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Old 09-03-2006, 09:41 PM   #1
Registered: Sep 2005
Location: Boynton Beach, FL
Distribution: Slackware
Posts: 801

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How to make an initrd for dmraid

I've just recently gone through the confusing and difficult process of getting Linux 2.6.17 running with Slackware and my Promise FastTrack 374 (PDC20378).

Since there is no Promise RAID driver for version 2.6 I had to use dmraid to find and map the RAID sets with the device-mapper in Linux.

I had another problem because the standard Linux SATA driver for the PDC20378 did not support the Parallel ATA (PATA) port. I posted another thread about solving that problem here.
Solution for Promise FastTrack PDC20378 PATA or RAID with 2.6.17 kernel

Once I got dmraid to work, my problem was how to make a working initrd RAM disk image to start up Slackware. It turned out to be quite a bit more complicated than I expected.

I had to create a new "linuxrc" script to map the RAID volumes and mount the root filesystem. I also had to write a script to include additional files in the initrd RAM disk image.

Here is the new "linuxrc" script.
# Boot parameters:
# real_root=rootdev	Replace "rootdev" with real root device
# root_fs=rootfs	Replace "rootfs" with real root filesystem type
# real_init=rl		Repalce "rl" with a run level, EX: 4
# single		Start in single user mode, I.E. "real_init=-s"
# auto			Pass auto flag to init, I.E. "real_init=-a"



parse_opt() {
	case "$1" in
			echo "$1" | cut -f2 -d=

# Remount ram disk read/write
mount -n -t ext2 -o remount,rw /dev/ram0 /
# Mount /proc:
mount -n -t proc proc /proc
# Mount system filesys
mount -n -t sysfs sysfs /sys 

# Get default root device and file system
if [ -r /rootdev ]; then
  ROOTDEV=`cat /rootdev`
if [ -r /rootfs ]; then
  ROOTFS=`cat /rootfs`

# Scan CMDLINE for any specified real_root etc.
CMDLINE=`cat /proc/cmdline`
for x in ${CMDLINE}
	case "${x}" in
			ROOTDEV=`parse_opt "${x}"`
			ROOTFS=`parse_opt "${x}"`
			REAL_INIT=`parse_opt "${x}"`

# Change root filesystem type to a mount parameter

# Load kernel modules:
if [ ! -d /lib/modules/`uname -r` ]; then
  echo "No kernel modules found for Linux `uname -r`."
elif [ -x ./load_kernel_modules ]; then # use load_kernel_modules script:
  echo "/boot/initrd.gz:  Loading kernel modules from initrd image:"
  . ./load_kernel_modules
else # load modules (if any) in order:
  if ls /lib/modules/`uname -r`/*.*o 1> /dev/null 2> /dev/null ; then
    echo "/boot/initrd.gz:  Loading kernel modules from initrd image:"
    for module in /lib/modules/`uname -r`/*.*o ; do
      insmod $module
    unset module

# Initialize LVM:
if [ -x /sbin/vgscan ]; then
  vgscan --mknodes
  sleep 10
  vgchange -ay

# Find any hardware RAID volumes
dmraid -ay

# If /rootdev isn't set, we'll have to trust exiting to work here.
# It's harder to clean up the initrd without a pivot_root,
# so it's a good idea to set rootdev (and rootfs) properly.
if [ "$ROOTDEV" = "" ]; then
  exit 0

# Switch to real root partition:
mount -n -o ro $ROOTFS $ROOTDEV /mnt
if [ ! "$ERR" = "0" ]; then
  echo "ERROR:  mount returned error code $ERR.  Trouble ahead."
  exit $ERR
unset ERR
# OK, in case there's no initrd directory:
if [ ! -d /mnt/initrd ]; then
  mount -n -o remount,rw $ROOTFS $ROOTDEV /mnt
  mkdir -p /mnt/initrd
  mount -n -o remount,ro $ROOTFS $ROOTDEV /mnt
umount /sys
umount /proc
cd /mnt
# bye now
echo "/boot/initrd.gz:  OK exiting"
pivot_root . initrd
exec <dev/console >dev/console 2>&1
exec /sbin/init ${AUTO_OPTION} ${REAL_INIT}
exit 0
Here is the script that creates the initrd image. It uses the existing Slackware "mkinitrd" to do part of the work.

