Xen虚拟化实战指南：艺术与应用

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"《Xen虚拟的艺术指南》是一本由Jeanna N. Matthews等人编著的专业书籍，由Prentice Hall出版社于2008年出版。本书详细介绍了如何下载、构建、部署和管理Xen虚拟化实施，是Xen社区在生产环境中实现虚拟化的实用指南。" Xen是一种开源的虚拟化技术，它允许在一个物理服务器上运行多个独立的操作系统实例，即虚拟机。这本书《Xen虚拟的艺术指南》深入浅出地提供了关于Xen虚拟化的全面知识，帮助读者掌握如何在各种环境中创建和管理高性能的Xen虚拟机。书中的内容涵盖了以下几个关键知识点： 1. **安装与配置**：介绍如何在不同的操作系统平台上安装Xen，包括必要的硬件需求和软件依赖。书中可能包括了对主机操作系统的选择，如Linux发行版，以及对Xen内核的定制和配置。 2. **虚拟机管理**：详细讲解如何创建、启动、停止和迁移虚拟机。这涉及到虚拟机的配置文件，如Xen配置文件（xen.cfg）的编写，以及如何分配资源如CPU、内存和磁盘空间。 3. **网络虚拟化**：介绍Xen如何实现虚拟网络设备，使虚拟机能够互相通信以及与外部网络连接。可能涉及桥接、NAT和VLAN等网络模型的设置。 4. **存储管理**：讨论Xen支持的不同类型的存储后端，如本地磁盘、网络存储（如iSCSI和NFS）以及块设备驱动。这部分可能会讲解如何配置存储池和虚拟磁盘。 5. **性能优化**：提供关于如何最大化虚拟机性能的策略，包括CPU调度、内存管理和I/O优化。此外，可能还包括针对特定工作负载的调优建议。 6. **高可用性与灾难恢复**：介绍如何构建高可用性集群，以及如何利用Xen进行灾难恢复规划，确保业务连续性。 7. **安全考虑**：讲解Xen虚拟环境中的安全最佳实践，如隔离策略、访问控制和虚拟机加密。 8. **自动化与脚本化**：介绍如何使用脚本和工具自动化Xen管理任务，如使用XenAPI和其他管理工具。 9. **案例研究与实战经验**：通过真实世界的案例分析，展示Xen在实际生产环境中的应用和挑战。 10. **故障排查**：提供故障排除的技巧和方法，帮助读者诊断并解决Xen虚拟化环境中可能出现的问题。《Xen虚拟的艺术指南》不仅适合初学者理解Xen的基本概念，也适合有经验的IT专业人员深入学习和提升他们在虚拟化领域的技能。通过阅读本书，读者可以充分利用Xen虚拟化技术的优势，降低IT成本，提高资源利用率，并简化运维流程。

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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