Linux Virtual Machines with Xen

Sean Reifschneider

tummy.com, ltd.

What is Xen

• Virtual Machine Monitor
• Multiple virtual machines on one computer
• Similar to VMWare or User Mode Linux or IBM S390
• Very fast
• University of Cambridge

Uses for Xen?

• Useful for building packages for multiple distros
• Crash and burn machines are very easy
• Partitioning machines by application for security
• Hosting of small applications
• Trying out new software or distributions
• Demo machines
• VPN hub machine

Hardware Support

• Supports only x86 32-bit
• 64-bit support is in development
• Can run on SMP machines, but guests only get UP
• Upcoming CPUs from Intel and AMD support enhancements

Unique features

• Balloon driver to scale memory use
• Save/restore virtual machines
• Live migration

Performance

• Worst case: Xen is 8% slower than native
• UML and VMWare are as much as 88% slower
• Many loopback devices are slow
• Up to 128 virtual machines supported

Behind the Scenes

• Boot up Xen hypervisor
• It loads the "xen0" guest
• xen0 has direct access to the hardware
• xen0 can create and control many xenU guests
• xenU guests see partitioned environment

Demo

Setup on FC4

• Packages are in FC4, but I've never had any luck with them
• yum install python-twisted curl bridge-utils
• Download and unpack Xen
• ./install.sh
• mkinitrd with --omit-scsi-modules
• Set up grub.conf:

  title Xen 2.0 / XenLinux 2.6
     kernel /xen-2.0.gz dom0_mem=100000 console=vga
     module /vmlinuz-2.6-xen0 root=/dev/hda2 ro console=tty0
     module /initrd-2.6.11.12-xen0.img

• Reboot into Xen and Xen0

OS Support

• OSs have to be ported to Xen
• Linux 2.4 and 2.6
• NetBSD-current
• FreeBSD 5 (guest only)
• Plan 9 (guest only)
• ReactOS (in development)
• Support for unmodified guests on upcoming hardware

References

• http://www.cl.cam.ac.uk/Research/SRG/netos/xen/
• http://lwn.net/Articles/139964/
• http://www.fedoraproject.org/wiki/FedoraXenQuickstart