[Beowulf] reboot without passing through BIOS?

Robert G. Brown rgb at phy.duke.edu
Wed Jul 30 12:49:15 PDT 2008

On Wed, 30 Jul 2008, David Mathog wrote:

> Robert G. Brown wrote:
>> There are several ways to accomplish this, and they can be automated.
>> For example, run a script at boot time that runs a script like
>> /etc/specialboot if it exists.  Then put:
>> #!/bin/sh
>> # cp /boot/grub/grub.conf.default /boot/grub/grub.conf
>> # cp /boot/grub/grub.conf.special /boot/grub/grub.conf
> These all work great for controlling from the linux side, not so well
> for controlling from the Windows site.  I have seen methods like this
> for Windows, but they depend upon putting grub.conf in a location where
> Windows can write it.

Windows?  What is this Windows?  I don't do Windows...;-)

Hmmm, I did think about this once upon a time when I lusted after
nighttime use of a Windows cluster in a lab here as a linux cluster.  I
remember that there was an automatable solution, but I don't remember
what it is.  Windows can definitely execute commands on a timed basis,
so that was no problem.  I'll think about it and see if I can remember
-- it was close to a decade ago that these fantasies occurred, before it
became clear that I wasn't going to get permission.

But since Linux is perfectly happy mounting /boot as root-writeable
msdos or ntfs, what's wrong with this solution as a boot toggle between
the two?  XP Pro should be able to at least do a copy exactly like the
one above as a .bat file.

Or maybe somebody has a better way.

Oh, one last way that is VERY easy is to set up dhcp to direct your
boot.  That is, boot ONLY via PXE/DHCP (or at the very least, put it as
the first default boot target and boot from it if it says to). Tell
dhcp.conf what kernel you want it to boot, and boot it, then change it
serverside.  There is a fallthrough solution like this as well -- put a
pxe boot target on the server like "windows" that you type in by hand at
the pxe/dhcp prompt (not the grub prompt), with the default after a
timeout of falling through to linux.  But you could do that with grub
now if you wanted to, so I'm guessing that the idea is to do this
unattended.  Unattended suggests server side control, and you have
(presumed) absolute control over the dhcp server on a time-dependent

Hope this helps -- I missed the early part of why you are doing this so
I'm firing blind, but if you (re)describe what you're trying to
accomplish I can try to be more directed.  In my copious spare time --
I'm teaching physics summer school at Beaufort (if anybody wants to see
my summer "house", check out:


and zoom in to the max possible.  My house is the southwest corner
building on the island.  That's me waving at you, fishing from the
dock...:-).  So I'm working 14-16 hour days.


> Ideally grub/lilo would look in the MBR, or some other block in the
> first track on the boot disk, for a "boot once special" flag and field.
> If the flag was set it would read the field and then clear the flag.
> Then the tool on pretty much any OS to enable this would just be:
> read a block from disk (from some known special location)
> set the flag and field
> write the block back to disk
> Easiest if it is the MBR, not hard though for any other block in the
> first track. Heck, if lilo and grub could coordinate they could even use
> the exact same flag and field for this purpose, so that only one tool
> would be needed to accomplish it.
> Regards,
> David Mathog
> mathog at caltech.edu
> Manager, Sequence Analysis Facility, Biology Division, Caltech

Robert G. Brown                            Phone(cell): 1-919-280-8443
Duke University Physics Dept, Box 90305
Durham, N.C. 27708-0305
Web: http://www.phy.duke.edu/~rgb
Book of Lilith Website: http://www.phy.duke.edu/~rgb/Lilith/Lilith.php
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