GRUB multiboot with redhat ES and DL380

[Matti_Kurkela · Accepted solution]

You'll have to read the GRUB documentation and do some customizing. The BIOS is so restricted by PC legacy that using it to control multi-booting tends to be rather painful.

First, GRUB gets all its knowledge of disks from the BIOS. When a computer is booting, the BIOS will make a list of all the disks it can access. The BIOS will identify each hard disk by a hexadecimal number, starting from 0x80 (the numbers of the floppy disks start from 0x00). This numbering is created dynamically each time system boots up: if you add or remove disks, the numbers of the other disks may/will change.

How does this relate to GRUB's disk identifiers? Well, if you don't confuse the situation with "map" directives, BIOS's 0x80 = GRUB's (hd0), 0x81 = (hd1) and so on. With Linux, you should not need any "map" directives at all.

If the BIOS has a facility to select a disk to boot from, that facility usually works by making that disk become the disk 0x80 in the BIOS numbering, and thus (hd0) for GRUB. What happens to the numbers of the other disks is "implementation-specific" - i.e. a likely cause for headaches: this is why I don't recommend using BIOS settings to handle complex multi-boot situations.

The BIOS loads the Master Boot Record from disk 0x80, i.e. the first hard disk on its list. The Master Boot Record might contain the boot loader (the usual way with GRUB), or just a simple program to load the real boot loader from the beginning of the active partition of the same disk (the Microsoft way, although GRUB can be used like this too).

If the boot loader needs to load some more parts of itself (like GRUB's stage2), it usually expects to find them on BIOS disk 0x80, unless specifically configured otherwise. Because the Master Boot Record is always read from disk 0x80, there is not much point to place the bootloader to any other disk: it would just increase the possibilities for failure.

When the Linux kernel starts up, it re-identifies all the disks. There is no general way to ensure that the detection order of Linux matches the detection order of the BIOS if you have multiple controllers, but if you have just a single SmartArray controller you can usually trust that
if disk slot 1 = 0x80 = (hd0) = /dev/cciss/c0d0,
then disk slot 2 = 0x81 = (hd1) = /dev/cciss/c0d1
and so on.
If you have multiple disk controllers, the problem may become a lot more challenging.

When you get this figured out, you can start writing extra entries in the GRUB configuration file. For the simplicity's sake, I recommend that you make the first disk, i.e. /dev/cciss/c0d0 as your "master" bootloader disk. (Otherwise you'll need to get your BIOS make some other disk get the number 0x80, which makes the entire numbering scheme more difficult to predict... see above.)

The /boot/grub/grub.conf (or in some distributions, /boot/grub/menu.lst) file will contain the GRUB boot configurations for all the OSes you have in this system.

Look at the entries for your first OS (c0d0) in the grub.conf file. They should be similar to this:

title
root (hd0,0)
kernel /vmlinuz. root=
initrd /initrd.img.

The "title" line begins each boot menu item definition and defines the text to display in the boot menu.

The "root" line is a bit of a misnomer: it has nothing at all to do with any root filesystem. It just identifies the partition (=filesystem) the next lines will refer to. In a typical Linux installation, it will refer either to the root filesystem or the /boot filesystem.

Note: if /boot is a separate filesystem, the file paths on the "kernel" and "initrd" lines will not have /boot prefixed to them, because GRUB is just looking at the detached /boot filesystem, not the complete tree seen by Linux.

The "kernel" and "initrd" lines will identify the files GRUB will load into the system memory whenever this menu item is selected. On the "kernel" line, you can also specify Linux kernel options, of which the "root=" option is probably the most important one here. RedHat normally uses "root=LABEL" or "root=UUID" style root definitions, but nothing is preventing you from using a simple "root=/dev/cciss/c0d1" here.

To get the correct path names for writing the boot entry for e.g. c0d1 when running the OS of c0d0, just manually mount the necessary partition(s) of c0d1 to /mnt and take a peek.
The GRUB entry to boot from c0d1 should look something like this:

title "boot from (hd1)"
root (hd1,0)
kernel /vmlinuz root=/dev/cciss/c0d1
initrd /initrd.img

To help you to identify the correct GRUB disk names, you could create an uniquely-named file to each installation's root directory (or /boot directory, if /boot is a separate filesystem - that is what we want to find when booting a system). You could then put GRUB in command prompt mode and use its "find" command to find out the correct GRUB disk identifier. (It's sad GRUB does not have a "ls" command.)

For example, if you're running the OS of c0d0 and have manually mounted the /boot partition of the c0d1 OS to /mnt:

touch /mnt/system_c0d1
umount /mnt
shutdown -r now

find /system_c0d1

GRUB should respond with something like:

(hd1,0) /system_c0d1

Now you know for sure that GRUB sees the /boot partition of the c0d1 installation currently as (hd1,0). If you move disks around, the number might change.

Because only the c0d0 installation's GRUB configuration is actually meaningful, you'll probably need to update it manually whenever you update the kernel on any of the other installations. The alternative is to replace the standard boot configuration auto-update system with a script you make yourself: if you update the kernels in this system frequently, it might be worth the effort... or not.

MK

[Marko Krstic]

Hello Bill,

My opinion is that this shouldn't be that complicated. The logic should be very similar to the situation when You have Windows and Linux on the same HDD and GRUB gives You the possibility to choose among them.

When You install the first RH(RH1) on the first HD (HD1), the GRUB is installed there. When You install second RH2 on HD2, You can boot only RH1 (RH2 installed it's grub in MBR of HD2). So, You need to change the menu.lst file on the HD1 and to tell him that You have another OS on another HD (HD2). This way, You'll have unique GRUB file, allways on the fisrt disk, and every time You reboot a machine, this file will be checked.



title RH1
root (hd0,2)
kernel
initrd /boot/initrd.img-2.6.20-16-generic
quiet
savedefault

title RH2

# You should put here another disk,
# ie. (hd1,2)
root (hd1,2)

kernel
initrd /boot/initrd.img-2.6.20-16-generic



For the details about naming hard disks in GRUB, check the manual :-)

When You make changes to menu.lst file, after reboot You should be able to see the GRUB menu with available OS and You should see something like this:

RH1
RH2

See You soon in Belgrade ;-)

Best Regards,
Marko