Dave's Mess > Blog

<<< Distribution and layers Galumph went the little green frog one day. >>>

A tale of duelling GRUBs and boots.

8am, 26th May 2007 - Geek, Linux, Sysadmin

Last night I finally coerced GRUB into doing what I wanted it to do. It's not that GRUB was particularly stubborn but more that it had a particular place that it liked to look for it's instructions on what to do and wouldn't do anything at all if it didn't find them in that place. Of course, no one tells you what this file is called... you don't really understand GRUB until you have searched half the internet trying to figure it out.

Let me start from the start. I'm installing Linux because I have a low spec box (650MHz, 128MB, 16GB) and I think I can get better performance out of a tightly controlled Linux installation than a sprawling, all-encompassing Windows XP installation. Jane, however, has a fear of Linux, possibly caused by some sort of Linux based trauma while she was at University and hence I can't get rid of Windows just yet. It would be nice to install Ubuntu and just be done with it but unfortunately the two different versions of Ubuntu I have tried both require more RAM than I have. They would both boot from the Live CD if you had about half an hour to spare but as soon as you tried to do anything more complicated such as opening a program the system would grind to a halt. Hence I settled on Damn Small Linux, a distro designed for it's small size and low reqirements while still being flexible enough to grow into a full distribution once installed. I could have my cake and eat it too !

After burning the LiveCD (it seems such a waste that only 50MB of the CD was actually used) and throwing it in my computer, it booted straight into a desktop that was only using 18MB of RAM. Even browsing the Net with Firefox I was still only using 33MB. DSL looked like it was going to fit into my tight RAM requirements quite well. I told it to install itself to my hard drive and then looked up a tutorial on how to dual boot Linux and Windows, having never done that before.

Inevitably, you will end up on the first link from Google for "dual boot Linux Windows" which is Ed's software guide on Linux. Unfortunately, Ed's guide seems a little out of date and not all that accurate. He suggests using fips for your partitioning needs but the link he supplies is no longer current. I found that burning a gparted live CD provided more than just the best tool money can buy for free. The particular CD I burned has Grub installed with about as many options for booting as you can imagine, so once you have started messing around with GRUB and can't boot your own system any more, you can still put in the gparted CD, load GRUB and tell it to boot "The first partition from the second disk" or wherever your favourite OS is. His instructions also seem convoluted and unnecessary. In the end he seems to be using the Windows boot loader to give the choice between loading Windows normally or loading GRUB which will then load Linux. My final result was a little different. I loaded GRUB from a 40MB boot partition and then gave GRUB the choice between Linux and Windows. The advantage of this way is that the process of getting it all working is actually less complicated than Ed's instructions. The most useful guide to GRUB (and hence dual booting) I found was GRUB from the ground up from Troubleshooters.

Now, for the list of gotchas and caveats. Firstly, I used gparted to shrink my Windows partition by 40MB and move it to the right so I could fit a 40MB partition in before it. The reason I did this is because of that famous 1024 cylinder limit. I don't know if my computer can boot from beyond the first 1024 cylinders but I didn't feel like taking the chance. Linux was going to be on a seperate physical drive anyway so I would need a boot loader on the first drive (The BIOS calls it the C: drive.) to load Linux from the second. This took about two hours for a 50% full 16GB drive. One for a read-only test and one for the real thing. Your mileage may vary. I also noticed some flags you could set for each partition in gparted but while reading Ed's guide I was a little reluctant to mess with them because he makes it sound like you'll need to reformat everything and re-install if you screw up your MBR. My computer persistently kept on booting into Windows until I changed the "boot" flag from the Windows partition to my brand new boot partition. It seems obvious now but it just goes to show how a little knowledge is a dangerous thing.

Once I had changed that "boot" flag, I was presented with the GRUB command line when my system started up. It looks scary but it's not really all that bad if you have used the Unix command line before. You can use it to interrogate your hard drives and search for files (such as the Linux kernel) or figure out what hard drives you have or, if you're feeling wild and rebellious, you could use it to boot into one of your available operating systems. Until I figured out the GRUB configuration file I was booting Linux by typing three commands at this boot prompt every time I restarted. The commands I typed are explained in detail on the Troubleshooters page I linked to earlier and are included in my sample GRUB menu file at the bottom. The first command tells GRUB where it can find the rest of itself. GRUB's first stage can only be 512 bytes and if it's doing anything more complicated than handing control over to Windows then it will need to load some more code. root (hd0,1) refers to the second partition (,1) of the first hard drive (hd0) and this is the location of GRUB's stage 2. The second command kernel /boot/linux24 root=/dev/hdb5 tells GRUB where to find the Linux kernel (kernel /boot/linux24) and where Linux should mount it's own root filesystem from when it loads (root=/dev/hdb5). If those two commands print confirmation messages rather than error messages then the third command is simply boot.

To make things a little easier, you can tell GRUB some of these commands in advance and group them together under convenient labels such as "Windows" and "Linux". If you are installing a new Linux kernel you can set up a new group called "Linux with new kernel" and give it a different kernel statement. There seems to be some confusion over whether these instructions should go in a file called grub.conf or a file called menu.lst. I found that grub.conf didn't work for me no matter where I put it but once I put the same contents into a menu.lst file, it worked straight away. Below is what I have in my menu.lst file.

timeout 10

default 1

title Linux
    root (hd0,1)
    kernel /linux24 root=/dev/hdb5

title Windows
    rootnoverify (hd0,0)
    chainloader +1

The location of this file and others needed by GRUB may cause some confusion. It is common to mount your boot partition under /boot on a Linux system however it is not strictly necessary to mount it at all after you have created it. In fact, you can create it without mounting it but I wouldn't recommend that until you are an expert. The confusion arises because when GRUB mounts your boot partition, it doesn't mount it as /boot. GRUB will mount it as / which means that any path you specify to GRUB that has /boot at it's start will not work. Some people suggest using a symlink in your boot partition that looks like this: (boot -> .) This symlink will allow the same path to work for GRUB when the partition is mounted at / as for Linux when the partition is mounted at /boot. Another option would be to install all of GRUB's files in /boot/boot/grub/ and symlink (grub -> boot/grub) to make Linux do the symlink chasing rather than GRUB. In either case, if GRUB can load then you can put your files (grub.conf or menu.lst) in the same place as GRUB's stage1 and stage2 files and feel safe in the knowledge that GRUB will find them.

I hope my little story has made it easier for you to get your system dual booting or simply booting at all.