Launching Gentoo VMs on

October 23, 2012

Update: Corrected fallocate and parted commands, and removed diratime mount option. Thanks to axil

Long time, no post.

For about a year now, I’ve been working at GRNET on its (OpenStack API compliant) open source IaaS cloud platform Synnefo, which powers the ~okeanos service.

Since ~okeanos is mainly aimed towards the Greek academic community (and thus has restrictions on who can use the service), we set up a ‘playground’ ‘bleeding-edge’ installation ( of Synnefo, where anyone can get a free trial account, experiment with the the Web UI, and have fun scripting with the kamaki API client. So, you get to try the latest features of Synnefo, while we get valuable feedback. Sounds like a fair deal. :)

Unfortunately, being the only one in our team that actually uses Gentoo Linux, up until recently Gentoo VMs were not available. So, a couple of days ago I decided it was about time to get a serious distro running on ~okeanos (the load of our servers had been ridiculously low after all :P). For future reference, and in case anyone wants to upload their own image on or ~okeanos, I’ll briefly describe the steps I followed.

1) Launch a Debian-base (who needs a GUI?) VM on

Everything from here on is done inside our Debian-base VM.

2) Use fallocate or dd seek= to create an (empty) file large enough to hold our image (5GB)

fallocate -o $((5 * 1024 * 1024 *1024)) -l 1 gentoo.img

3) Losetup the image, partition and mount it

losetup -f gentoo.img
parted /dev/loop0 mklabel msdos
parted /dev/loop0 mkpart primary ext4 2048s 5G
kpartx -a /dev/loop0
mkfs.ext4 /dev/mapper/loop0p1
losetup /dev/loop1 /dev/mapper/loop0p1 (trick needed for grub2 installation later on)
mount /dev/loop1 /mnt/gentoo -t ext4 -o noatime

4) Chroot and install Gentoo in /mnt/gentoo. Just follow the handbook. At a minimum you’ll need to extract the base system and portage, and set up some basic configs, like networking. It’s up to you how much you want to customize the image. For the Linux Kernel, I just copied directly the Debian /boot/[vmlinuz|initrd|] and /lib/modules/ of the VM (and it worked! :)).

5) Install sys-boot/grub-2.00 (I had some *minor* issues with grub-0.97 :P).

6) Install grub2 in /dev/loop0 (this should help). Make sure your inside the Gentoo chroot looks like this:

(hd0) /dev/loop0
(hd1) /dev/loop1

and make sure you have a sane grub.cfg (I’d suggest replacing all references to UUIDs in grub.cfg and /etc/fstab to /dev/vda[1]).
Now, outside the chroot, run:

grub-install --root-directory=/mnt --grub-mkdevicemap=/mnt/boot/grub/ /dev/loop0

Cleanup everything (umount, losetup -d, kpartx -d etc), and we’re ready to upload the image, with snf-image-creator.

snf-image-creator takes a diskdump as input, launches a helper VM, cleans up the diskdump / image (cleanup of sensitive data etc), and optionally uploads and registers our image with ~okeanos.

For more information on how snf-image-creator and Synnefo image registry works, visit the relevant docs [1][2][3].

0) Since snf-image-creator will use qemu/kvm to spawn a helper VM, and we’re inside a VM, let’s make sure that nested virtualization (OSDI ’10 Best Paper award btw :)) works.

First, we need to make sure that kvm_[amd|intel] is modprobe’d on the host machine / hypervisor with the nested = 1 parameter, and that the vcpu, that qemu/kvm creates, thinks that it has ‘virtual’ virtualization extensions (that’s actually our responsibility, and it’s enabled on the servers).

Inside our Debian VM, let’s verify that everything is ok.

grep [vmx | svm] /proc/cpuinfo
modprobe -v kvm kvm_intel

1) Clone snf-image-creator repo

git clone

2) Install snf-image-creator using setuptools (./ install) and optionally virtualenv. You’ll need to install (pip install / aptitude install etc) setuptools, (python-)libguestfs and python-dialog manually. setuptools will take care of the rest of the deps.

3) Use snf-image-creator to prepare and upload / register the image:

snf-image-creator -u gentoo.diskdump -r "Gentoo Linux" -a [ username] -t [ user token] gentoo.img -o gentoo.img --force

If everything goes as planned, after snf-image-creator terminates, you should be able to see your newly uploaded image in, inside the Images container. You should also be able to choose your image to create a new VM (either via the Web UI, or using the kamaki client).

And, let’s install kamaki to spawn some Gentoo VMs:

git clone

and install it using setuptools (just like snf-image-creator). Alternatively, you could use our Debian repo (you can find the GPG key here).

Modify .kamakirc to match your credentials:

enable = on
url =
cyclades_extensions = on
enable = on
url =
colors = on
token = [token]
enable = on
url =
account = [username]
container = pithos
enable = on
pithos_extensions = on
url =

Now, let’s create our first Gentoo VM:

kamaki server create LarryTheCow 37 `kamaki image list | grep Gentoo | cut -f -d ' '` --personality /root/.ssh/authorized_keys

That’s all for now. Hopefully, I’ll return soon with another more detailed post on scripting with kamaki (vkoukis has a nice script using kamaki python lib to create from scratch a small MPI cluster on ~okeanos :)).


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