Xen Notes

Background

  • Vanilla installation of Xen v3.0.3 on hypervisor.example.com.
    All defaults.
  • Platform is CentOS 5. Paravirtualization is not supported on CentOS
    6
    .
    It’s possible to make it work,
    but I think you should get a newer processor and run KVM if using
    CentOS 6 to save yourself the trouble.

Glossary

Not meant to be complete.

Term Explanation
Type I Hypervisor Runs directly on hardware. Virtual machines don’t know they’re virtualized.
Type II Hypervisor Hypervisor (Xen) runs in OS (RHEL/CentOS). The virtual machines ‘‘know’’ they’re being run in a virtual environment
HVM (“Hardware Virtual Machine” or “Hardware-assisted Virtualization”) Not entirely sure about this. Certain processor technology (e.g. Intel VT-x) allows “complete simulation of underlying hardware.” VMs don’t know they’re virtualized.
dom0 The hypervisor itself
domU A single virtual instance
xm Xen-provided tool to manage domU’s
virsh A Red Hat-designed shell to manage VM’s. Differs from xm in that it can manage QEMU and HVM-based domU’s as well since it’s based on the libvirt API.
virt-install and virt-manager Management and provisioning tools based on libvirt/

Installation

yum groupinstall Xen  
yum install python-virtinst qemu*

The first installs the Xen-enabled kernel, Xen daemon, virtualization
libraries, etc. Make sure that (a) SELinux is disabled, and (b) that you
reboot into the Xen kernel before doing anything else.

The First VM

Preparing the dom0

  • My VMs will be running CentOS 6. So I
    downloaded and
    loop-mounted the latest CentOS 6 ISO. I then offered the mount via
    HTTP for VM installation.
  • I then created logical volumes for use as storage by the VMs. You
    can also format and use disk
    images
    .

Creating the VM

virt-manager is the easiest way to do things. You can do a
command-line install via virt-install. Here’s a sample command that
creates a 64-bit VM called “devel1” running CentOS 6 with two virtual
CPUs and 1.2GB of RAM. Observe that I explicitly specify the MAC
address.

virt-install \  
--name=devel1 \  
--arch=x86_64 \  
--vcpus=2 --check-cpu \  
--ram=1200 \  
--disk path=/dev/xenspace/devel1 \  
--mac=00:0C:29:1A:98:D5 \  
--os-type=linux \  
--os-variant=rhel6 \  
--location=http://hypervisor.example.com/install/6/x86_64/ \  
--debug \  
--nographics

Once the VM is installed, it’s a good idea to save the kickstart files.
Here’s a sample:

# Modified by Nikhil Anand 
install
url --url http://hypervisor.example.com/install/6/x86_64/
lang en_US.UTF-8
keyboard us
network --device eth0 --bootproto dhcp
rootpw --iscrypted $1$9P2b0WZe$CSd.fBGCVjjUfzlZ6m5Rk1
firewall --enabled --port=22:tcp
authconfig --enableshadow --enablemd5
selinux --enforcing
timezone --utc America/Chicago
bootloader --location=mbr --driveorder=xvda
# The following is the partition information you requested
# Note that any partitions you deleted are not expressed
# here so unless you clear all partitions first, this is
# not guaranteed to work
clearpart --linux --drives=xvda
part /boot --fstype ext3 --size=100 --ondisk=xvda
part pv.6 --size=0 --grow --ondisk=xvda
volgroup VolGroup00 --pesize=32768 pv.6
logvol / --fstype ext3 --name=LogVol00 --vgname=VolGroup00 --size=1024 --grow
logvol swap --fstype swap --name=LogVol01 --vgname=VolGroup00 --size=528 --grow --maxsize=1056

%packages
@base
@core
keyutils
iscsi-initiator-utils
trousers
fipscheck
device-mapper-multipath

If you ever wanted to reinstall the VM, you can now append a flag with
the (HTTP downloadable) path to the kickstart file:

-x "ks=http://hypervisor.example.com/kickstarts/centos-6.ks"

HVM Support

You can find if your processor supports HVM by issuing

egrep '^flags.*(vmx|svm)' /proc/cpuinfo

Network Topologies

Xen offers the following:

  • Bridged
  • NAT-ted
  • Routed

It’s unusual (and crazy) to use all three on a given dom0 instance. The
default is bridged networking. The brctl command is used to manage
network bridges.

In our case, the router hands out DHCP leases depending on MAC
addresses. This is why I didn’t have to do anything other than specify
the MAC address in a domU’s config:

vif = [ "mac=00:50:56:78:0a:1b,bridge=xenbr0,script=vif-bridge" ]

More exotic configurations are possible. You can, for example, specify
two virtual interfaces (vif’s), with public and private IPs. In this
case, the route and iptables commands become important, since you’ll
have to set up routes and masquerading.

Edit /etc/xen/xend-config.sxp to set up these configs. For instance,
if you only had a routed config, you’d comment out every other
network-script and vif-script other than these:

#(network-script network-route)  
#(vif-script     vif-route)

PyGRUB

virt-install removes the kernel and ramdisk lines from a domU’s
config file and adds this instead:

bootloader = "/usr/bin/pygrub"

PyGRUB itself will look for the first partition or LVM container that
contain the kernel and init image
.

I made an error of using the CentOS project-supplied kernel and
ramdisk
, which
were good for an install, but useless when the domU was rebooted.
They’re built specifically for installation :)

“Could not connect to localhost:8000”

You may see this when using virt-install or virt-manager. Edit
/etc/xen/xend-config.sxp and make sure these lines are uncommented:

(xend-http-server yes)  
(xend-port 8000)  
(xend-address localhost)

And restart the Xen daemon.

Logging

You’re supposed to be able to edit /etc/sysconfig/xend, uncomment this
line and see logs in /var/log/xen/console

XENCONSOLED_LOG_DIR=/var/log/xen/console

Didn’t work for me.

Miscellaneous

“Guest name already in use”

virsh undefine <guestname>