NOTE: Things you are likely to need to change are in bold.
LINUXVER="2.6.17"		# Linux modules version
CLIBVER="2.3.5"			# C library version
ROOTFS="/boot/initrd-tree"	# Location of root filesystm
# Get most of the needed programs form the normal mkinitrd
mkinitrd -k $LINUXVER -c -r /dev/mapper/pdc_bbbffffihj3 -f ext3
# Create directories
for dir in \
   "bin" "dev" "etc" "mnt" "proc" "sbin" "sys" "usr" \
   "tmp" "var" "var/lock" "var/log" "var/run" "var/tmp"
   if [ ! -d "$ROOTFS/$dir" ] ; then
      mkdir -p "$ROOTFS/$dir"
# Create devices
pushd "$ROOTFS/dev" > /dev/null
# Remove existing devices
rm -Rf *
# Required devices
mknod -m u=rw,g=,o= console c 5 1
chown root:tty console 
mknod -m u=rw,g=rw,o= ram0 b 1 0
chown root:disk ram0 
mknod -m u=rw,g=r,o= mem c 1 1
chown root:kmem mem 
mknod -m u=rw,g=r,o= kmem c 1 2
chown root:kmem kmem 
mknod -m u=rw,g=rw,o=rw null c 1 3
chown root:root null 
mknod -m u=rw,g=rw,o=rw zero c 1 5
chown root:root zero 
mkdir vc
chmod u=rwx,g=rx,o=rx vc
chown root:root vc
mknod -m u=rw,g=rw,o= vc/1 c 4 1
chown root:tty vc/1 
ln -s vc/1 tty1
# IDE Disks (up to 20) max 64 partitions per disk
if [ $drives -gt 0 ] ; then
   majors=( 3 22 33 34 56 57 88 89 90 91)
   for drv in `seq 0 $(($drives-1))` ; do
      minor=$(( ($drv%2) * 64 ))
      mknod -m u=rw,g=rw,o= $dev b $major $minor
      chown root:disk $dev
      if [ $partitions -gt 0 ] ; then 
         for i in `seq 1 $partitions` ; do
            mknod -m u=rw,g=rw,o= $dev$i b $major $(($minor+$i)) 
            chown root:disk $dev$i
# SCSI Disks (0 to 127) max 16 partitions per disk
if [ $drives -gt 0 ] ; then
   majors=( 8 65 66 67 68 69 70 71)
   for drv in `seq 0 $(($drives-1))` ; do
      if [ $drv -lt 26 ] ; then
      minor=$(( ($drv%16) * 16 ))
      mknod -m u=rw,g=rw,o= $dev b $major $minor
      chown root:disk $dev
      if [ $partitions -gt 0 ] ; then 
         for i in `seq 1 $partitions` ; do
            mknod -m u=rw,g=rw,o= $dev$i b $major $(($minor+$i)) 
            chown root:disk $dev$i
# Floppy disks A and B
for i in `seq 0 1` ; do
   mknod -m u=rw,g=rw,o= fd$i b 2 $i 
   chown root:floppy fd$i
# Device mapper for "dmraid"
mkdir mapper
chmod u=rwx,g=rx,o=rx mapper
chown root:root mapper
mknod -m u=rw,g=rw,o= mapper/control c 10 63
chown root:root mapper/control
# Done with devices
popd > /dev/null
# Copy scripts and programs
cp -p linuxrc "$ROOTFS"
chmod u=rwx,g=rx,o=rx "$ROOTFS/linuxrc"
cp -p /sbin/dmraid "$ROOTFS/sbin"
cp -p /bin/cut "$ROOTFS/bin"
for lib in \
   "" "" \
   "" "" \
   "ld-$" "libc-$"
   if [ -e "/lib/$lib" ] ; then
      cp -Pp "/lib/$lib" "$ROOTFS/lib/$lib"
      echo "Library file not found \"/lib/$lib\""
      exit 1
# Make the compressed image file
I had no luck at all getting "lilo" to work with a RAID device created by dmraid. I was able to make "grub" work, but I had to do a "native" installation. To do a "native" installation you create a bootable floppy or bootable CD. After booting the floppy or CD, you can use "grub" to install itself to the RAID array. It makes calls to the BIOS, which is why it can correctly access the RAID array. I was not able to install "grub" from Linux, though there may be a patch for that. I didn't bother looking since "grub" only has to be installed once (unlike lilo).

