OS Virtualization Principles

Principles

Run multiple instances of OSes simultaneously
Multitasking at the OS level
Garantee isolation between OSes
Provide controlled resources sharing

Applications of OS Virtualization

Wide range of use cases:

Server consolidation
Colocation of services
Security, isolation, maintainance
Regression tests
...

Virtual Machines and Hardware

Ideally virtualization is transparent to guest OSes

Common principle in mainframe design
Nearly impossible with i386 design
Running unmodified OSes is horribly tricky
- Trapping priviledged calls
- Dynamic code rewrite
Those tricks kills performances in some scenarii

Paravirtualization and Xen

Xen comes from the University of Cambridge

Do not hide the virtualization from the guest OSes

Port the OS to work with Xen
Reuse the OS drivers, but monitor hardware access
Linux, NetBSD. FreeBSD, OpenSolaris and Plan9 worked on.

Xen's performances

Borrowed from their paper at SOSP

comparative performances of Xen on bencharks

Example of use

ISP server consolidation

One VM per customer
Dynamic creation of VM
Migration of VM in a cluster

Smooth upgrades of critical servers

Xen for Linux

Xen is been integrated in Linux upstream

this is a separate architecture (ARCH=xen)
Xen runs as an hypervisor
kernel instances call the hypervisor for resources
the hypervisor uses linux for the drivers

Xen Architecture

multiple domain running in Xen

Xen in Fedora

This is a work in progress

Fedora Core 4 has the basics
Package xen-2:
- Control command xm/xend
- performance CLI tools
- Python bindings and libraries
- /boot/xen
Package kernel-xen0 (2.6.11...)

Booting Xen

From grub.conf:

title Fedora Core Xen (2.6.11-1.1369_FC4xen0)
        root (hd0,0)
	kernel /boot/xen.gz
	module /boot/vmlinuz-2.6.11-1.1369_FC4xen0 ro root=LABEL=/12 rhgb quiet selinux=0
	module /boot/initrd-2.6.11-1.1369_FC4xen0.img

Xen runs in ring 0 (approx 32MB)

Linux runs in ring 2/3