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Keywords: Virtualisation, virtual machines, Linux Title: User Mode Linux Author: Jeff Dike Publisher: Prentice Hall PTR ISBN: 0131865056 Media: Book Level: Intermediate/Advanced Verdict: Recommended

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Virtualization (or virtualisation depending on which side of the Atlantic you're on), is big news these days, hardly a day goes by without some announcement of a new product, service or improved hardware or operating system support. While VMware still leads the way, there are plenty of other well known contenders in the market, such as Microsoft's Virtual Server/PC, Sun's Solaris Containers and Xen for the Linux world. However, Xen isn't the only virtualization product available for Linux, a lesser known option is User Mode Linux (UML - no relation to the Unified Modelling Language).

Written by Jeff Dike, founder and lead developer of UML, this is an authoritative and useful resource for anyone interested in using the software in anger. It describes both the genesis of the software - and it's instructive for those who are interested in how such projects grow and develop over time - and also a technical reference that describes what it's used for and how to get it up and running properly.

Unlike VMware and other virtual machine technologies, UML does not create a separate machine image under which 'guest' operating systems can be installed. Instead UML takes the approach that Linux is installed and run as a user process under Linux. From this simple starting point UML has progressed to the point where it can be used to run multiple copies of Linux concurrently on a single machine. Each Linux system operates as though it's a standalone system, unaware that it's just another Linux process as far as the 'real' operating system is concerned.

Once installed, a UML can set up networking both with the main operating system and with the broader network. Virtual file systems can be installed, what is a physical file on the hard disk appears as a complete file system from within a UML. In short, a UML is a real live Linux system.

The book covers both installation and configuration of a UML, going in to considerable detail. Core functionality, such as file system management, networking, setting up UML servers (both large and small), all get complete chapters in addition to being introduced in the opening couple of chapters. The book also describes a number of key usage scenarios, from development and test environments to systems used in educational settings to using UML to test upgrades and installations before letting them go live.

The book is well written, and is well pitched at the experienced Linux sysadmin, super-user or developer. It's decidedly not a book for someone who is new to Linux, or who has only ever used a Linux desktop such as GNOME or KDE.

If there's a criticism it's not of the book, which is essential reading for anyone seriously interesting in User Mode Linux, but of the title for the book and the software. UML is so firmly associated with the modelling language that trying to use it for something else in IT is bound to cause confusion. However, whatever it's called, there's no doubt that it's a useful approach to virtualization, and this book is highly recommended.