Multiple Personalities With The Linux Kernel

Virtualization is all the rage these days. Taking a single computer system and installing multiple “guest” operating systems on it. The benefits are a reduced footprint and better utilization of existing resources. There is a danger, however, in having many systems dependent on a single piece of hardware. The solution, of course, is to use multiple pieces of hardware and allow your “guests” to be moved between the individual hardware units, thus making the system more resilient to failure.

I’ve started playing a bit with virtualization, specifically, KVM virtualization. For my purposes, I’m using CentOS 6.x on a 64-bit capable system.

The hypervisor itself is a standard CentOS base install with the addition of kvm and various management packages. I installed the hypervisor on a RAID1 LVM, allowing me some room to grow if necessary, and reserving the remainder of the hard drive for virtual hosts. While you can use binary blobs for virtual disk, I prefer using a raided LVM which gives me the ability to grow the disk if necessary as well as minor bumps in speed.

Using yum, adding KVM to an existing installation is a pretty straightforward process :

yum install virt-manager libvirt libvirt-python python-virtinst

That should take care of any dependencies required to get KVM virtualization up and running.

Next up, we need to tackle networking. There are many, many different configurations, far too many to go through here. So, I’m going to keep it simple. For my purposes, I need a single connection to the outside network, all in the same VLAN, as well as a local NAT for some VMs that I need local access to, but that don’t need to be accessed via the Internet.

Setting this up is brilliantly simple. First, copy the /etc/sysconfig/network-scripts/ifcfg-eth0 file to /etc/sysconfig/network-scripts/ifcfg-br0. Next, edit the ifcfg-eth0 file. You’ll need to remove a bunch of lines and add a BRIDGE line as follows :





Next, edit the ifcfg-br0 file. All you really need to do here is change the DEVICE= line to reflect br0. I also recommend disabling NM_CONTROLLED … NetworkManager shouldn’t be installed anyway since you used a base install, but better safe than sorry. In the end, the ifcfg-br0 file should look something like this :









Restart networking and you’ll be all set. The NAT portion of this is handled by KVM itself, so there’s nothing to do there. And networking should be all ready to go.

Without guests, however, all you have is a basic Linux system with a few extra packages taking up space. The real magic starts when you create and install your first VM. My recommendation is to start with creating a template system you can clone later rather than hand-installing every single VM. To install the template, first decide on the base disk size. I’m using 15 GB volumes which is more than enough for the base install and leaves room for most basic server configurations. If you need more space, you can attach additional disks later.

I’m not going to go into how I set up LVM, there are plenty of tutorials out there. For the purposes of this article, I have a volume group names vg_libvirt where I plan to store all of the virtual machines. So first we create the disk necessary for the template :

lvcreate -L15G -n template_base vg_libvirt

Next we install the OS. virt-install is essentially a wrapper script that sets all the necessary values within KVM to get you going. After the settings are configured and the VM is started, girt-installer will automatically attach you to the VM console. The full command I used to install is as follows :

virt-install –accelerate –hvm –connect qemu:///system –network bridge:bra –name template –ram 512 –disk=/dev/mapper/vg_libvirt-template_base –vcpus=1 –check-cpu –nographics –extra-args=”console=ttyS0 text” –location=/tmp/CentOS-6.2-x86_64-bin-DVD1.iso

Since this is effectively a text install, you do run into a bit of a problem. Namely, you can’t configure the drives the way you want. There is a way around this, though it takes a bit of work. Of course, since you’re creating a template, the little bit of work now is easily made up for later. So, here’s how I handled the drive configuration.

First, run through a basic install using the above install method. Once you’re up and running, log into the new VM and head to the root home directory. In that directory you’ll find a kickstart file called anaconda-ks.cfg. Make a local copy of that file and shut down the VM.

The kickstart file gives you the basic parameters that CentOS used to configure the system. You can edit this file and use it yourself to automatically install and configure systems. For our purposes, we’re interested in editing the drive configuration and then using the kickstart file to create the template. So, edit the file and set the parameters as you see fit. An example is as follows :

# Kickstart file automatically generated by anaconda.




lang en_US.UTF-8

keyboard us

network –onboot no –device eth0 –noipv4 –noipv6

rootpw –iscrypted somerandomstringthatiwontrevealtoyoubutnicetry

firewall –service=ssh

authconfig –enableshadow –passalgo=sha512

selinux –enforcing

timezone –utc America/New_York

bootloader –location=mbr –driveorder=vda –append=” console=ttyS0 crashkernel=auto”

# 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 –all –drives=vda

part /boot –fstype=ext4 –size=500

part swap –size=2048

part pv.253002 –grow –size=1

volgroup VolGroup –pesize=4096 pv.253002

logvol / –fstype=ext4 –name=lv_root –vgname=VolGroup –size=4096

logvol /tmp –fstype=ext4 –name=lv_tmp –vgname=VolGroup –size=2048

logvol /var –fstype=ext4 –name=lv_var –vgname=VolGroup –size=4096

logvol /home –fstype=ext4 –name=lv_home –vgname=VolGroup –size=2048

#repo –name=”CentOS” –baseurl=cdrom:sr0 –cost=100

%packages –nobase



Once you have this, you can re-run the girt-install command from above with a slight tweak to make the install use the kickstart file you created (I named it kick1.ks) :

virt-install –accelerate –hvm –connect qemu:///system –network bridge:bra –name template –ram 512 –disk=/dev/mapper/vg_libvirt-template_base –vcpus=1 –check-cpu –nographics –initrd-inject=/path/to/kick1.ks –extra-args=”ks=file:/kick1.ks console=ttyS0 text” –location=/tmp/CentOS-6.2-x86_64-bin-DVD1.iso

This will nuke the existing VM and replace it with one configured with the drive partitions as set in the kickstart file. And now you almost have a template.

You could use this new VM as a clone, but if you’ve set an IP on it, you’ll run into duplicate IP problems. SSH keys on the machine will be cloned, making all of your systems contain the same keys. And other machine-specific settings will be cloned as well. This can be worked around, though.

I recommend that you first configure this new template with the basic settings you want on all of your VMs. For instance, if you’re using Spacewalk for server management, you can install all of the necessary spacewalk binaries. You can configure a standard iptables template for the system. Maybe you have some standard security software you use such as OSSEC. And, of course, create the standard users on the system so you don’t have to create them each time you clone the VM. Once everything is installed and running how you want it, perform the following actions to make the template :

touch /.unconfigured

rm -rf /etc/ssh/ssh_host_*


The VM will power down and you’ll have your template. Cloning this to a new VM is quick and simple. First, create the new logical volume as we did before. Next, clone the VM to the new drive :

virt-clone -o template -n new_vm -f /dev/mapper/vg_libvirt-new_vm_base

Simple enough, right? Run this command and when it completed, you can start the VM and connect to the console. You’ll be greeted with the standard first boot process and then dropped at a login prompt. Congratulations, you now have a VM. Set the IP, configure whatever services you need, and you’re off to the races.

If you need to modify the RAM, number of CPUs, etc., then use the virsh command on the hypervisor. You’ll need to shut down the VM and restart it in order for these changes to take effect.

And that’s really all there is to it. The VMs themselves can be treated as self-contained systems with no special care necessary … One note, however. If you reboot the hypervisor, the VMs are paused before rebooting and resumed after reboot. This leads to an interesting problem in that the uptime on a VM can easily exceed that of the hypervisor. Be aware of this and don’t depend on a VMs uptime to be accurate.

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