Alternative Boot Loaders

[Colin Watson]

On Fri, May 02, 2008 at 04:40:49PM +0100, Matt Darcy wrote:
> I'm trying to establish the reasoning behind including alternative boot 
> loaders such as lilo on the ubuntu alternative and server cds ?
> 
> Was this a technical decision ?
> Was this a policy decision ?
> 
> could someone please explain the decision and the reasons behind it.

This was a technical decision (inherited from Debian, but very much
supported by me). The circumstances in which the installer will install
LILO are precisely as follows:

  * /boot is on an LVM volume
  * /boot is on an XFS file system (and the
    grub-installer/install_to_xfs debconf question was not explicitly
    preseeded to true)
  * the grub-installer/skip debconf question was explicitly preseeded to
    true

The XFS item has been disputed from the start, and I can only repeat
what I've always said regarding it. The problem here is that XFS skates
close to the edge of what the POSIX specification allows a file system
to do; in particular it cannot be forced to write data to disk by any
action short of unmounting and remounting the file system (or possibly
remounting the file system read-only, but I seem to remember that that
was also problematic). GRUB requires that data be physically on disk in
order for grub-install to work. To answer a frequently asked question
up-front, 'xfs_freeze -f' at least used not to be sufficient to
guarantee that data had been written to disk; I tested this extensively
some time ago.

This problem is a race condition, and so it is entirely possible that
some users may never see it, while other users will always see it, and
still others will randomly see it or not. Installing LILO, whatever its
user interface deficiencies, is much better than the boot loader
installation process crashing.

Ubiquity does not at present support installing LILO (which is a bug
we'll need to fix at some point). In this environment, LVM partitioning
is not yet supported, and attempting to put /boot on an XFS file system
will warn you that this may cause problems.

Regards,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]