Book review: Xen Virtualization

Someone from Packt Publishing asked me if I was interested in reviewing the Xen Virtualization book by Prabhakar Chaganti that they recently published. I said yes and it was in my mailbox a few days letter.

The sub-title is “a fast and practical guide to supporting multiple operating systems with the Xen hypervisor” and it turns out that the operating word is “fast”. It’s a short book (approx 130 pages, many filled with screen captures and console output listings). It is best used as an introduction to Xen for people who understand computer administration (especially Linux) but are new to virtualization.

The book contains a brief overview of virtualization, followed by a description of the most common tasks:

  • the Xen install process (from binary and source) on Fedora core 6
  • creating virtual machines (using NetBSD plus three different flavors of Linux)
  • basic management of Xen using the xm command line or the XenMan and virt-manager tools
  • setting up simple networking
  • setting up simple storage
  • encrypting partitions used by virtual machines
  • simple migration of virtual machines (stopped and live)

For all of these tasks, what we get is a step by step process that corresponds to the simple case and does not cover any troubleshooting. It is likely that anyone who embarks on the task described will need options that are not covered in the book. That’s why I write that it is an introduction that shows the kind of thing you need to do, rather than a reference that will give you the information you need in your deployment. You’ll probably need to read additional documentation, but the book will give you an idea of what stage you are in the process and what comes next.

Even with this limited scope, it is pretty light on explanations. It’s mostly a set of commands followed by a display of the result. Since it’s closer to my background I’ll take the “managing Xen” chapter as an example. There is nothing more basic to management than understanding the state of a resource. The book shows how to retrieve it (“xm list”) and very briefly describes the different states (“running”, “blocked”, “paused”, “shutdown”, “crashed”) but you would expect a bit more precision and details. For example, “blocked” is supposed to correspond to “waiting for an external event” but what does “external” mean? Sure the machine could be waiting on I/O, but it could also be on a timer (e.g. “sleep(1000)”) or simply have run out of things to do. I don’t think of a cron job as an “external event”. Also, when running “xm list” you should expect to always see dom0 in the “running” state (since dom0 is busy running your xm command) and on a one-core single-CPU machine (as is the case in the book) that means that none of the other domains can be in that state. That’s the kind of clarification (obvious in retrospect) that goes one step beyond the basic command description and saves some head scratching but the book doesn’t really go there. As another example, We are told in the “encryption” section that LUKS helps prevent “low entropy-attacks” but if you’re the kind of person who already knows what that means you probably don’t have much to learn from the “encryption” chapter of the book. In case you care, it is a class of attacks that take advantage of poor sources of random numbers and you can read all the details of how entropy is defined in this classic 1948 paper (it doesn’t have much to do with how the term is defined in physics).

Among the many more advanced topics that are not covered I can think of: advanced networking, clustering, advanced storage, Windows guests (even though it’s not Xen’s strong point), migration between physical and virtual, relationship to other IT management tasks (e.g. server and OS management), performance aspects, partitioning I/O so domains play well together, security considerations (beyond simply encrypting the file system), new challenges introduced by virtualization…

Xen documentation on the web is pretty poor at this point and the book provides more than most simple “how-to” guides on installing/configuring Xen that you can Google for. And it brings a consistent sequence of such “how-to” guides together in one package. If that’s worth it to you then get the book. But don’t expect this to cover all your documentation needs for anything beyond the simplest (and luckiest) deployment. I would be pleased to see the book on the desk of an IT manager in a shop that is considering using virtualization, I would be scared to see it on the desk of an IT administrator in a shop that is actually using Xen.

[UPDATED on 2008/02/01: Dan Magenheimer, a Xen expert who works on the Oracle VM, highly recommends another Xen book that just came out: Professional Xen Virtualization by William von Hagen. I haven’t seen that book but I trust Dan on this topic.]

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