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 classic 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
• New Intel and AMD server chip will help virtualization

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 guests

Xen's performances

Borrowed from their paper at SOSP in 2003

Xen's performances II

Borrowed from this year presentation at OLS

Xen's performances III

Borrowed from this year presentation at OLS

Xen for Linux

Xen is being integrated in Linux upstream

• as a sub arch of the supported architectures (i386, x86_64)
• Xen runs as an hypervisor
• kernel instances call the hypervisor for resources
• the hypervisor uses linux for the drivers

Xen Architecture

Xen in Fedora

This is a work in progress

Rik van Riel and Jeremy Katz

• Fedora Core 4/Rawhide has the basics
• Package xen-2:
  • Control command xm/xend
  • performance CLI tools
  • Python bindings and libraries
  • /boot/xen
• Package kernel-xen0 (2.6.12...)
• Package kernel-xenU (2.6.12...)
• Track updates and Rik's page

Booting Xen

From grub.conf:

title Fedora Core Xen (2.6.12-1.1454_FC4xen0)
        root (hd0,0)
	kernel /boot/xen.gz
	module /boot/vmlinuz-2.6.12-1.1454_FC4xen0 ro root=LABEL=/ rhgb quiet
	module /boot/initrd-2.6.12-1.1454_FC4xen0.img

Xen runs in ring 0 (approx 32MB)

Linux runs in ring 2/3

Creating guests

Per guest description file in /etc/xen/

kernel="/boot/vmlinuz-2.6.12-1.1454_FC4xenU"
memory=128
name="fc4"
nics = 1
disk = ['file:/u/fc4.img,sda1,w']
root = "/dev/sda1"
extra = "ro selinux=0 3"

Reduce the Xen0 memory, launch and check:

xm mem-set 0 512
xm create fc4
xm list

Current state (FC4)

It works with basic core tools

• Start up/Stop a domain "xm create/shutdown ..."
• Basic resource control "xm baloon/max-mem/set-mem ..."
• Saving/restarting domains

But we are still in stabilization mode for Xen-3.0

Future tools

Higher level tools

• /etc/xen profiles maintainance
• Integration with anaconda/yum
• Freeze/migration of domains
• UI (Web) front-end

Xen limitations

This is a relatively new technology

Some limitations are being worked on

• Non-SMP guests
• architecture: i386 only, x86_64 in i386 mode
• x86_64 actively worked on, ppc and itanium too

Xen Research

Work being done for Xen 3.0

• Migration and clustering
• Debugging and fault tolerance
• Security
• Performances aspects
• Intel hardware virtualization support

Xen's migration data

Borrowed from this year presentation at OLS

Useful Links

Fedora Xen Kickstart

Rik's page

Fedora Virtualization

Xen FAQ Wiki

Cambridge Lab. Xen page

XenSource