计算机专业时言语选读之四

Virtual Servers(2)

Virtualized servers do all the good and bad things regular servers do. They boot up, power down, suspend, hang, and even crash. If a guest OS or a device driver it uses is buggy, the virtual PC will crater. But not the physical computer, and that's key.

If your OS crashes or an application hangs, or even if you install a software fix that requires a reboot, nothing happens to the hardware. One virtual machine can fail over to another in a purely virtual sense or in a way that's closer to the real thing. Even if certain hardware devices have malfunctioned, so long as the fail-over target is configured to use a secondary network adapter and an alternate path to storage, the fail-over will work exactly as it would if the virtual PCs were physical PCs.

In most cases, an enterprise management system will monitor and react to a virtual fail-over as if it were the real thing. Solutions such as HP OpenView see and interact with virtual servers the same way they do with physical ones. The reported configurations of the servers will change after they're virtualized, but it's entirely likely that the day-to-day management of your shop will experience little change.

In addition, most virtualization systems bundle solution-specific management software, allowing an administrator to sit at a central console and manipulate all the virtual servers in an enterprise. It's quite an eye-opener to swap out a virtual Ethernet card without ever touching the hardware.

A virtualization solution's management console gives you a degree of control over your virtual PCs that surpasses what administrators can do with traditional tools. From a central location, you can boot and shut down virtual PCs as needed. It's also possible to pause them, which harmlessly freezes them in their current state, or hibernate them, putting them in a deep freeze by saving their state to a file on disk. By overwriting the disk file, you can restore PCs from a backed-up state and roll back changes that rendered the guest inoperable, all from a terminal session.

In environments with a mix of operating systems——a common condition that turns even simple consolidation into a messy affair—— one solution would be to host each OS in its own VM. For example, on a PC server running one of VMware's virtualization solutions, you can run any combination of Windows 2003 Server, Windows 2000, Windows NT 4.0, various flavors of Linux, and FreeBSD. You can even use VMs to host different versions of the same OS. Linux software is infamous for dependence on specific versions and vendor distributions of Linux. Virtualization is the only way to run applications designed for Red Hat 7.2 and Suse 9.0 simultaneously on a single server.

Virtualization is magnificent stuff, but it doesn't cure all ills. You can never create a virtual PC that outperforms the physical system underneath. You will learn much about your applications' system requirements from moving them to a virtual environment. They'll likely surprise you, either with how little of the original server they used——that's the typical case——or how piggish they are. If necessary, you can throttle the nasty ones down.

And while one of the great benefits of virtualization is security——it's hard to accomplish much by cracking a system that doesn't exist——a virtualized PC can still be compromised. Fortunately, the cure is to overwrite the virtual PC's disk image with one that's known to be clean, but managing virtual servers still requires vigilance.

Ultimately, hardware consolidation is only one reason to opt for server virtualization, and it has wide appeal. Still, depending on each department's unique needs, IT managers are sure to find innumerable ways that virtualization can benefit your enterprise. (The End)

时文选读

虚拟服务器 (2)

虚拟化的服务器会做正常服务器会做的所有好事和坏事。它们会自举、宕机、挂起、暂停、甚至崩溃。如果它的客 OS或者设备驱动程序有错误，虚拟PC机将崩溃。而物理的计算机不会那样，这是关键。

如果 OS崩溃或者应用程序暂停，甚至如果你安装需要重新引导的软件补丁，对硬件不会发生任何影响。一台虚拟机能以完全虚拟的方式或者以更近似于真实的方式，将故障转移到另一台虚拟机。即便某个硬件装置有故障，只要配置好故障转移的目标，能使用第二网卡和到存储设备的替代通道，故障转移就能正确地工作，如同虚拟机就是一台物理PC机那样地工作。

在多数情况下，企业管理系统将对虚拟的故障转移进行监视和做出反应，好像就是真实的东西。诸如 HP OpenView一类的解决方案对虚拟服务器做出的反应和对物理服务器做出的反应是一样的。在虚拟化之后，报告的服务器配置将改变，但是整体上，数据中心日常管理几乎没有什么变化。

此外，多数虚拟化系统与特定的解决方案管理软件是捆绑在一起的，允许管理员坐在中央控制台，管理整个企业的所有虚拟服务器。在不用碰硬件的情况下，就能置换以太网卡，真让人大开眼界。

虚拟化解决方案的管理控制台让你一定程度上控制虚拟 PC机，这种控制程度超过了用传统工具能做到的程度。从一个集中点，就能按需要引导和关闭虚拟PC机。也能暂停它们，无害地将它们冻结在当前的状态，或者使它们休眠，通过将它们的状态保存到磁盘上的一个文件中，将它们深度冻结。通过重写该磁盘文件，就能将PC机从备份状态中恢复过来，从最后一次会话开始将使之不工作的改变全部回滚。

在多种操作系统混用的环境下(这是一种常见的情况，会把简单的服务器集中弄得乱七八糟)，一种解决办法就是将每个 OS在各自虚拟机上运行。例如，在一台运行VMware虚拟方案的PC服务器上，你能运行Windows 2003 Server、 Windows 2000、 Windows NT 4.0、各种版本的Linux以及FreeBSD的各种组合。甚至能运行同一OS的不同版本。Linux因对特定版本和厂商有依赖性而搞得名声狼藉。虚拟化是惟一的方法，让一台服务器上同时运行为Red Hat 7.2 和Suse 9.0设计的应用程序。

虚拟化是个好东西，但它不能包治百病。你永远不能建立一个虚拟 PC机，其性能超过它所在的物理系统。将应用程序移到虚拟环境，你能从中学到很多应用程序的系统要求。它们可能让你吃惊，不是它们只用了原来服务器非常少的一部分(这是一种很典型的情况)，就是它们是多么的贪婪。如果有必要，你就能关闭那些讨厌的程序。

虚拟化最大的好处之一是安全(破坏实际不存在的系统是难以实现的)，同时虚拟化的 PC机还能做出妥协。办法是把虚拟PC机的磁盘映像重写到已知是干净的磁盘上，但是管理虚拟服务器仍需要警惕。

归根结底，硬件集中不是选择服务器虚拟化的惟一理由，它还有很广泛的需求。依据每个部门不同的要求， IT 管理人员一定能找到数不清的方法，让虚拟化为企业服务。