Chapter 12. Virtualization with KVM

Table of Contents

Virtualization using KVM
Why virtualize
Deployment and uses
Offline operations
Bare metal recovery (snapshot backups)
Integrity validation (offline AIDE scans)



Virtualization using KVM

When possible, we will use virtualization. As virtualization platform, we will choose KVM as it offers many interesting features (both for development as well as larger enterprise usability) and is quite popular in the Linux development (and user) community. Other virtualization techniques that can be used are Xen or Virtualbox. Within Gentoo Linux, KVM is a popular and much-used virtualization technology.

Why virtualize

Virtualization has been around for quite some time. Early mainframe installations already had a sort of isolation that is not far off from virtualization nowadays. However, virtualization is not just a mainframe concept anymore - most larger enterprises are already fully working on the virtualization of their stacks. Products like VMWare have popularized virtualization in the enterprise, and other hypervisors like KVM, VirtualBox, Xen and more are trying to get a piece of the cacke.

To help administrators manage the virtual guests that are running on dozens of hosts, frameworks have emerged that lift some of the management tasks to a higher level. These frameworks offer automated generation of new guests, simplified configuration of the instances, remote management of guests. Some of them even support maintenance activities, such as moving guests from one host to another, or monitor the resource consumption of guests to find a good balance between running guests and available hosts.

In Gentoo, many of these virtualization frameworks are supported.

The first one is app-emulation/libvirt and is RedHat's virtualization management platform. The hypervisor systems run the libvirt daemon which manages the virtual guests as well as storage and other settings, and the administrator remotely connects to the various hypervisor systems through the app-emulation/virt-manager application. It has support for SELinux through its s(ecure)Virt(ualization) feature. To do so, it does require the MCS SELinux policy. Libvirt is also being used by many other frameworks (like oVirt, Archipel, Abiquo and more).

Another one that is gaining momentum is app-emulation/ganeti and is backed by Google. It is foremost a command-line driven method but is well capable of handling dozens and dozens of hypervisor systems. It supports simplified fail-over on top of DRBD and makes an abstraction of the running hosts versus the guests. It bundles a set of hosts (which it calls nodes) in logical groups called clusters. The guests (instances) are then spread across the nodes in the cluster, and the administrator manages the cluster remotely.

Using virtualization has a whole set of advantages of which I'll try to mention a few in the next paragraphs. I will use the term host when talking about the host operating system (the one running or directly managing the hypervisor software) and guest for the virtualized operating system instances (that are running on the host).

High Availability

In a virtualized environment, guests can be quickly restarted when needed. The host is still up and running, so rebooting a system is a matter of restarting a process. This has the advantage that caching and other performance measures taken by the hypervisor are still available, which makes bootup times of guests quite fast.

But even in case of host downtime, given the right architecture, guests can be quickly started up again. By providing a hardware abstraction in the virtualization layer, these technologies are able to start a guest on a different hardware environment (even with different hardware, although there are limits to this - the most obvious one being that the new hardware must support the same architecture and virtualization). Many virtualization solutions can host guest storage (the guests' disks) in image files which can be made highly available through high-performance NFS shares, cluster file system or storage synchronisation. If the host crashes, the guest storage is quickly made available on a different host and the guest is restarted.

Resource optimization

For most organizations, virtualization is one of the most effective ways to optimize resources in their data rooms. Regular Unix/Windows servers generally consume lots of power, cooling and floor space while still only offering 15% (or less) of actual resource consumption (CPU cycles). 85% of the time, the system is literally doing nothing but waiting. With virtualization, you can have resources utilized better, going towards a healthy 80% while still allowing room for sudden resource burst demands.

Flexible change management

The virtualization layer also offers a way to do flexible change management. Because guests are now more tangible, it is easier to make snapshots of entire guests, copy them around, upgrade one instance and, if necessary, roll back to the previous snapshot - all this in just a few minutes or less. You can't do this with dedicated installations.

"Secure" isolation

In a security-sensitive environment, isolation is a very important concept. It ensures that a system or service can only access those resources it needs to, while disallowing (and even hiding) the other resources. Virtualization allows architects to design the system so that it runs in its own operating system, so from the viewpoint of the service, it has access to those resources it needs, but sees no other. On the host layer, the guests can then be properly isolated so they cannot influence each other.

Having separate operating systems is often seen as a thorough implementation of "isolation". Yes, there are a lot of other means to isolate services. Still, running a service in a virtualized operating system is not the summum of isolation. Breaking out of KVM has been done in the past, and will most likely happen again. Other virtualization have seen their share of security vulnerabilities to this level as well.

Simplified backup/restore

For many organizations, a bare-metal backup/restore routine is much more resource hungry than regular file-based backup/restore. By using virtualization, bare-metal backup/restore of the guests is a breeze, as it is now back a matter of working with files (and processes). Ok, the name "bare-metal" might not work anymore here - and you'll still need to backup your hypervisor. But if your hypervisor (host) installation is very standardized, this is much faster and easier than before.

