Using and Understanding Xen4Centos


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Delivered by Russell Pavlicek at CentOS Dojo, Denver, CO, April 10. 2014.

A basic introduction to Xen4CentOS: What it provides, how to install it, and where it is going.

Published in: Technology

Using and Understanding Xen4Centos

  1. 1. Using and Understanding Xen4CentOS Russell Pavlicek, Xen Project Evangelist A summary of the Xen4CentOS project, use, and status
  2. 2. First thing… A personal word of appreciation to the CentOS team
  3. 3. The Problem • Red Hat made a business decision to focus on KVM • In RHEL 6, they disabled support for the Xen Project Control Domain (aka Dom0) in their kernel • Many Xen Project users on CentOS 5 users want to upgrade to CentOS 6, but don’t want to change hypervisors and tools to accomplish it • So there is a need to reintroduce a Xen Project hypervisor option in CentOS 6
  4. 4. What Xen4CentOS Provides • Xen Project 4.2.x • Linux Kernel 3.10.x ( LTS kernel) • blktap 2.5 userspace tools • • blktap 2.5 drivers in the Centos6 Xen 3.4.x dom0 kernel • libvirt 0.10.x with drivers for both xm/xend and xl/libxl toolstacks
  5. 5. Quicky HOWTO • Start with a minimal CentOS installation • Include the usual Xen Project considerations (LVM, etc.) • Set up the installation repository: • yum install centos-release-xen • Install the software: • yum install xen • Install the grub boot entries: • /usr/bin/
  6. 6. Quicky Checkout • Check /boot/grub/grub.conf for entry like: title CentOS (3.10.34-11.el6.centos.alt.x86_64) root (hd0,0) kernel /xen.gz dom0_mem=1024M,max:1024M loglvl=all guest_loglvl=all module /vmlinuz-3.10.34-11.el6.centos.alt.x86_64 ro root=/dev/mapper/vg_xen01-lv_root rd_LVM_LV=vg_xen01/lv_swap rd_NO_LUKS KEYBOARDTYPE=pc KEYTABLE=uk rd_NO_MD LANG=en_GB rd_LVM_LV=vg_xen01/lv_root SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_NO_DM rhgb quiet module /initramfs-3.10.34-11.el6.centos.alt.x86_64.img • Reboot and do: • uname -r ; xm info
  7. 7. That’s About All It Takes Really… It’s about that simple. It’s not hard. And it’s not rocket science.
  8. 8. Issues and Limitations • SELinux support is disabled • You might need to disable SELinux on the dom0 for some operations; primarily when using qemu-xen and blktap backed storage. • For the time being, only Xen4CentOS on CentOS-6/x86_64 is available. • 32 bit may eventually be added, if there is enough demand • If you need it, speak up and join in!
  9. 9. Issues and Limitations (continued) • Serial Console • Must be setup manually if you intend to re-route the dom0 console as well • libvirt and xl/xm compatibility • Both xl and xm are supported and tested on command line • Currently, libvirt is only completely functional with xm (which needs xend running); native xl support in libvirt is expected in a future release
  10. 10. Issues and Limitations (continued) • Ballooning • A bug in Linux's balloon driver can be worked around with a Xen Project command line option; see CentOS bug 6893 report • No UEFI support • Xen Project hypervisor 4.2 does have UEFI support, but the Linux 3.4 dom0 kernel doesn't have UEFI support yet • Thus, Secure Boot is unsupported
  11. 11. Issues and Limitations (continued) • The Xen4CentOS LTS kernel is not the Red Hat kernel • If you rely on Red Hat-specific modifications to your kernel, you won’t find them here • But the focus of this kernel is to run the Control Domain (Dom0) only; the work normally assigned to Dom0 is primarily control of the hypervisor, which should not require any “special sauce”
  12. 12. References and Resources • Release notes: • • Wiki: • • QuickStart: • • History and Motivations Video, by Karanbir Singh: •
  13. 13. Getting Help • IRC: • #centos-virt irc channel on • Mailing List: • centos-virt list on
  14. 14. The Future • Original intent of updating Xen4CentOS for every other release • Increased supportability using regular CentOS and Xen Project communities
  15. 15. The Future (continued) • The CentOS Virtualization SIG (March 5, 2014) • • “The Virt-SIG aims to deliver a user consumable full stack for virtualization technologies that want to work with the SIG. This includes delivery, deployment, management, update and patch application ( for full lifecycle management ) of the baseline platform when deployed in sync with a technology curated by the Virt-SIG.” • “The CentOS Virt-SIG wants to build upon the already successfully released Xen4CentOS project. Moving forward the Xen4CentOS codebase will be shifted over to the Virt-SIG resources.” • Interest regarding other virtualization technologies like OpenVZ, KVM, etc.
  16. 16. Upcoming Meetings • If you use or hack Xen4CentOS, consider attending or speaking at one of these upcoming meetings: • Xen Project Developer Summit • Co-located with LinuxCon NA, Chicago IL, Aug 18-19 • CFP is open until May 2 • Xen Project Users Summit • New York City, Sept 15 • CFP is open until May 31 • The perfect time for a long weekend in Manhattan! • Check out
  17. 17. Questions? @RCPavlicek