there, you need to start it yourself by switching to root with su - and then running service xend start. Now run the command xm list, which prints out a list of all the virtual machines that are running and how much RAM they have allocated — you should see Domain-0, which is your current system, in the list.
Setting Up Guest Operating Systems
The output from xm list probably shows that domain 0 is taking up all the RAM on your system, which means there is no room to create a new guest OS. Fortunately, you can resize that memory usage downward to make space: Run the command xm mem-set Domain-0 256 to have domain 0 use 256MB RAM. This is the bare minimum for a Fedora install, so expect some slowdown — if you have more than 512MB RAM, we recommend you allocate more to each VM.
Creating a domU VM on Fedora is handled with the xenguest-install.py script, which you should run as root. You are asked to do the following things:
1. Give your virtual machine a friendly name (for example, FCUnleashed) so that you can differentiate between it and other virtual machines.
2. Allocate it some RAM, with the minimum being 256MB; more is better.
3. Choose where it should save its files. Xen uses a loopback filesystem so that all of a VM's files are stored in just one file on domain 0. Enter something like /home/paul/vms/fcu.img.
4. Select how big the virtual disk should be. For a basic install, around 4.0GB should be enough.
5. Set the install location (the place from which Fedora should install). This needs to be an online resource, so choose a server from http://mirrors.fedoraproject.org/publicist/Fedora/8.
Now sit back and wait. Downloading the necessary files can take quite a while, depending on your connection speed.
Once your files have been downloaded, the normal Fedora installer (Anaconda) will start, and will ask whether you want to install using text mode or VNC — choose Start VNC so that you have a graphical install. Xen's guests do not have direct access to the hardware, which means they have nowhere to display graphics. VNC lets you have your Xen VM render its graphics to your dom0 display inside a window, which means you can work with multiple VMs simultaneously. When you select Start VNC, you are prompted to enter a password. Click OK and you see the VNC address to which you need to connect. This address should look something like 10.0.0.1:1, where the :1 is the number of the VNC display.
Back on dom0, you should have installed VNC Viewer at the start of this chapter, so go to Applications, Accessories, VNC Viewer. Now enter that address (including the :1 or what ever it is for you) and click Connect. Enter your password when prompted. VNC starts and you see the Fedora installer. Depending on the resolution of your screen, the Fedora installer might not fit entirely on the screen; in that case, you have to use the scrollbars to get around.
The installer is as normal from here on in, except of course that it is using the virtual disk you created earlier so you have only a small amount of space. It might refer to your hard disk as something like /dev/xvda — do not worry about that.
Runtime Configuration
Now that your guest OS is up and running, you can start trying out more of the features of the xm command on dom0. You have already seen xm mem-set, which alters the amount of memory allocated to a machine. This works because the domU VM is aware of its virtualized state and can therefore handle having memory taken away. This extends further: You can use xm shutdown yourvm to have Xen politely request the VM to shutdown. On Linux, this goes through the whole shutdown sequence properly, ensuring that the machine is cleanly terminated. If you want an immediate shutdown, use xm destroy yourvm, but make sure the virtual machine is in a safe state first — if you have a text file open and unsaved, for example, it will be lost.
The xm command can also be used to save snapshots of a virtual machine, rather than just switching them off. To do this, use xm save . That command essentially saves the RAM of the yourvm to whatever you called your virtual machine) to a file and then turns off the VM. To restore a saved state, just use xm restore yourvm.state. If you want to create a virtual machine from a configuration file, use xm create -c yourconfig. Note that Xen searches the directory /etc/xen for configuration files, and that each VM must have a unique name assigned to it and set in the configuration file.
You can connect to the console of any virtual machine by running xm console yourvm. To exit from a console, press Ctrl+] (Control + right bracket). This does not shut down the VM; the VM continues to run, but you are no longer connected to it and have to use xm connect to reconnect.
The configuration files in /etc/xen are in text format and so are easily edited. For example, if you want to change the number of CPUs a VM sees, look for the vcpus setting. Note that these are virtual CPUs rather than real ones — you can set this to 8 and have your guest see eight CPUs, even if your actual machine has just one. This is a great way to test a cluster without going beyond your desktop!
The following commands are useful for working with Xen on Fedora:
> virt-manager — Red Hat's new graphical Xen management system
> vncviewer — Lets you connect to the graphical output of a Xen VM
> xend — Starts and stops the Xen daemon without using the service command
> xenguest-install.py — A helpful script that generates configuration files for you
> xm — Lets you manipulate the state of virtual machines while they are running
Reference
> http://www.xensource.com — A company (run by some of the Xen engineers) that offers help and support for Xen, as well as produces XenOptimizer to ease enterprise deployment.
> http://www.cl.cam.ac.uk/Research/SRG/netos/xen — The Xen home page at the Cambridge computer laboratory.
PART IV
