Ceci n'est pas une -EPIPE

Life as a F/OSS Zealot, Debianite, Pythonista and Lisper

Linode pv-grub chaining

by

Paul Tagliamonte

I've been using Linode since 2010, and many of my friends have heard me talk about how big a fan I am of linode. I've used Debian unstable on all my Linodes, since I often use them as a remote shell for general purpose Debian development. I've found my linodes to be indispensable, and I really love Linode.

The Problem

Recently, because of my work on Docker, I was forced to stop using the Linode kernel in favor of the stock Debian kernel, since the stock Linode kernel has no aufs support, and the default LVM-based devicemapper backend can be quite a pain.

I tried loading in btrfs support, and using that to host the Docker instance backed with btrfs, but it was throwing errors as well. Stuck with unstable backends, I wanted to use the aufs backend, which, dispite problems in aufs internally, is quite stable with Docker (and in general).

I started to run through the Linode Library's guide on PV-Grub, but that resulted in a cryptic error with xen not understanding the compression of the kernel. I checked for recent changes to the compresson, and lo, the Debian kernel has been switched to use xz compression in sid. Awesome news, really. XZ compression is awesome, and I've been super impressed with how universally we've adopted it in Debian. Keep it up! However, it appears only a newer pv-grub than the Linode hosts have installed will fix this.

After contacting the (ever friendly) Linode support, they were unable to give me a timeline on adding xz support, which would entail upgrading pv-grub. It was quite disapointing news, to be honest. Workarounds were suggested, but I'm not quite happy with them as proper solutions.

After asking in #debian-kernel, waldi was able to give me a few pointers, and the following is very inspired by him, the only thing that changed much was config tweaking, which was easy enough. Thanks, Bastian!

The Constraints

I wanted to maintain a 100% stock configuration from the kernel up. When I upgraded my kernel, I wanted to just work. I didn't want to unpack and repack the kernel, and I didn't want to install software outside main on my system. It had to be 100% Debian and unmodified.

The Solution

Left unable to run my own kernel directly in the Linode interface, the tact here was to use Linode's old pv-grub to chain-load grub-xen, which loaded a modern kernel. Turns out this works great.

Let's start by creating a config for Linode's pv-grub to read and use.

sudo mkdir -p /boot/grub/

Now, since pv-grub is legacy grub, we can write out the following config to chain-load in grub-xen (which is just Grub 2.0, as far as I can tell) to /boot/grub/menu.lst. And to think, I almost forgot all about menu.lst. Almost.

default 1

timeout 3

title grub-xen shim
root (hd0)
kernel /boot/xen-shim
boot

Just like riding a bike! Now, let's install and set up grub-xen to work for us.

sudo apt-get install grub-xen
sudo update-grub

And, let's set the config for the GRUB image we'll create in the next step in the /boot/load.cf file:

configfile (xen/xvda)/boot/grub/grub.cfg

Now, lastly, let's generate the /boot/xen-shim file that we need to boot to:

grub-mkimage --prefix '(xen/xvda)/boot/grub' -c /boot/load.cf -O x86_64-xen /usr/lib/grub/x86_64-xen/*.mod > /boot/xen-shim

Next, change your boot configuration to use pv-grub, and give the machine a kick. Should work great! If you run into issues, use the lish shell to debug it, and let me know what else I should include in this post!

Hack on!