Migrating Xen to KVM#

Wed, 03 Jun 2009 16:06:29 +0000

Until very recently I ran this blog site, and a couple of others (test sites for work, that kind of thing) in a Xen virtual host on the pc sitting on my desk. While it worked ok for that purpose, it really didn't coexist nicely with much else: the Xen kernels available in debian have a showstopper bug for running X11, are missing modules for e.g. cpu temperature monitoring, etc. And the process to build ones own is not usefully documented. (I apt-get sourced my kernel image, and it downloaded a completely different kernel version). I don't think the stuff is unusually buggy, it probably just doesn't have enough eyeballs on it to find all the weird interactions with other components or to patch them when they do turn up.

So when my four year old Athlon motherboard died recently, the opportunity to rejoin the mainstream and start using kvm was the silver lining in the "ew, hardware" cloud.

A hard ware is gonna fall

I decided to go Intel this time despite my AMD-supporting underdog instincts, chiefly because I figured that (a) every Intel cpu on the planet should have VT support by now, (b) onboard graphics are well-supported in xorg, (c) everything I read about the E5300 CPU said it had very good price/performance ratio and overclocking it someday sounded like a fun project. So basically I bought that and the first G45 board I found that had onboard graphics (this is the distinction between G45 and P45 which doesn't), three SATA ports and was made by someone I'd heard of. It's the Asus P5Q-EM, and it apparently also is a good one for overclocking.

After putting it all together, I found that it didn't work. Important lesson here: for 'market segmentation' reasons - and I was certainly pretty cut up about it, though that may be not the kind of segmentation they mean - Intel only implement the necessary VT bits on CPUs that cost north of £100, not on the cheap and lovely E5x00 series even though they have the (arguably equally "enterprise niche") 64 bit support. So, gah. So I ended up with an E8200 as well, and a spare cpu+fan that is probably going on Ebay soon.

This walk is made for booting

KVM now runs, which is a start. Note that you need the kvm module and the kvm-intel module, or it will complain that you have no CPU support. You may also have to fiddle with your bios settings, although I didn't.

To convert a Xen disk image into an image that kvm understands is still a bit fiddly: in brief, xen images are filesystem (= partition) images, whereas kvm wants entire disks and they need to be bootable (i.e. contain grub and a kernel). Here's how I did it, prefaced by the obligatory "it worked for me, but it's not my fault if it doesn't for you. Back up first" caveat

Obligatory caveat: it worked for me, but it's not my fault if it doesn't for you. Back up first.

1) Identify your disk image. On a Debian box it will be in some place like /usr/local/lib/xen/domains/vitrual.4a.telent.net/disk.img

cd /usr/local/lib/xen/domains/no.stargreen.com
file disk.img disk.img: Linux rev 1.0 ext3 filesystem data, UUID=e6143cef-c3ed-4c21-94d9-32d4ba186910 (large files)
ls -l disk.img -rw-r--r-- 1 root staff 4294967296 2009-06-03 14:33 disk.img Looking promising. So let's create a qemu image.

dd if=/dev/zero of=mbr bs=8225280 count=1
cat mbr disk.img mbr | cp --sparse=always /dev/stdin qemudisk.raw Why these numbers? It's a throwback to ancient technology: ye olde PC accessed disks using cylinder/head/sector technology, and the upper bounds on each bit are 16383, 16, 63. In my testing, KVM ignores the CHS settings in the disk image and assumes 16 heads/63 sectors, so we're going to make it easy for ourselves by adopting that geometry ourselves, and since this makes each cylinder 8225280 bytes long we'll stick that much blank space on the front of the image for MBR/partition table/etc so the first partition is at cylinder 1

And the additional space on the end? In my first few attempts I ran into trouble with e2fsck complaining that the filesystem was bigger than the disk image. This is, of course, impossible, and I imagine was due to some kind of rounding error, but padding it out this way is easier than investigating.

The sparse flag to cp might save us a bit of disk space if we have any great big expanses of zero in the image. Mine didn't, as it turned out, but YMMV.

Next up, partitioning. sfdisk accepts input on stdin and uses it to destroy your data, which makes it in some ways the perfect Unix-philosophy tool. GNU Parted will never give you this kind of buzz, so be very careful here.

