Hackery

Basic KVM on CentOS 5

Fri 24 Oct 2008

Tags: linux, centos, kvm, virtualisation

I've been using kvm for my virtualisation needs lately, instead of xen, and finding it great. Disadvantages are that it requires hardware virtualisation support, and so only works on newer Intel/AMD CPUs. Advantages are that it's baked into recent linux kernels, and so more or less Just Works out of the box, no magic kernels required.

There are some pretty useful resources covering this stuff out on the web - the following sites are particularly useful:

There's not too much specific to CentOS though, so here's the recipe I've been using for CentOS 5:

# Confirm your CPU has virtualisation support
egrep 'vmx|svm' /proc/cpuinfo

# Install the kvm and qemu packages you need
# From the CentOS Extras repository (older):
yum install --enablerepo=extras kvm kmod-kvm qemu
# OR from my repository (for most recent kernels only):
ARCH=$(uname -i)
OF_MREPO=http://www.openfusion.com.au/mrepo/centos5-$ARCH/RPMS.of/
rpm -Uvh $OF_MREPO/openfusion-release-0.3-1.of.noarch.rpm
yum install kvm kmod-kvm qemu

# Install the appropriate kernel module - either:
modprobe kvm-intel
# OR:
modprobe kvm-amd
lsmod | grep kvm

# Check the kvm device exists
ls -l /dev/kvm

# I like to run my virtual machines as a 'kvm' user, not as root
chgrp kvm /dev/kvm
chmod 660 /dev/kvm
ls -l /dev/kvm
useradd -r -g kvm kvm

# Create a disk image to use
cd /data/images
IMAGE=centos5x.img
# Note that the specified size is a maximum - the image only uses what it needs
qemu-img create -f qcow2 $IMAGE 10G
chown kvm $IMAGE

# Boot an install ISO on your image and do the install
MEM=1024
ISO=/path/to/CentOS-5.2-x86_64-bin-DVD.iso
# ISO=/path/to/WinXP.iso
qemu-kvm -hda $IMAGE -m ${MEM:-512} -cdrom $ISO -boot d
# I usually just do a minimal install with std defaults and dhcp, and configure later

# After your install has completed restart without the -boot parameter
# This should have outgoing networking working, but pings don't work (!)
qemu-kvm -hda $IMAGE -m ${MEM:-512} &

That should be sufficient to get you up and running with basic outgoing networking (for instance as a test desktop instance). In qemu terms this is using 'user mode' networking which is easy but slow, so if you want better performance, or if you want to allow incoming connections (e.g. as a server) you need some extra magic, which I'll cover in a "subsequent post":kvm_bridging.