XEN

Från Minitruth

(Texten är kopierad från ett projekt som xor var med i, det är därför den är på engelska. Texten är såklart Public Domain som allt annat på wikin.) XEN is a paravirtualizer, meaning that the guests kernels will often need to be modified to run on it. Paravirtualizing is a tiny bit faster than ordinary virtualization.

Innehåll

1 Setup
2 issues
3 Regular usage
4 Virtual networks
- 4.1 Default looks something like this
  - 4.1.1 This is a real example from legion.
- 4.2 Adding more virtual OSI2-networks

Setup

You need to modify your kernel
- aptitude install xen-..something xen-tools
- reboot
  - if it fails, boot with your old kernel and fix the problem
goto /dev/xen/ and edit the xen-config.cfg file
- it should say (network-script network-bridge) instead of (network-script network-dummy) if you want networking to exist.. READ THE COMMENTS for more info.
You need to create a domU (= virtualized host, the operating system that runs on the real hardware is called dom0.)
1. type "man xen-create-image" and read how the /etc/xen-tools/xen-tools.cfg file should be written. _you need to understand this_.
2. edit your /etc/xen-tools/xen-tools.cfg file
3. xen-create-image --hostname meh --ip 1.2.3.4 --memory 512M --passwd --size 10G
  - ..will create a computer with hostname meh, ip 1.2.3.4, RAM=512M, hdd=10G (read the man page for more info on how to tweak.)
  - enter the root password when you are prompted for it
4. goto /etc/xen/
5. add extra = 'xencons=tty' to the bottom of /etc/xen/meh.cfg (meh is the hostname)
6. xm create meh.cfg
7. xm console meh
8. install udev at the machine and reboot it ("aptitude install udev")
9. ssh to it
done.

issues

Fixes for random problems i encountered..

you want guests to start when the xen hosts boots up

Put symbolic links to the guests startup scripts in /etc/xen/auto/, you might need to create the directory.

ln -s /etc/xen/domU-name.cfg /etc/xen/auto/domU-name

/dev/loop runs out after 4 virtual guests

in debian: create the file /etc/modprobe.d/local-loop if it does not exists, then add "options loop max_loop=256" to the bottom of the file. (lets hope it works, i have not restarted the xen host yet.....)

to fix the problem without reboots, put this in a file and run it:

#!/bin/bash
for ((i=8;i<255;i++)); do
  [ -e /dev/loop$i ] || mknod -m 0600 /dev/loop$i b 7 $i
done

terminal fuckup at login

IFF it fails because it complains about stty not existing, or something similiar:

xm shutdown meh
edit /etc/xen/meh.cfg, add extra = 'xencons=tty' to the bottom of the file.
mount -o loop /path/to/your/disk/disk.img (default: /home/xen/domains/meh/disk.img)
vim /mnt/etc/fstab
- add "none /dev/pts devpts defaults 0 0" to the file, without the "
mkdir /dev/pts (sometimes the installer forgets to create this directory?)
umount /mnt
cd /etc/xen/; xm create meh.cfg; xm console meh
- xm console meh brings you inside the domU/guest..
aptitude install udev (this is really important if you want ssh to work..)
DONE. Now, you should be able to log into it with ssh too.

Regular usage

xm list --- shows what virtual machines are running
xm shutdown meh --- shuts down machine meh
xm destroy meh --- pulls the power plug for machine meh, shuts it down imideatly
xm create meh.cfg --- starts the machine described in ./meh.cfg (domU scripts are located in /etc/xen/, go there first.)
xm console meh --- gives you a console to machine meh (detaching from that console is rather difficult. use ssh if you can.)
...

Virtual networks

By default, xen does not provide any networking. By changing that as described above, all machines will be connected to a virtual ethernet bridge (all guests are connected to a switch inside your machine.)

Default looks something like this

(Default does not look like this, but I assume that you changed to the configuration described above.. The real default is a desolate place, where only the host machine is connected to any network.)

peth0 <--> veth0
             |
             +-- host machine
             |
             +-- virtual machine 0
             |
             +-- virtual machine 1
             |
             +-- virtual machine 2
             |
           (etc)

All machines, including the host (dom0) is connected to the virtual bridge. But the host has control over peth0, veth0, and all the virtual machines interfaces, vifX.Y. (where X is machine ID and Y is its ethY that the guest will see. vif5.3 is virtual machine 5's eth3 interface, as seen by the host.)

This is a real example from legion.

brctl is a program for handling OSI2-networking in linux. It shows the ASCII graph just above as seen from the host machine.

# brctl show
bridge name	bridge id		STP enabled	interfaces
eth0		8000.0023545e6a18	no		peth0
							vif10.0
							vif11.0
							vif18.0
							vif21.0
							vif47.0
							vif51.0
							vif52.0
							vif54.0
							vif56.0

Adding more virtual OSI2-networks

It is possible to set up NAT, routing and bridging networks with the default scripts. I have only tried bridging.. And when I open the script it is filled with very complex stuff. One almost has to master the program sed to know what it does, it seems.

I need to learn:

moar sed (again!)
brctl (omg, just found this program. can be used to create virtual bridges on the fly. bridges are used to handle OSI2-layer networking and operates on ethernet frames. like a switch. linux bridging supports STP, the spanning tree protocol, so it is compatible with other hardware equipment.)

XEN