Installing and testing NixOS


NixOS is a minimal and flexible Linux distribution which doesn’t use any of the existing package manager.

NixOS is a Linux distribution with a unique approach to package and configuration management. Built on top of the Nix package manager, it is completely declarative, makes upgrading systems reliable, and has many other advantages.

I wanted to test NixOS and see if it could be a candidate for a very minimal KVM hypervisor running just Qemu, libvirt and Apache CloudStack.

With this post I just wanted to share how you can quickly install NixOS inside a VirtualBox VM.


On my desktop and laptop I usually use VirtualBox to quickly test something inside Virtual Machines. In this case I downloaded the NixOS minimal 64-bit ISO and created a VM:

  • 1024MB of memory
  • 8GB SATA disk
  • NixOS ISO attached


After you start the VM it will boot from the ISO. You will then find yourself in a root prompt saying just nixos.

The first step is to format your disk and mount it under /mnt.

parted /dev/sda mklabel msdos
parted /dev/sda mkpart primary 0% 100%
mkfs.xfs /dev/sda1
mount /dev/sda1 /mnt

If you have that done you can run:


This will generate /mnt/etc/nixos/configuration.nix from where you can configure your OS.

This is what I used as my configuration:

{ config, pkgs, ... }:

  imports = [

  boot.loader.grub.enable = true;
  boot.loader.grub.version = 2;
  boot.loader.grub.device = "/dev/sda";

  boot.kernelPackages = pkgs.linuxPackages_4_1;

  time.timeZone = "Europe/Amsterdam";

  networking.firewall.enable = false;

  environment.systemPackages = with pkgs; [
    wget git screen ceph

  services.openssh.enable = true;
  services.openssh.permitRootLogin = "yes";

  virtualisation.libvirtd.enable = true;
  virtualisation.libvirtd.extraOptions = ["-l"];
  virtualisation.libvirtd.extraConfig = "listen_tls = 0\nlisten_tcp = 1";

  system.stateVersion = "15.09";

A minimal installation with just OpenSSH and libvirt installed.

Now you can actually install NixOS:


After a few minutes you will be prompted for a root-password and that’s it!

Reboot and you have a running NixOS installation 🙂

Using TRIM/DISCARD with Ceph RBD and libvirt


Using TRIM/DISCARD you can give back free space to a Ceph cluster. Normally, any thin provisioned block device will keep on growing until its maximum size while being used. Using the DISCARD command a underlying block device can be instructed to discard blocks which do not contain data.

In the case of Ceph’s RBD we can shrink our RBD images again which gives us back free space in our Ceph cluster.


Using this feature is only supported if you use VirtIO-SCSI and not if you use plain VirtIO.

Some searching brought me to this XML for my Ubuntu 15.10 guest:

<disk type='network' device='disk'>
  <driver name='qemu' type='raw' cache='none' discard='unmap'/>
  <auth username='admin'>
    <secret type='ceph' uuid='f94812dd-f06f-48f6-9839-1edf7ee8f8d6'/>
  <source protocol='rbd' name='libvirt/image1'>
    <host name=''/>
  <target dev='sda' bus='scsi'/>
  <controller type='scsi' index='0' model='virtio-scsi'/>

Inside the guest

I tried a Ubuntu 15.10 guest but this should be supported in any other modern Linux guest.

lspci shows me:

root@ubuntu1510:~# lspci 
00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
00:01.2 USB controller: Intel Corporation 82371SB PIIX3 USB [Natoma/Triton II] (rev 01)
00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)
00:02.0 VGA compatible controller: Cirrus Logic GD 5446
00:03.0 Ethernet controller: Red Hat, Inc Virtio network device
00:04.0 SCSI storage controller: LSI Logic / Symbios Logic 53c895a

And I have a sda block device which my guest uses:

root@ubuntu1510:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            230M     0  230M   0% /dev
tmpfs            49M  4.6M   45M  10% /run
/dev/sda1       9.3G  1.3G  7.6G  15% /
tmpfs           245M     0  245M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           245M     0  245M   0% /sys/fs/cgroup
tmpfs            49M     0   49M   0% /run/user/0

Now I can run fstrim which will trim the block device:

root@ubuntu1510:~# fstrim -v /
/: 128 MiB (134217728 bytes) trimmed

Rebuilding libvirt under CentOS 7.1 with RBD storage pool support

If you want to use CentOS 7.1 for your hypervisors with Apache CloudStack and Ceph’s RBD as Primary Storage you need to rebuild libvirt.

CloudStack requires libvirt to be built with RBD storage pool support. It uses libvirt to manage RBD volumes. By default libvirt under CentOS is not built with this support. (On Ubuntu it is btw).

Rebuilding from source

First we need to install a couple of packages:

$ yum install -y rpm-build gcc make ceph-devel

Now we need to download the sRPM:

$ wget

Create a rpmbuild directory:

$ mkdir /root/rpmbuild

Now edit /root/.rpmmacros so that it contains:

%_topdir    /root/rpmbuild

Install the sRPM:

$ rpm -i libvirt-1.2.8-16.el7.src.rpm

Open the /root/rpmbuild/SPECS/libvirt.spec file and look for:

    %define with_storage_rbd      0

Change this to:

    %define with_storage_rbd      1

Now build the RPM:

$ cd /root/rpmbuild
$ rpmbuild -ba SPECS/libvirt.spec

After a couple of minutes you should have RPMs with RBD storage pool support enabled!