Ravi writes "There is commercial grade software like VMware (which does full virtualization) as opposed to open source approaches like Xen which do partial virtualization. But besides these two, there is a very interesting piece of software called User Mode Linux (UML), which can be used to create virtual Linux machines within a Linux computer. Unique to UML is that support for it has been incorporated into the mainstream Linux kernel tree and it is now a fully supported part of the official Linux kernel. On this note, UML gains a lot of significance. UML has been created and maintained by Jeff Dike, who is known throughout the Linux technical community. And when someone who has created a popular software decides to author a book on the subject, then that book is sure to hold a lot of promise. So when I came across the book titled User Mode Linux, authored by Dike under Bruce Perens' Open Source Series, I just couldn't resist getting my hands on it." Read on for the rest of Ravi's review.