Initrd for Raid-Boot

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If you are going to have Debian and a RAID on your Linkstation, you'll need a initrd for setting up the root filesystem.

Note: An initrd is only required of you have to set up some things to make the system boot, like starting the RAID, loading special modules or starting dm-crypt. If you do not have any of these needs, there is simply no need for an initrd, since the kernel will happily boot from hard disk.

This page is work in progress. While all relevant information is already present, the description is not yet complete. Please have a look here again in a few days.


Some ideas are taken from Debian Lenny on LS-WXL#Create an initrd.

What you need beforehand

This guide assumes you already opened your box and bootstrapped Debian as described in Debian Squeeze on 'V' and 'X' Series#Debootstrap. You may easily adapt this guide to other directory names or Debian versions. Or you may build the initrd on another device.

The Guide

Basic steps are:

  1. Create an empty file and mount it as a filesystem to be able to copy data into
  2. Into this mounted filesystem
    1. copy a prepared linuxrc script
    2. copy all binaries required by linuxrc plus the libraries required by the binaries
    3. create all required directories and device nodes required
  3. unmount the filesystem-file
  4. compress the file and make an initrd out of

For that you will

  1. Create and edit a text-file/script linuxrc.txt
  2. Create a text-file/script make-initrd.txt
  3. Copy both files to your LS
  4. Fix the line endings
  5. Run the script on your LS

Note: This Guide will install additional software into your debootstraped environment. Thus you may really want to create hddrootfs.buffalo.updated (as descriped in Debian_Squeeze_on_'V'_and_'X'_Series#Personalize the deboostrap) prior to building the initrd.

Required linuxrc.txt

Put this into a text-file called linuxrc.txt. You'll need to uncomment the line matching you root device.

#!/bin/busybox sh
# linuxrc - to be put into the inted

# Mount the /proc and /sys filesystems.
mount -t proc none /proc
mount -t sysfs none /sys

echo 'DEVICE /dev/sd??*' > /etc/mdadm/mdadm.conf
mdadm -Eb /dev/sd??* >> /etc/mdadm/mdadm.conf
mdadm -As --force

# use /dev/md1 as root
echo "0x901" > /proc/sys/kernel/real-root-dev
# use /dev/md2 as root
#echo "0x902" > /proc/sys/kernel/real-root-dev
# use /dev/sda6 as root
# echo "0x806" > /proc/sys/kernel/real-root-dev
# use /dev/sdb6 as root
# echo "0x822" > /proc/sys/kernel/real-root-dev

# Clean up.
umount /proc
umount /sys

File make-initrd.txt

Put this into a textfile called make-initrd.txt.

chroot debian-squeeze
# install
apt-get install --no-install-recommends busybox uboot-mkimage makedev

cd /tmp
rm initrd.gz

dd if=/dev/zero of=initrd bs=1k count=0 seek=3K
mke2fs -F -m 0 -b 1024 initrd
tune2fs -c0 -i0 initrd
mkdir INITRD
mount -o loop initrd INITRD

mkdir -p INITRD/{bin,lib,dev,etc/mdadm,proc,sbin}
cp -a /dev/{null,console,tty,sd{a,b,c,d}?,md*} INITRD/dev/
# ensure sd? and md exits, even if chrooted installation does not have them
cd INITRD/dev/
MAKEDEV sda sdb sdc sdd md
cd -

cp -aL /bin/busybox INITRD/bin/
cp -aL /sbin/mdadm /sbin/pivot_root INITRD/sbin
# find required libs and install into INITRD/lib
libs2install=$( ldd INITRD/*bin/* | grep -v "^INITRD/" | sed -e 's/.*=> *//'  -e 's/ *(.*//' | sort -u )
cp -aL $libs2install INITRD/lib
ln -s INITRD/bin/busybox INITRD/bin/sh

cp ../linuxrx.txt INITRD/linuxrc
chmod +x INITRD/linuxrc

umount INITRD
gzip initrd
mkimage -A arm -O linux -T ramdisk -C gzip -a 0x0 -e 0x0 -n initrd -d initrd.gz initrd.buffalo