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.


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 described in Debian_Squeeze_on_'V'_and_'X'_Series#Personalize the deboostrap) prior to building the initrd.

Detailed description

  1. Create a text-file/script linuxrc.txt with the content shown below
  2. Uncomment the line echo "0x..." > ... which matched the device to be used as root
  3. Create a text-file/script make-initrd.txt with the content shown below
  4. Change the DEBOOTSTRAP=... line at the top to match your debootstraped environment.
  5. Copy both files to your LS.
  6. Now log into your LS and continue there.
  7. Find the place on your LS where you copied the files to:
    If you used the share "share", this should be either /mnt/disk1/share or /mnt/array1/share
    If you used scp, winscp or so, you'll know where :-)
  8. Run the script on your LS. Assuming you copied the file to /mnt/disk1/share, the command will be
sh /mnt/disk1/share/make-initrd.txt
  1. The initrd file will be initrd.buffalo in your debootstraped environment.

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 initrd

# 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.

# Licensed under GPL v3 or later
# make-initrd.txt -- This script will be executed on the LS

# edit this line as needed

if [ "$1" != chrooted ] ; then
  if [ ! -d $DEBOOTSTRAP ] ; then
    echo "debootstrap directory not found, please edit DEBOOTSTRAP in $0"
    exit 1
  DIR=$( cd $(dirname $0) ; pwd)
  cp "$0" "$DIR/linuxrc.txt" $DEBOOTSTRAP
  scriptname=$( basename "$0" )
  # rerun script in chrooted environment
  chroot $DEBOOTSTRAP bash -x "$scriptname" chrooted
  exit $?
# install some required stuff
apt-get install --no-install-recommends busybox uboot-mkimage makedev dos2unix

cd /tmp
[ -f initrd.gz ] && 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 ../linuxrc.txt INITRD/linuxrc
dos2unix 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