Viraj's Weblog

These days the buzzword seems to be "hypervisor". It's a general term for what VMware is. One thing I've always found to be a good use case for virtualization is testing new and possibly broken configurations. These past few days I've been food poisoned and working from home recovering, but it did give me a chance to play around with the latest Linux virtualization: KVM.

KVM in this sense is not that hardware device to connect one monitor to multiple machines, so the choice of naming is unfortunate. It is based on QEMU, however there are kernel hooks to make things go faster by utilizing virtualization features on certain processors. What's great is recent distributions have it simply as a kernel module that doesn't require any kernel patching.

First some background. Why am I interested in this? Well in my case I run a website on Apache (who doesn't). I've been hearing some good things about lighttpd and I wanted to see what the fuss was about. However, I didn't want to muddy my current Apache server with lighttpd. I simply wanted another system to play around with, preferably one that didn't require hardware. So this seemed like a perfect case to try out the completely free KVM.

Now my host OS (the OS that will run KVM) is a Ubuntu 7.10 server. It actually runs on a closed laptop, so essentially I have no display on it and only connect to it remotely. Most virtualization software these days are GUI apps unless you buy the horribly expensive 'server' products. I was somewhat pleasantly surprised as to how KVM worked in my scenario. Ideally I wanted to run a guest OS the same as my host OS, Ubuntu 7.10, so that's what I started with.

First I installed the kvm package:

sudo aptitude install kvm

Easy enough, next I modprobe the right modules:

sudo modprobe kvm
sudo modprobe kvm_intel

I should point out you need to have a processor that supports KVM. See their web page for more info. Now it get's a bit confusing. In some documentation there is mention of running "qemu-system-x86_64", however on my box this is the unaccelerated version that makes no use of KVM's kernel modules. I'm not sure what's going on here, but I believe at some point QEMU and KVM will merge code, and maybe this is the reason for the documentation discrepancy. Anyhow, for my case, I had to use "kvm" to start up my virtual machine. So I downloaded the Ubuntu 7.10 server ISO and begun my journey. First I needed to create a disk image. Interestingly, these disk images only take up space as you add to them, similar to VMware:

qemu-img create -f qcow vdisk.img 10G 

This creates a 10G-maximum disk image. Now we're ready to begin the installation:

sudo kvm -hda vdisk.img \
  -cdrom ubuntu-7.10-server-i386.iso \
  -boot d -m 384 -vnc 192.168.1.137:0.0 \
  -no-acpi

This simply boots the VM with the Ubuntu server CD and 384M of RAM. It's recommended to use -no-acpi, so I did. Now BAM I get an "exception 6" and a crash immediately (as described here). My foray is not starting off well . After more searching I came across this bug which hinted that this was maybe fixed in a newer version of Ubuntu. Huh? So something in Ubuntu is causing this crash? Searching further I found this thread which gives more info:

Confirmed here with kvm-intel and KVM 39. Invalid opcode (#UD) is probably caused by the boot spash code which may be using big real mode code.

So indeed, something in Ubuntu was doing it. So I decided to try grabbing the bleeding edge Ubuntu "Hardy Heron" server. After some thumb twiddling it now boots!

Now remember how I have no display on this box? Well the convenient -vnc argument creates a VNC server that I can connect to remotely! This is all great, but kvm also supports X, and actually I do most of my work from an OS X X terminal. Why not just use the X interface? Well on OS X, the keyboard becomes completely unusable in KVM for some reason. It could be something retarded in the X server. Anyhow, for VNC, there is Chicken of the VNC, by far the stupidest package name ever. It generally works well though, however when I tried to connect to my VM I was getting some invalid rectangle error. It seems everything was against my trying to get this working . After much experimentation, I found that I had to simply disable Hextile encoding in the VNC connection profile and voila, I can connect.

Now I thought to myself, I'm going to be running a webserver on this, don't I need some kind of network setup? Well once I confirmed things are booting ok, I did some research on KVM networking. Essentially what I wanted is my VM to appear just like a separate machine on the network with full network access. KVM has all sorts of networking possibilities, but here is my setup. First I updated /etc/network/interfaces to look like:

auto lo
iface lo inet loopback

auto br0
iface br0 inet dhcp
  bridge_ports eth0
  bridge_maxwait 2
  up /sbin/ifconfig eth0 inet 0.0.0.0 promisc

auto eth0
iface eth0 inet static
  address 172.16.5.0
  netmask 255.255.255.0

This generally came from this Ubuntu KVM doc. The IP above is actually a bogus one that won't be used. Rebooting (alas, I did have to reboot) gave me a br0 and eth0 device listed via 'ifconfig'.

My host machine still worked so that's good.

Now how to start the VM with networking? Simple:

sudo kvm -hda vdisk.img -m 384 \
  -vnc 192.168.1.137:0.0 \
  -no-acpi -net nic -net tap

I went through the install with no problems at all and it got an IP via DHCP on my router. Sweet.

Now one thing that was bothering me about all of this is that spiny sudo prefix. I would've liked to not use sudo. This forum thread mentions a possible solution but I had no luck with tunctl and I didn't feel like spending too much time on it.

So I got my cool VM working, now what? For what reason am I on this Earth? On my main web server I have a Django site running under Apache. What I wanted to try is running this with FastCGI under lighttpd. Now I have to admit, lighttpd configuration is a much nicer experience than Apache configuration.

Take a look at my lighttpd.conf. My virtualhost is defined at the bottom, and is taken mostly from the Django fastcgi docs. lighttpd has a very simple and elegant configuration. Note that I had to modify some of the modules loaded, and the Django docs seem to indicate the ordering is important. Once I started up my Django site in fcgi mode my site was instantly accessible, and FAST. At least, very fast for a virtual machine!

So what have I learned from all of this? Well KVM is cool, and one of these days it is going to beat out its commercial counterparts. Also, lighttpd is cool. Cooler than Apache I must say. It will definitely be coming to more web servers near you.