From Xen
Revision as of 07:43, 10 November 2011 by Kyl191 (talk | contribs) (How?)


PyGrub enables you to start Linux DomUs with the kernels that lie in the filesystem of the DomU instead of with a Kernel that lies in the filesystem of the Dom0. This enables easier kernel update from DomU or migration of HVM'ed Linuxes.


It is used as a Bootloader and provided with Xen. PyGrub behaves like Grub and reads the standard Grub menu.lst (or Grub2's grub.cfg) to provide the xen create process with the required parameters and SXP stanzas.

  • Question is: Does it copy the req'd kernel and initd temporarily out to Dom0 for it to access it there or does Dom0 access the kernel and initd in their filesystem? Anyone?

Therefore usage is quite easy.


In your domU

  1. Install Grub
  2. Install a linux-image-xen of your choice.
  3. Create the /boot/grub directory if it is not present.
  4. Let it create its default menu.lst (update-grub in Debian)

In your dom0

  1. Change your domU.cfg:
  • remove kernel and initrd stanza
  • add bootloader stanza, e.g. bootloader = '/usr/bin/pygrub'

Note: You need to know where in the filesystem pygrub lies. For example, debian uses /usr/lib/xen-$version/bin/pygrub or something like this, instead of the path reflected above

Important Tip / Caveat

On your disk line, e.g.

disk = ['file:/vserver/mail/guest_base2G.img,hda1,w','file:/vserver/mail/guest_swap256M.img,hda2,w','phy:data/mail,hda3,w']

make sure that the partition/lvm/container that contains the kernel and initrd comes first. Xend passes the *first* entry to pyGrub. (At least for Version 3.2.1) If the first disk entry does not contain a kernel you will most likely get

Error: Boot loader didn't return any data 

If you get this error have a look at /var/log/xen/xend.log . You can read here which parameters were given to pyGrub, try them by Hand and see if pyGrub outputs the right stanzas for domU startup. (They look a little like grub entries LISPified, SXP-Format).

Thanks to (The following is taken from and own experience) for first pyGrub Reference.