KfWiki: Backup MBR

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Backup MBR

The first 512b contains partition table and the MBR. First 448b contains the MBR only.
ref http://www.tuxation.com/mbr-tricks-with-linux.html

dd if=/dev/sda of=mbr.img bs=448 count=1

Rescue / emergency boot disc

It is a good idea to make such a boot disc especially when you have custom kernel with extra filesystem and storage controllers enabled. On your running system, run the following to create an iso, which can then be cut to a cd disc:

mkbootdisk --device /rescue.iso --verbose --iso 2.6.28.1

Fixing boot record

Sometimes, MBR (master boot record) may be messed up if you have a dual-boot system. Do the followings and get your boot loader fixed:

# boot up your system with your installation CD, enter **linux rescue** at boot prompt. Most Linux LiveCD (like Ubunto) will do.

mkdir /mnt/mysys

# My root partition is on /dev/vg0/lv_root

mount -t ext3 /dev/vg0/lv_root /mnt/mysys

mount -t proc none /mnt/mysys/proc

mount -o bind /dev /mnt/mysys/dev

# chroot to the mysys mount point

chroot /mnt/mysys /bin/bash

# mount the boot partition

mount -t ext3 /dev/hda2 /boot

# run grub install to fix the boot loader

grub-install /dev/hda

grub-install --recheck /dev/dev/hda

# Reboot

Fix grub in grub prompt

[root@www root]# grub

grub> find /boot/grub/stage1
(hd0,0)

grub> root (hd0,0)
Filesystem type is ext2fs, partition type 0x83

grub> setup (hd0)

Boot from grub prompt

The key is to point grub to the right partition. If the root parameter points to a partition other than /, system will halt with a kernel panic error.

grub> root (hd0,1)
grub> kernel /vmlinuz root=/dev/cciss/c0d0p1 ro vga=791
grub> boot

Setting up a boot-once grub session

It's very useful to setup grub to boot to a specific kernel only once, to perhaps test out a newly compiled kernel. Setup your grub.conf like this

	 default saved        # This is important!!!
	 timeout 10
	 
	 title the old kernel
	 root (hd0,0)
	 kernel /old_kernel
	 savedefault
	 
	 title the new kernel
	 root (hd0,0)
	 kernel /new_kernel
	 savedefault 0         # This is important!!!

Essentially, when entry #1 is booted, it will instruct grub to boot from entry #0 on next boot. Save grub.conf and then do a

grub-set-default 1

You can also configure grub to do fallback boot. See Grub's manual http://www.gnu.org/software/grub/manual/grub.html#Booting-once_002donly

Messing with initrd

If you clone / migrate your system to a different box which uses a different storage card, you may need to edit the initrd file to add load the necessary modules.

# back it up first!!
cp initrd-2.x.x.img initrd-2.x.x.img-bak
# extract the image to temp
mkdir temp
cd temp
zcat ../initrd-2x.x.img | cpio -idmv
# do what you have to. e.g. edit init to load a certain module, copy that module to temp/lib
# then pack the image
find . | cpio --create --format=newc > /boot/initrd-2.x.x-new.img

KfWiki : LinuxBoot

Useful boot time parameters

Grub2