Xen虚拟化实战指南：构建与管理高效虚拟环境

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"Running Xen: A Hands-On Guide to the Art of Virtualization 是一本由Jeanna N. Matthews等人编写的关于Xen虚拟化技术的入门书籍，由Prentice Hall出版，于2008年4月发布。这本书旨在提供下载、构建、部署和管理Xen实施所需的所有知识，并在生产环境中实现虚拟化的实际应用。" Xen是一种开源的虚拟化技术，它允许在一个物理主机上运行多个独立的操作系统实例，称为虚拟机（VM）。在Xen的架构中，有两类虚拟机：Dom0（Domain 0）和DomU（Domain U）。 1. Dom0：是Xen环境中的特权虚拟机，具有直接访问硬件的权限。它是Xen hypervisor与用户空间管理工具之间的桥梁，负责管理其他非特权虚拟机（DomU）以及硬件资源。 2. DomU：是非特权虚拟机，它们依赖于Dom0来分配硬件资源，如CPU、内存、磁盘和网络接口。DomUs可以运行不同的操作系统，如Linux或Windows。 3. 安装Xen：安装Xen涉及配置和编译内核，确保选择支持Xen的选项，然后安装Xen hypervisor。同时，还需要为Dom0安装一个特别定制的操作系统，以支持作为虚拟化层的Xen。 4. xm命令：xm是Xen管理工具的一部分，用于控制和管理Xen虚拟机。这些命令包括启动（xm create）、停止（xm shutdown）、列出（xm list）虚拟机，以及调整资源分配等。书中详细介绍了如何使用Xen创建高性能的虚拟机，包括： - **虚拟机的创建与配置**：指导读者如何创建DomU，设置内存、CPU分配，挂载磁盘和网络设备。 - **性能优化**：讨论如何最大化虚拟机的性能，减少虚拟化开销，例如通过使用para-virtualized（半虚拟化）驱动。 - **资源管理**：讲解如何动态调整虚拟机的资源，如内存和CPU份额，以适应不断变化的工作负载需求。 - **网络虚拟化**：介绍桥接、路由和网络地址转换（NAT）在Xen环境中的实现，确保虚拟机间的通信和与外部网络的连接。 - **存储虚拟化**：探讨Xen支持的各种存储后端，如本地硬盘、网络存储（iSCSI、NFS等），以及如何配置存储池。 - **高可用性与灾难恢复**：介绍如何通过Xen实现故障切换和灾难恢复策略，确保业务连续性。 - **安全性**：讨论虚拟化环境中的安全问题，如隔离、防火墙和安全策略的设置。 - **自动化管理**：讲解如何使用脚本和自动化工具来简化大规模Xen集群的管理。这本书不仅适合初学者，也对有经验的系统管理员提供了深入的见解和实践建议，无论是在数据中心、云计算环境还是个人服务器中，都能帮助读者有效地利用Xen虚拟化技术。

Virtual machine monitors also provide a uniform interface to the hardware. This uniform interface shields guest

systems from some lower level details of the physical computing resources and provides portability, which is

another key benefit of virtualization. In fact, many modern hypervisors allow guest systems to move from one

physical machine to another without interruption. Guest system configurations can easily be developed on one

machine and then deployed on many systems. This eases the job of managing or deploying software on a

collection of machines with different hardware characteristics. Guest systems can even migrate from one

physical computer to another while running. Xen calls this live migration. Some benefits of virtualization are as

follows.

Debugging operating systems is time consuming and requires exceptionally skilled programming.

Virtualization can ease the burden by allowing a developer to test new operating systems as a guest on a

more stable host. This technique has been used for many years and has proven effective. Similarly,

security researchers can create guest operating systems that are isolated from one another as well as the

host. Such guests allow researchers to study the effects of worms, Trojans, and viruses, without affecting

the host system. These isolated guests are colloquially referred to as being "sandboxed." Sandboxed

guests can also be used to test updates or cutting-edge software before applying them to production

systems.

Another benefit of virtualization is the ability to recover quickly from software problems caused by

deliberate malicious attacks or accidental malfunctions. By maintaining a copy of a stable guest image,

recovering from an attack can be as simple as rolling back to this trusted saved point.

Virtualization can provide higher availability by relocating guests when a server machine is inoperable.

Server environments can have many physical machines, each running a number of guest systems. Guest

systems can be moved seamlessly between physical machines to balance the load dynamically, thus using

the aggregate resources most efficiently. Many enterprise customers have enjoyed these benefits on

exotic hardware platforms for many years. Xen now provides these advantages to a wider audience.