Fast deployment

By leveraging the use of guest images, it is easy to create a single image and then use it as a master template for other instances. Need a web serving cluster? Set up on, replicate and boot. Need a few more during high load? Replicate and boot a few more. It becomes just that easy.


To support the various advantages of virtualization mentioned earlier, we will need to take these into account in the architecture. For instance, high availability requires that the storage, on which the guests are running, is quickly (or even continuously) available on other systems so these can take over (or quickly boot) the guests.

The underlying storage platform we focus on is Ceph, and will be discussed in a later chapter.

Also, we will opt for regular KVM running inside screen sessions. The screen sessions allow us to easily manage KVM monitor commands. The console itself will be launched through VNC sessions. The sessions will by default not be started (as to not consume memory and resources) but can be started through the KVM monitor when needed.

All the other aspects of the hypervisor level are the same as what we will have with our regular operating system design (which is defined further down). This is because at the hypervisor level, we will use Gentoo Linux as well. The flexibility of the operating system allows us to easily manage multiple guests in a secure manner (hence the secure containers displayed in the above picture). We will cover these secure containers later.


Managing the guests entails the following use cases:

  • IMACD operations on the guest

  • Operate a guest (stop, start, restart)

The regular IMACD operations are based on image management. Basic (master) guest images are managed elsewhere, and eventually published on a file server. The hosts, when needed, copy over this master image to the local file system. When a guest needs to be instantiated, it either uses a copy of this file (long-term guest) or a copy-on-write (short-term guest) stored in the proper directory after which it is launched.

Similarly, removal of guests entails shutdown of the guest and removal of the image from the system. All this is easily scriptable.


Monitoring guests will be integrated in the monitoring component later. Basically, we want to be notified if a guest is not running anymore but should (process monitoring), or if a guest is not responsive anymore (ping reply monitoring).


We mentioned secure containers before, so let's describe this bit in more detail.

What we want to accomplish is that the virtual guests cannot interfere with each other. This is both permission-wise (for which we will use SELinux) as well as resources (for which we will use Linux cgroups).

Permissions will be governed through SELinux' categories. The guests all run inside a SELinux domain already, so that vulnerabilities within the Qemu emulator (the user-space part of the hypervisor) cannot influence the host itself. However, if they all run inside the same SELinux domain, then they could influence each other. So what we will do is run each of them within a particular SELinux category, and make sure that their images also have this category assigned.

Resources are governed through Linux cgroups, allowing administrators to put restrictions on various resources such as CPU, I/O, network usage, etc. On a hypervisor level, we want to support guests with differentiated resource requirements:

  • guaranteed resources (so the CPU shares and memory shares are assigned immediately)

  • prioritized resources (so the CPU shares and memory shared are, when needed, preferably assigned to members of this group)

  • shared resources (the rest ;-)

Deployment and uses

Let's make all this a bit more tangible and look at how this would be accomplished.

File system structure

Each host uses the same file system structure (standard) so that you can move images from one system to another without changing your underlying procedures. Let's say the images are stored in /srv/virt using the following structure:

+- pending
+- base
`- guests
   +- hostname1
   |  +- rootfs.img
   |  +- otherfs.img
   |  +- settings.conf
   |  `- computed.conf
   +- hostname2
   `- ...

The pending location is where base image copies are placed first. Images in that directory are still in process of being transferred, or have been interrupted somewhere along. Once a copy is complete, the image will be moved from the pending to the base location. Guests then use copies (of copy-on-write) images from the base images, although this is of course not mandatory - new empty images can be created as well for other purposes. The base images allow you to quickly setup particular systems or prepared file systems for other purposes.

The settings.conf file contains the information related to the guest. This includes MAC address information, guest image information (and options on how to attach them), etc. It is also wise to make this file easily manageable by both humans (so don't use special syntax) and tools (so don't use human language). The other configuration file, computed.conf, contains settings specific for running this guest on this host and is generated and updated by the scripts we use to manage the virtual images.

## Example settings.conf
MEM_DEFAULT=1536 # 1.5 Gbyte of memory
RESOURCES=guaranteed # for cgroup management

## Example computed.conf
VNC_PORT=14 # real port is 5914
MEM=2048 # 2 Gbyte of memory

Interacting with KVM monitors

The KVM monitor (actually Qemu/KVM monitor) allows administrators to manage the virtual guest specifics, such as hot-adding (or removing) devices, taking snapshots, etc. As we placed the KVM monitor on the standard output within screen, all the admin has to do (after logging in) is to attach to the monitor of the right virtual guest.

$ screen -ls
There are screens on:
   4872.kvm-pg1     (Detached)
   5229.kvm-www1    (Detached)
  14920.kvm-www2    (Detached)
$ screen -x kvm-pg1

Offline operations

Bare metal recovery (snapshot backups)

Integrity validation (offline AIDE scans)