From vservers to KVM

2010-01-08 4-minute read

Given the impending deprecation of vservers, I’ve decided to make the switch to KVM on my laptop. Although lxc is a closer approximation to vservers, I decided to go with KVM due to it’s support in Virtual Machine Manager.

My first step was to confirm that my CPU would support kvm:

egrep -o "svm|vmx" /proc/cpuinfo

If that command outputs either svm or vmx (depending on whether you have Intel or AMD hardware) then your CPU supports virtualization.

I’m working on a host machine called chicken, which has a logical volume called vg_chicken0. All vservers on chicken operate on a root filesystem that is backed by their own logical volume.

In this post, I’ll describe the steps to convert the vserver hobo (which operates on a filesystem mounted on the host in /var/lib/vservers/hobo and is backed by the logical volume called vg_chicken0-hobo_root).

Both chicken and hobo are running debian squeeze.

vservers don’t have a kernel installed or grub. KVM virtual servers need both.

I was hoping I could simply enter the vserver, install both a kernal and grub and be ready to go. However, grub installation will fail miserably because grub can’t figure out how to install on the underlying disk (which is hidden from the vserver).

Next, I tried launching a kvm instance, passing a debirf generated ISO with the -c (cdrom) option. However, grub recognized that it was being installed onto a device that did not have a partition table (the logical volume was directly formatted with a file system).

So, since I had disk space to spare, I created a new logical volume:

lvcreate --size 5GB --name hobo_root_new vg_chicken0

I then added a gpt partition table (why not prepare for the coming 2TB disks?) and created two partitions. One partition for grub2 and one for everything else:

parted /dev/mapper/vg_chicken0-hobo_root_new mklabel gpt
parted /dev/mapper/vg_chicken0-hobo_root_new unit s mkpart biosboot 2048 4095 
parted /dev/mapper/vg_chicken0-hobo_root_new set 1 bios_grub on 
parted /dev/mapper/vg_chicken0-hobo_root_new unit s mkpart primary 4096 

When prompted for the end of the last partition, choose: -1 and accept the adjustment.

I had to eyeball cat /proc/partitions to figure out which dm device was the second partition (dm-19).

I then created a file system:

mkfs -t ext3 /dev/dm-19

Mounted it:

mount /dev/dm-19 /mnt

And rsync’ed the data:

rsync -a /var/lib/vservers/hobo/ /mnt/

With the data in place, I chroot’ed and installed the packages I needed. When prompted, I chose not to install grub to the disk, because I wanted to wait until I had an environment in which the proper disk would be available to grub as it will when the virtual server boots (see below):

chroot /mnt
mount /proc
aptitude install linux-image-2.6-amd64 grub2
umount /proc

Then, I cleaned up:

umount /mnt
umount /var/lib/vservers/hobo
lvremove vg_chicken0/hobo_root
lvrename vg_chicken0/hobo_root_new hobo_root
dmsetup remove /dev/mapper/vg_chicken0-hobo_root_newp1
dmsetup remove /dev/mapper/vg_chicken0-hobo_root_newp2
kpartx -d /dev/mapper/vg_chicken0-hobo_root

And I removed it from /etc/fstab.

Next, I created a new kvm virtual server, using the disk /dev/mapper/vg_chicken0-hobo_root and passing a debirf cd image with -c:

virt-install –name hobo –ram 512 –disk /dev/mapper/vg_chicken0-hobo_root -c /usr/local/share/debian/ISOs/debirf-rescue_squeeze_2.6.32-5-vserver-amd64.iso

After logging in, I installed grub2 (aptitude update; aptitude install grub2) and then I installed grub:

mount /dev/sda2 /mnt/
grub-install --no-floppy --root-directory=/mnt /dev/sda

After running grub-install, edit /mnt/boot/grub/ so it reads:

(hd0) /dev/sda

Then, rerun grub-install command.

I tried generating the grub.cfg file, but got an error message indicating that grub-probe would not detect the device providing / (because I was running on a ram file system from debirf).

I added the following to /mnt/etc/fstab:

/dev/sda2  /     ext3 errors=remount-ro 0 1
proc       /proc proc defaults          0 0

And then re-generate the initrd image:

chroot /mnt
mount /proc
update-initramfs -u

So, I rebooted the virtual machine by typing:


This dropped me into a grub shell. I manually typed:

root (hd0,gpt2)
linux /vmlinuz root=/dev/sda2 ro
initrd /initrd.img

Once booted, I logged in a completed the task with: