Pygrub boot iso

This article covers creating a DomU on Xen 4. To demonstrate, we setup a paravirtualized CentOS 7 bit domain. For many candidate DomU distributions, installation via the xen-tools xen-create-image eg. This works great for Debian, Ubuntu, and CentOS 5 — but other OSes takes a bit of work, or may not be easily possible without installation from its original media.

Here, we will look at installing an operating system on to a Xen domain by first creating an HVM-based domain which will run during our install of the OS. Then, once installation is completed, we can convert the domain to boot via PyGrub so that it can benefit from the increased speed that comes with paravirtualization PV over full hardware virtualization HVM.

This article looks at installing CentOS 7 bit , but the process can be used for other OSes as well. Here, I am using Xen 4. Now complete the installation steps. Go through the localization portions as needed, important bits outlined below:. Make sure to set a root password while the installation is proceeding.

Then, once the install is complete, choose the Reboot option. Let's break this GRUB configuration down a bit. The first points to a preseed configuration file and starts a partially automated Debian install, while the other just boots the Debian installer with no preseed file and acts like an "Expert" mode so you can choose every install option by hand.

Next in each menu, you define a loopback device GRUB will use:. The next two lines should look pretty familiar to someone who has worked with Linux GRUB configuration before, but with a twist:. In the first line, you define what kernel to boot and what options to pass it, and in the following line, you point GRUB to the initrd file you want it to use.

The main difference here though is that you precede each file path with loop to instruct GRUB to look in that loopback filesystem for the file. Once this configuration finds its way into grub. Now the first time I tried to boot the most recent Debian installer this way, I ran into a bit of a problem.

It turns out that the initrd that comes on the ISO itself does not contain the installer scripts you need to install from an ISO on a hard drive. Because of that, I discovered I had to download a different Debian installer initrd and put it on the rescue disk for things to work. I was able to find an initrd that worked here. The following figure outlines the boot process with PVGrub.

Unlike Pygrub it runs an adapted version of the grub boot loader inside the created domain itself, and uses the regular domU facilities to read the disk mounted as root directory, fetch files from network, etc. It also eventually loads the PV kernel and chain-boots it. PVGrub allows host admins to configure what guests and kernel versions a guest admin can install: this is also one of the main drawbacks.

It can also be used for PXE booting. Xen does not come with PVGrub: you will need to install an appropriate distro package make sure that Xen support is enabled or build it from source.

PyGrub enables you to start Linux domUs with a kernel inside the DomU instead of a kernel that lies in the filesystem of the dom0. This means easier management: each domU manages its own kernel and initrd, meaning you can use the built-in package manager to update the kernels, instead of having to track and update kernels stored in your dom0.

The following figure outlines the boot process with PyGrub:.