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2 The Book of Xen Table of Contents FOREWORD ACKNOWLEDGMENTS INTRODUCTION Virtualization: A Brief History So What's Xen Again? (And Why Should I Use It?) Xen's Limitations So, Why Should I Use Xen? Overview of the Book But I Am Impatient! 1. XEN: A HIGH-LEVEL OVERVIEW Virtualization Principles Virtualization Techniques: Full Virtualization Virtualization Techniques: OS Virtualization Paravirtualization: Xen's Approach How Paravirtualization Works Xen's Underpinnings: The Gory Details Scheduling Interrupts Memory I/O Devices Networking Block Devices Putting It Together 2. GETTING STARTED Hardware Compatibility Installing CentOS Getting Familiar with Your Xen System Management with xm Making a DomU Anatomy of a Domain Configuration File Configuring the DomU You're Finished. Have a Cookie. 3. PROVISIONING DOMUS A Basic DomU Configuration Selecting a Kernel Quick-and-Dirty Install via tar Why This Is Not the Best Idea Stuff to Watch Out For Using the Package Management System with an Alternate Root Red Hat, CentOS, and Other RPM-Based Distros Debootstrap with Debian and Ubuntu 2

3 QEMU Install virt-install Red Hat's One-Step DomU Installer Converting VMware Disk Images Mass Deployment Manual Deployment QEMU and Your Existing Infrastructure Installing pypxeboot Automated Installs the Red Hat Way And Then 4. STORAGE WITH XEN Storage: The Basics Varying Types of Storage Basic Setup: Files LVM: Device-Independent Physical Devices Enlarge Your Disk File-Backed Images LVM Copy-on-Write and Snapshots LVM and Snapshots Storage and Migration Network Storage Closing Suggestions 5. NETWORKING Xen's Network Setup Process Defining Virtual Interfaces Naming Virtual Interfaces Autoconfiguration Specifying a MAC Address Manipulating vifs with xm Securing Xen's Virtual Network Specifying an IP Address Firewalling the Dom0 Networking with network-route Networking with network-bridge Networking with network-nat Configuration Variables Custom Network Scripts Multiple-Bridge Setups Bridged and Routing Further Thoughts 6. DOMU MANAGEMENT: TOOLS AND FRONTENDS Tools for the VM Provider Xen-tools libvirt, virsh, and virt-manager Administering the Virtualized Data Center Administration for the VM Customer 3

4 Xen-shell 7. HOSTING UNTRUSTED USERS UNDER XEN: LESSONS FROM THE TRENCHES Advantages for the Users Shared Resources and Protecting Them from the Users Tuning CPU Usage Scheduling for Providers Controlling Network Resources Storage in a Shared Hosting Environment Regulating Disk Access with ionice Backing Up DomUs Remote Access to the DomU An Emulated Serial Console A Menu for the Users PyGRUB, a Bootloader for DomUs Making PyGRUB Work Wrap-Up 8. BEYOND LINUX: USING XEN WITH OTHER UNIX-LIKE OSS Solaris Getting Started with Solaris Solaris Dom0 Setting Up Xen Solaris SMF Creating a Solaris DomU Creating a Solaris DomU Under Linux OpenSolaris DomU Postinstall Configuration NetBSD NetBSD's Historical Xen Support Installing NetBSD as a Dom0 Installing NetBSD as a DomU Beyond Paravirtualization: HVM 9. XEN MIGRATION Migration for Troglodytes Migration with xm save and xm restore Cold Migration Live Migration How It Works Making Xen Migration Work Migrating Storage ATA over Ethernet iscsi Quo Peregrinatur Grex 10. PROFILING AND BENCHMARKING UNDER XEN A Benchmarking Overview UnixBench Analyzing Network Performance 4

