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)

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虛擬服務器(1)

服務器虛擬化是少有的幾種技術之一，它們聽上去太好而好像不真實，但實際上是真的。其早期的用途是減少使用率不足的服務器硬件，(將處理任務)集中到數(shù)量更少的機器上。自這些早期應用以來，它已經(jīng)發(fā)展成一種多目的的解決方案，實現(xiàn)更高的可靠性、更好的管理、以及成為企業(yè)數(shù)據(jù)中心管理員不可或缺的工具。

按照非常簡化的定義，虛擬服務器是僅僅利用軟件來模擬獨立計算機的行為和能力。

虛擬化這個術語是很簡單的。(服務器虛擬化)軟件棧集的底部是直接安裝在服務器硬件上的普通操作系統(tǒng)。其上是處理重新定向和仿真的虛擬層，而重新定向和仿真構(gòu)成了虛擬計算機。這底部的兩層合起來被叫做宿主機。宿主機提供大家熟悉的 PC機BIOS ROM的全部工作，按照用戶的選擇，利用改變用戶定義的配制而生出很多獨立的PC機。

作為物理的服務器，虛擬 PC機只有安裝了操作系統(tǒng)才有用。安裝在虛擬宿主機上的操作系統(tǒng)稱做客OS。安裝客操作系統(tǒng)(OS)非常簡便，只需從OS的安裝光盤上引導過來。這和PC機上安裝OS沒有什么兩樣。一般來說，如果你不知道虛擬化是如何運轉(zhuǎn)的，這就是答案：就像一臺PC機。

事實上，在全 Windows環(huán)境下，你很容易找不到北：是在看Windows宿主機OS，還是在看剛剛安裝的四個Windows客OS中的一個?你自己可能搞不清，但你的客OS及其應用程序是從來不會弄錯的。每個客OS認為它擁有整個機器。在某種意義上，它是擁有整個機器。

基于軟件的硬件

運行在虛擬服務器上的操作系統(tǒng)和應用程序不直接控制內(nèi)存、硬盤和網(wǎng)絡端口等資源，而是由位于 OS和應用程序之間的VM(虛擬機)截取與硬件交互的請求并在認為合適時對它們進行處理。

真正將此技術變成接近于魔術的是 VMware公司的ESX Server等世界級的虛擬化解決方案，這類解決方案能合成整個硬件的配置，而它與基礎的硬件幾乎沒有關系。例如，宿主機可以最詳細地模擬SCSI控制器的初始化過程，讓客OS相信即使不存在物理的SCSI控制器時初始化也在被執(zhí)行。它能使IDE磁盤機看上像SCSI磁盤機，能將網(wǎng)絡共享的磁盤機轉(zhuǎn)換成裝在本機內(nèi)的存儲設備，將一個以太網(wǎng)卡變成幾個網(wǎng)卡，并能在較老式的操作系統(tǒng)和它不支持的新式硬件(如光通道卡)之間生成網(wǎng)關。你可以建立完全適合應用程序需要的屬于你自己的服務器，這時你用的是鼠標而不是用改錐。

在每次需要時把 OS 和軟件裝到物理的 PC 服務器是很煩人的。幸虧有了虛擬化你就不必這么做。在完全按你的喜好調(diào)好虛擬硬件的配置后，你就能把服務器磁盤映像保存到文件中，對其他的客系統(tǒng)而言它可以當做樣板用。在實際中，它是一件賞心悅目的好事。你能通過拷貝該文件將虛擬服務器備份。復制該文件就能創(chuàng)建新的服務器 —— 而拷貝 Windows 需要重新激活和相應的許可證，或者把已有的服務器移到不同的物理硬件上。