看刚刚安装的四个Windows客OS中的一个?你自己可能搞不清，但你的客OS及其应用程序是从来不会弄错的。每个客OS认为它拥有整个机器。在某种意义上，它是拥有整个机器。

基于软件的硬件

运行在虚拟服务器上的操作系统和应用程序不直接控制内存、硬盘和网络端口等资源，而是由位于 OS和应用程序之间的VM(虚拟机)截取与硬件交互的请求并在认为合适时对它们进行处理。

真正将此技术变成接近于魔术的是 VMware公司的ESX Server等世界级的虚拟化解决方案，这类解决方案能合成整个硬件的配置，而它与基础的硬件几乎没有关系。例如，宿主机可以最详细地模拟SCSI控制器的初始化过程，让客OS相信即使不存在物理的SCSI控制器时初始化也在被执行。它能使IDE磁盘机看上像SCSI磁盘机，能将网络共享的磁盘机转换成装在本机内的存储设备，将一个以太网卡变成几个网卡，并能在较老式的操作系统和它不支持的新式硬件(如光通道卡)之间生成网关。你可以建立完全适合应用程序需要的属于你自己的服务器，这时你用的是鼠标而不是用改锥。

在每次需要时把 OS 和软件装到物理的 PC 服务器是很烦人的。幸亏有了虚拟化你就不必这么做。在完全按你的喜好调好虚拟硬件的配置后，你就能把服务器磁盘映像保存到文件中，对其他的客系统而言它可以当做样板用。在实际中，它是一件赏心悦目的好事。你能通过拷贝该文件将虚拟服务器备份。复制该文件就能创建新的服务器 —— 而拷贝 Windows 需要重新激活和相应的许可证，或者把已有的服务器移到不同的物理硬件上。