After I discovered GitLab CI, I was a huge fan of Docker and its containers.
I wanted to dig deeper into the business of virtualization and look at
alternative approaches. Recently, I thought I’d give Xen a try. I’m also rather new to CentOS, but I
planned to setup a CentOS
dom0 and a para-virtualized
domU anyway. It didn’t go as
planned. Whenever I tried to boot the installer for the guest OS with
I received an error starting with something like
This was really puzzling to me. What I was trying to do was basic and I expected that everything works out-of-the-box. It turned out that this has been already reported as a bug, which (according to the discussion at the bug report) will be eventually fixed in CentOS 7.5. Until that version of the OS is released, the solution seems to be to use the CentOS’ kernel-plus repositories which use a fixed version of the kernel. However, this task is rather cumbersome if this is your first Xen+CentOS installation.
Using the information posted by PryMar56 on the bug report and by Johnny Hughes and PJ Welsh on the CentOS-virt mailing list, I will explain in the following how to install CentOS 7.4 with kernel-plus as a Xen PV guest. The post is intended for people who are new to Xen and CentOS.
As discussed in the threads mentioned above, the problem stems from a bug in the kernel used in CentOS 7.4. CentOS/RHEL’s policy is to not update the kernel of a released version of the OS. It is, however, possible to install the centos-plus repository which ships with a newer version of the kernel. So, the task is
- to modify the installer such that it boots with the kernel-plus, and
- to install the kernel-plus by default, such that the installed OS is able to boot with the new kernel.
Throughout this guide, I’m using
mirror.certos.org. You should probably
replace these parts with another
1. Setting up the installer
First, we need to make sure that lorax is installed. Lorax is a tool to build boot images. We will use it to build the anaconda (the centos installer) boot image with the kernel-plus.
The default build options are specified in
/usr/share/lorax/runtime-install.tmpl. Open this file with your favorite text
editor and change the line
installpkg kernel to
To build the image, disable SElinux with
setenforce 0 and run
Lorax loads the required packages from the CentOS mirror, builds the installer
images and puts the result in the directory
centos7_installer. It is generally
a bad idea to deactivate SElinux. While judging for yourself, if you can make an
exception, consider that we are still in permission mode, so any violation will be
logged and you can switch to enforcement mode immediately after the lorax
All the files in the output directory have to be available during the installation process. The easiest method is to expose them via HTTP. If you are not in a hostile network environment, you can make the files available with
For later reference, let’s assume the IP address of this host where we publish
the installer image is
It is not necessary to run the server on your
dom0. This can be any other
server on your network. Check that files are accessible by directing your browser to
http://10.1.1.100:8000/. You might have to modify your firewall settings to
allow traffic to go to port 8000.
2. Install guest OS with kernel-plus
If we run
virt-install now with the
--location argument pointing to our newly
created installer image, the installer should boot. However, if we then
proceeded, we would install the guest OS with the default kernel.
We can configure the installation process with a so-called kickstart file.
Create a new directory
centos7_kickstart which we will later also expose
via HTTP. Open
centos7_kickstart/kickstart.cfg and enter
This file instructs anaconda to install the kernel-plus package.
Again, we can expose this file via HTTP on port 8001. Port 8000 might be in use by the other simple HTTP server.
Finally, we can create the guest machine with a custom
argument to use our installer image with plus kernel and a custom
to tell anaconda about our kickstart file.
During the installation (I recommend to start VNC to have more options), make
sure that the centos-plus repository is enabled. Furthermore, point the main
installation source to a public mirror, e.g.
While working on this document, I stumbled over other inconveniences
In all the examples, I have used
--nographics, because my
dom0 is not
running an X server. Connecting with VNC from another computer over a local
network usually doesn’t work out-of-the-box, because the packages are not
forwarded to the
domU. You can, however, use port forwarding
to forward any traffic to a local port to the
domU via ssh of the
By default, the installer used
xfs for the boot partition. Every time I tried
to reboot after the installer finishes,
pygrub complained that it could not
read the machine image or could not find the boot partition. Switching to
ext4 via VNC (or potentially via the kickstart file) solved the issue for
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