Other benefits of virtualization become especially clear in a server environment. One example is the ability

to consolidate many services on one physical machine while still allowing each service to be administered

independently. In a multihosting environment, a service provider may run guest systems belonging to

many different individuals or businesses on the same physical machine. Each entity could have its own

root or administrative access, make its own choices as to what software to run, and administer its own

virtual guest autonomously without any need to consult or coordinate with the owners of the other guest

systems.

Most of the advantages of virtualization, especially on commodity platforms such as x86, derive from the

abundance of computing power available on a single machine. As the power of the average system has

increased, the amount of computing power that goes underutilized is also increasing—especially with

multiprocessor and multicore systems. Virtualization provides a way to take advantage of this latent

computing power by consolidation onto today's increasingly powerful physical machines.

Hypervisors can be especially useful for developers, because the developers no longer need to restart

physical machines to switch between various operating systems. Multiboot configurations are just not

sufficient for these developers any longer. Developers requiring this functionality are becoming more

common as more applications are made multiplatform.

From a business perspective, virtualization can provide a reduced total cost of ownership (TCO). Hardware

is utilized more fully when multiple operating systems coexist on a single physical machine. Imagine

running just two virtual machines on each server a company owns. This would mean 50 percent of the

hardware would be needed for the same computing infrastructure. Now, we do not mean to imply every

computer should be running simultaneous virtualized guest operating systems, but often many machines

sit practically idle, and these computers are prime candidates for consolidation via virtualization. Training

costs for employees can be decreased when using virtualization because it allows several different training

configurations (operating systems and applications) to coexist on a single platform, thus fewer computers

are needed for training, and reconfiguration is minimized between different training sessions.

In many business environments, users are afforded the advantage of being able to virtualize legacy

operating systems and applications on modern hardware platforms. Typically, migration of these

applications to current architectures is too costly. Even if migration was successful, those applications

would need debugging for many years to be as robust as the original applications. With a virtual machine,

users are free to execute legacy products in a protected virtual environment without the fear of some

rogue legacy application bringing the system to a halt.

The final benefit of virtualization that bears mentioning is decreased power consumption and cooling

infrastructure. Servers running virtualized at higher utilization make more efficient use of power than

many systems functioning at low capacity. Because smaller space is occupied by the computing

infrastructure, there is more room to adequately cool today's very dense and very warm data centers. In

some cases a substantial cost savings for air conditioning can be realized.

Chapter 1. Xen—Background and Virtualization Basics

Xen is a virtual machine monitor (hypervisor) that allows you to use one physical computer to run many virtual

computers—for example, running a production Web server and a test server on the same physical machine or

running Linux and Windows simultaneously. Although not the only virtualization system available, Xen has a

combination of features that make it uniquely well suited for many important applications. Xen runs on

commodity hardware platforms and is open source. Xen is fast, scalable, and provides server-class features

such as live migration. This chapter discusses common uses and types of virtualization, describes the history of

virtualization and origins of Xen, provides a brief overview of the Xen architecture, and compares Xen with other

virtualization systems.

Common Uses and Benefits of Virtualization

Virtual machine monitors provide a convenient way to use the same physical computer hardware for many

different tasks. Operating systems have been doing this for years simply by enabling users to run many

different applications at once, such as Web browsers, database servers, and games. However, without

virtualization, the act of choosing an operating system and system configuration to run on your physical

computer has the unfortunate side effect of closing off many other options. For example, if you run Linux to

develop and test programs in that environment, you may not be able to run programs written exclusively for

Windows. Also, if you run the newest and fully patched version of Windows, reproducing problems experienced

by customers on earlier versions may be difficult. Additionally, if your Web server and database server require

different versions of a system library, they may not be able to run on the same system. Without virtualization in

each of these examples, you would need to maintain many physical machines, each with a special software

configuration, even if the computing resources in one machine are sufficient to run all of your applications at

once.

Virtual machine monitors (hypervisors) are becoming increasingly important in modern computing because they

allow many different operating systems and software configurations to exist on the same physical machine. The

hypervisor controls the underlying hardware, allowing it to be used by many guest systems at once, and gives

each guest system the illusion that it is running on its own private hardware.

The hypervisor abstracts the physical resources of the host computer into discrete virtual counterparts that can

be allocated for use by individual guests. Virtual guests treat their virtual hardware as if it were real, and the

hypervisor ensures that this illusion is seamless. Additionally, hypervisors must ensure some level of isolation

between guests. In a way, hypervisors act as both magician and traffic cop. Figure 1.1 illustrates the

relationship between the physical hardware, the hypervisor, and the guest virtual machines.

Figure 1.1. The hypervisor sits between the guest domains and the physical hardware.

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