# echo '1' | sfdisk qemudisk.raw # DO NOT MISTYPE THIS # sfdisk -l qemudisk.raw Disk qemudisk.raw: cannot get geometry

Disk qemudisk.raw: 524 cylinders, 255 heads, 63 sectors/track Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

Device Boot Start End #cyls #blocks Id System qemudisk.raw1 1 523 523 4200997+ 83 Linux qemudisk.raw2 0 - 0 0 0 Empty qemudisk.raw3 0 - 0 0 0 Empty qemudisk.raw4 0 - 0 0 0 Empty

At this point if you have a rescue cd image or something you could start kvm and have a poke around.

 # kvm -hda /usr/local/lib/xen/domains/no.stargreen.com/qemudisk.raw
   -cdrom trinity-rescue-kit.3.1-build-210.iso -boot d

But you still can't boot the hd image directly, because there's no bootloader. So let's fix that next. Note that your exact kernel package name may not be the same as mine.

 # mount qemudisk.raw /mnt -t ext2 -o loop,offset=8225280
 # chroot /mnt apt-get update
 # chroot /mnt apt-get linux-image-2.6.26-2-amd64 grub  
 # mkdir -p /mnt/boot/grub
 # cp /mnt/usr/lib/grub/x86_64-pc/* /mnt/boot/grub/
 # cat | grub --device-map=/dev/null
 device (hd0) qemudisk.raw 
 root (hd0,0)
 setup (hd0)
 # cat > /mnt/boot/grub/menu.lst
 title           Debian GNU/Linux, kernel 2.6.26-2-amd64
 root            (hd0,0)
 kernel          /boot/vmlinuz-2.6.26-2-amd64 root=/dev/hda1 
 initrd          /boot/initrd.img-2.6.26-2-amd64

apt-get may complain about having no initrd set up and ask if you want to to abort. I said "no", which works for me. We install grub by hand because grub-install doesn't work so well in this situation. Obviously you will need to muck about with pathnames in my minimal menu.lst if your system is not quite like mine.

That's basically it as far as getting a bootable image is concerned. You can start kvm with this image: I would probably suggest running update-grub inside it to replace this minimal config with all the Debian grub goodness

Take me to the bridge: networking

The kvm networking default is 'user' networking, which is in summary dead convenient if all you want to do in the virtual host is run clients, but a little bit slow and a little bit weird. If you want to run servers on your kvm instance, you will probably instead want to bridge its network with your real host so the outside world can see it.

I previously had a statically addressed eth0, now I have a statically addressed br0 which bridges that and a couple of tap devices (you will need one per kvm virtual host). The necessary debs are bridge-utils and vde2, and your /etc/network/interfaces stanza, on the real host, is something like this

 # The primary network interface
iface br0 inet static
    address 192.168.1.2
    netmask 255.255.255.0
    gateway 192.168.1.1
    pre-up /usr/bin/vde_tunctl -u www-data -t tap0
    pre-up /usr/bin/vde_tunctl -u www-data -t tap1
    pre-up ifconfig tap0 up
    pre-up ifconfig tap1 up
    bridge_ports all tap0 tap1
    post-down ifconfig tap0 down
    post-down ifconfig tap1 down
    post-down tunctl -d tap0 tap1

and then I start kvm with

 # kvm -hda qemudisk.raw -net nic -net tap,ifname=tap0,script=no

And there it is. Write yourself some fancy startup scripts: I invoke kvm with -daemonize and -vnc options so that it doesn't need to open a window, but there are plenty of different ways to do that. You will probably want to play with other options too: for example to change the kvm instances ram allocations or to give them a second disk image suitable for swapping on. If you have lots of kvm instances all running at once you should maybe also look into VDE networking rather than the tun/tap devices, but I haven't spent the time to work that out yet.

If you go googling to solve your own problems: you probably know this already ,but it bears repeating: KVM is in some sense a fork/branch/derivative of QEMU, and the two systems have largely or maybe even entirely common code in this area. So a lot of what you see about qemu booting is directly applicable to kvm too.

If you found this useful, feel free to get in touch. If I've left any step out, tell me that too and I'll edit it.