2 - Raspberry PI Community Projects

12.2. Virtualization
Virtualization is one of the most major advances in the recent years of computing. The term
covers various abstractions and techniques simulating virtual computers with a variable degree
of independence on the actual hardware. One physical server can then host several systems
working at the same time and in isolation. Applications are many, and often derive from this
isolation: test environments with varying configurations for instance, or separation of hosted
services across different virtual machines for security.
There are multiple virtualization solutions, each with its own pros and cons. This book will
focus on Xen, LXC, and KVM, but other noteworthy implementations include the following:
• QEMU is a software emulator for a full computer; performances are far from the speed
one could achieve running natively, but this allows running unmodified or experimental
operating systems on the emulated hardware. It also allows emulating a different hardware
architecture: for instance, an i386 system can emulate an arm computer. QEMU is
free software.
➨ http://www.qemu.org/
• Bochs is another free virtual machine, but it only emulates the i386 architecture.
• VMWare is a proprietary virtual machine; being one of the oldest out there, it's also one
of the most widely-known. It works on principles similar to QEMU. VMWare proposes
advanced features such as snapshotting a running virtual machine.
➨ http://www.vmware.com/
• VirtualBox is a virtual machine that is mostly free software (although some extra components
are under a proprietary license). Although younger than VMWare and restricted
to the i386 and amd64 architectures, it shows promise; it already allows snapshotting, for
instance. VirtualBox has been part of Debian since Lenny.
➨ http://www.virtualbox.org/
12.2.1. Xen
Xen is a “paravirtualization” solution. It introduces a thin abstraction layer, called a “hypervisor”,
between the hardware and the upper systems; this acts as a referee that controls access
to hardware from the virtual machines. However, it only handles a few of the instructions, the
rest is directly executed by the hardware on behalf of the systems. The main advantage is that
performances are not degraded, and systems run close to native speed; the drawback is that the
kernels of the operating systems one wishes to use on a Xen hypervisor need to be adapted to
run on Xen.
Let's spend some time on terms. The hypervisor is the lowest layer, that runs directly on the
hardware, even below the kernel. This hypervisor can split the rest of the software across several
domains, which can be seen as so many virtual machines. One of these domains (the first one
320 The Debian Administrator's Handbook