5 Measuring Disk Performance with Bonnie++ Application Benchmarks httperf: A Load Generator for HTTP Servers Another Application Benchmark: POV-Ray Tuning Xen for Optimum Benchmarking Profiling with Xen Xenoprof Getting OProfile Profiling Multiple Domains in Concert An OProfile Example Conclusion 11. CITRIX XENSERVER: XEN FOR THE ENTERPRISE Citrix's Xen Products The Benefits of Using Citrix XenServer Ten Minutes to Xen The Disadvantages of Using Citrix XenServer Getting Started Prerequisites Installing Citrix XenServer Citrix's Xen GUI: XenCenter Administering VMs with the XenCenter Installing DomU Images Installing from the Debian Templates Templated Linux VM Windows Install Creating DomU Images with P2V Converting Pre-existing Virtual or Physical Machines with XenConvert XenServer Tools in the DomU xe: Citrix XenServer's Command-Line Tool XenServer's Disk Management Xen Storage Repositories Emulated CD-ROM Access XenServer VM Templates XenServer Resource Pools Citrix XenServer: A Brief Review 12. HVM: BEYOND PARAVIRTUALIZATION Principles of HVM Device Access with HVM HVM Device Performance HVM and SMP HVM and Migration Xen HVM vs. KVM Working with HVM Creating an HVM Domain Interacting with an HVM Domain Getting the Standard Xen Console to Work 5

6 HVM Devices Paravirtualized Drivers And, for Our Next Trick 13. XEN AND WINDOWS Why Run Windows Under Xen? Windows on Xen: Prerequisites Windows on Xen: Installation Installing Windows Manually A Discussion of HALs Installing Windows the Red Hat Way Windows with the Virtual Framebuffer Et Voilà! Windows Activation Graphics Cards Paravirtualized Drivers for Windows Proprietary Windows PVM Drivers GPL Windows Paravirtualized Drivers Ongoing Development 14. TIPS Compiling Xen Compile-Time Tuning Alternate Kernels (Dom0 and DomU) paravirt_ops Dom0 paravirt_ops DomU The Xen API: The Way of the Future Managing Memory with the Balloon Driver PCI Forwarding GRUB Configuration The Serial Console Enabling Serial Output The Xen Hypervisor Console Xen and LILO The Virtual Framebuffer Use of the XenStore for Fun and Profit Automatically Connecting to the VNC Console on Domain Boot 15. TROUBLESHOOTING Troubleshooting Phase 1: Error Messages Errors at Dom0 Boot DomU Preboot Errors Creating Domains in Low-Memory Conditions Configuring Devices in the DomU Troubleshooting Disks VM Restarting Too Fast Troubleshooting Xen's Networking The DomU Interface Number Increments with Every Reboot iptables 6

7 Memory Issues Other Messages A Constant Stream of 4GiB seg fixup Messages The Importance of Disk Drivers (initrd Problems) XenStore Xen's Logs dmesg and xm dmesg Logs and What Xen Writes to Them Applying the Debugger Xen's Backend Architecture: Making Sense of the Debug Information Domain Stays in Blocked State Debugging Hotplug strace Python Path Issues Mysterious Lockups Kernel Parameters: A Safe Mode Getting Help Mailing Lists The Xen Wiki The Xen IRC Channel Bugzilla Your Distro Vendor xen-bugtool Some Last Words of Encouragement A. XM REFERENCE xm's Syntax xm Subcommands B. THE STRUCTURE OF THE XEN CONFIG FILE List of Directives HVM Directives Device Model Options 7

