[Pkg-xen-devel] Bug#672104: ITP: pv-grub-menu.lst

[Ian Campbell]

On Thu, 2012-05-24 at 09:08 +0900, Charles Plessy wrote:
> Le Fri, May 11, 2012 at 04:17:39PM +0100, Ian Campbell a écrit :
> > 
> > Can I just check I understand the motivation for this script properly.
> > 
> > There are two ways of setting up the disk for a VM.
> > 
> > The first is the "whole disk" scheme. In this configuration the VM
> > configuration contains the entire "xvda" which contains a partition
> > table in the usual way. In this configuration either grub-legacy or
> > grub-pc can be installed and "grub-install /dev/xvda" does the right
> > thing including setting up the MBR. This is useful because you can flip
> > quite easily from PV to HVM just by changing the VM config and
> > rebooting. (This is the setup I generally use myself, so I'm mostly
> > familiar with it)
> > 
> > The second scheme is the "split partitions" scheme. In this
> > configuration the VM config contains "xvda1" and "xvda2" etc which
> > appear to the guest OS as partitions but critically there is no overall
> > "xvda" and therefore no partition table. This means that "grub-install"
> > cannot work. This is the configuration which EC2 etc use and therefore
> > this update-grub variant is necessary.
> > 
> > Is that right?
> 
> Exactly.
> 
> When installing Debian on a EC2 volume with Debian Installer, the partition is
> a "whole disk", completely usable by grub.  But that means that when the newly
> prepared Debian system is booted, it is a root partition, which is "split
> partitions" in the Amazon cloud and grub hooks will fail when installing a new
> kernel (I did not have time to triplecheck).

So it is whole disk at install time and split partition at run time?
Wow! I guess I need to try amazon some time so I can see these quirks
for myself...

> Perhaps that could be solved by
> making the hooks checking for the availability of a MBR before running grub,
> but the grub packages are quite critical, and I am not sure how this additional
> complexity would be welcome.

Yes, I can see that being tricky to get right, without regressing the
normal cases.

> In addition there is a second problem.
> 
> Pv-grub needs a menu.lst file that is made by GRUB 1.

Ah, this is a wrinkle I hadn't considered before, I tend to think about
pygrub (which can do GRUB 2) and always forget that EC2 uses pvgrub!

>   But Debian's GRUB 1
> package is in maintainance-only mode, I do not know what are the plans to
> remove it eventually, but once GRUB 2 can replace it in all cases, this may
> happen.  So it is probably better to have a menu.lst-builder script for
> pv-grub maintained somewhere else.  The pkg-xen project on Alioth is a good
> idea indeed.

I've CC'd the pkg-xen list, perhaps waldi has an opinion...

> Or maybe pv-grub could be extended to parse menu.cfg files ?

There was mention of a project to turn grub2 into a pv-grub2 at one
point, but I'm not sure who that was or what the status is.

>   But I do not
> know how long it would take for this new function to propagate to Amazon.

Indeed.

> Have a nice day,
> 
> -- 
> Charles Plessy
> Tsurumi, Kanagawa, Japan
> 
> 
> 

-- 
Ian Campbell

The system will be down for 10 days for preventive maintenance.