8 The Book of Xen Copyright 2009 Chris Takemura Luke S. Crawford For information on book distributors or translations, please contact No Starch Press, Inc. directly: No Starch Press, Inc. 555 De Haro Street, Suite 250, San Francisco, CA phone: ; fax: ; Library of Congress Cataloging-in-Publication Data Takemura, Chris, The book of Xen : a practical guide for the system administrator / Chris Takemura and Luke S. Crawford. p. cm. Includes index. ISBN-13: ISBN-10: Xen (Electronic resource) 2. Virtual computer systems. 3. Computer organization. 4.Parallel processing (Electronic computers) I. Crawford, Luke S. (Luke Seidel), II. Title. QA76.9.V5C '.35--dc No Starch Press and the No Starch Press logo are registered trademarks of No Starch Press, Inc. "The Book of " is a trademark of No Starch Press, Inc. Other product and company names mentioned herein may be the trademarks of their respective owners. Rather than use a trademark symbol with every occurrence of a trademarked name, we are using the names only in an editorial fashion and to the benefit of the trademark owner, with no intention of infringement of the trademark. The information in this book is distributed on an "As Is" basis, without warranty. While every precaution has been taken in the preparation of this work, neither the authors nor No Starch Press, Inc. shall have any liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the information contained in it. No Starch Press 8

9 Dedication This one's for my dad, who first introduced me to text-based adventure games on a Z80-based Heathkit computer, and whose fault, therefore, all of this is. Chris Takemura To Sarah. I got what I wanted. Luke S. Crawford 9

10 FOREWORD Virtualization is cool. I've always had a soft spot for virtualization, since as a lifelong sysadmin I get pretty tired of the endless fine-tuning that goes into building a successful network "host." Especially when that fine-tuning evolves into upgrades involving screwdrivers, recabling, and dust. While Xen wasn't the first serious virtualization platform, it was the first serious open source virtualization platform, so it was the first that I was willing to invest my time in learning about, and the first I'd consider basing any production-level systems on. Open source isn't just a preference for me I dislike lock-in, so I hardly ever buy or deploy or depend on something that I couldn't replace with a different product offered by a competing vendor sometime in the future. Like any serious open source system, Xen has the power of an ecosystem in which anybody who wants to vend can pick a spot and start hacking, but Xen also has the backing of a strong company whose employees contribute to the open source version of their product. This kind of vertical openness makes it possible for anyone (a hobbyist or a Fortune 500 company) to jump into Xen, buy only what they want or need (or just get it all for free), and have it remain compatible with the rest of the ecosystem. Thank you, XenSource and Citrix, for all this. Confession time: I don't use Xen for any of my personal projects. I just don't have enough systems in any one location, nor can I plan far enough in advance I'm too small to be able to afford virtualization's efficiencies. Whenever I do need separation of privilege on the same physical servers, I've been able to get away with FreeBSD jails or User Mode Linux. I also do a fair amount of real-time work, in which I need my code to be close to the hardware for design and sometimes performance reasons. For professional work, my company uses a mixture of proprietary (VMware) and open source (Xen) virtualization and the results are outstanding. Whether it's to save money on hardware, save money on sysadmin time, or enable new kinds of computing, virtualization is a winner and it's here to stay. I've seen Amazon and Google build gigantic clouds of virtualized servers for their own use and for rental to customers, and this method has driven down IT costs for both new and established companies of all sizes. It probably saves power and lowers the industry's carbon footprint as well. I'm struggling to find a way to communicate how amazingly cool this is. We try to write programs that fit into a single process, but they end up taking a whole Unix system because of all the processes and databases and shell scripts and file systems and UIDs they slop over. So we end up dedicating physical servers to applications that have no performance- or security-related reason to be on dedicated servers; but each one takes up some rack space and some sysadmin time, and each one generates some minimum amount of heat, and so on. Then along comes virtualization, and we're back to adding physical servers only when we've got a good reason to do so, which usually means for capacity reasons. 10

INDEX. Numbers. Page numbers in italics refer to figures. 4GiB seg fixup messages, layer OSI model, interaction with, 65

INDEX. Numbers. Page numbers in italics refer to figures. 4GiB seg fixup messages, layer OSI model, interaction with, 65 INDEX Page numbers in italics refer to figures. Numbers 4GiB seg fixup messages, 230 7-layer OSI model, interaction with, 65 A addlabel subcommand, 244 addresses, physical and pseudophysical, 9 address