If you change the "grub" boot menu file "/boot/grub/menu.lst" it is not necessary to re-install "grub". Also, if you make a copy of the Linux/grub boot block for Windows XP to use, you don't have to update that file when you change boot entries (unlike lilo).

Here is an example boot menu using grub and the initrd image.

NOTE: Indented text is a wrapped line, not a separate line.
default 0
timeout 30

title Linux
root (hd0,2)
kernel /boot/vmlinuz vga=773 auto root=/dev/ram0 load_ramdisk=1 ramdisk_size=4096
 real_root=/dev/mapper/pdc_bbbffffihj3 root_fs=ext3 init=/linuxrc
initrd /boot/initrd.gz

title Linux Single User Mode
root (hd0,2)
kernel /boot/vmlinuz single root=/dev/ram0 load_ramdisk=1 ramdisk_size=4096
 real_root=/dev/mapper/pdc_bbbffffihj3 root_fs=ext3 init=/linuxrc
initrd /boot/initrd.gz

title Linux Old Version
root (hd0,2)
kernel /boot/vmlinuz.old single root=/dev/ram0 load_ramdisk=1 ramdisk_size=4096
 real_root=/dev/mapper/pdc_bbbffffihj3 root_fs=ext3 init=/linuxrc
initrd /boot/initrd.old.gz

title Linux Known Good
root (hd0,2)
kernel /boot/vmlinuz.kgd single root=/dev/ram0 load_ramdisk=1 ramdisk_size=4096
 real_root=/dev/mapper/pdc_bbbffffihj3 root_fs=ext3 init=/linuxrc
initrd /boot/initrd.kgd.gz

title Windows XP Pro
rootnoverify (hd0,0)
chainloader +1
The two key points when using the "initrd" image are these. You must specify the kernel parameter "root=/dev/ram0" since Linux cannot identify the RAID device even to tell that it is not the same as the boot device (RAM disk). You must specify the kernel parameter "init=/linuxrc" to make Linux execute the script even though the boot device (/dev/ram0) is the same as the root device (/dev/ram0). Normally when those two devices are the same, Linux does not run any "linuxrc" script.

You need the "load_ramdisk=1" to load the intird image, and you will probably want "ramdisk_size=" to specify the size of the RAM disk needed. All of the initrd images that I've built (without modules) are around 3 megabytes.

The script takes some additional parameters. You can override the root device given to the "mkinitrd" command by including the "real_root=" option. You can override the root filesystem type given to "mkinitrd" by including "root_fs=". You can specify a run level for "init" by including the "real_init=" option followed by a number. The "single" and "auto" options behave in the same way as normal, specifying single-user mode and indicating an automatic boot was done.

Finally, there are some messy details to deal with in the Linux init script, "rc.S". Unfortunately when "udev" is started by the script, it will recreate the entire list of devices. That is guaranteed to wipe out the contents of "/dev/mapper" even if you happen to copy them from the RAM disk to the root filesystem. It is necessary to leave the RAM disk mounted, and then copy the RAID device names after "udev" is done. That also means some of the "rc.S" script will have to refer to devices as "/initrd/dev/mapper/devicename" rather than "/dev/mapper/devicename". Here are the key places that must be changed in "rc.S".

# Initialize udev to manage /dev entries for 2.6.x kernels:
if [ -x /etc/rc.d/rc.udev ]; then
  if ! grep -w nohotplug /proc/cmdline 1> /dev/null 2> /dev/null ; then

# Enable swapping:
# NOTE: Have to wait until dmraid devices are found!
#/sbin/swapon -a
# Check the root filesystem:
if [ ! $READWRITE = yes ]; then
  if [ ! -r /etc/fastboot ]; then
    echo "Checking root filesystem:"
    /sbin/fsck $FORCEFSCK -C -a /initrd/dev/mapper/pdc_bbbffffihj3
Add the following code right after the end of the filesystem checking. I included the last line of that "if" statement. The new code is between the "fi # Done checking..." line and the "# Any /etc/mtab that exists..." line.

fi # Done checking root filesystem

# Copy device mapper devices from initrd
for file in /initrd/dev/mapper/*; do
  short=`basename $file`
  if [ "$short" != 'control' ]; then
    rm -rf /dev/mapper/$short
    cp -dpR $file /dev/mapper
# Might have found another swap device
/sbin/swapon -a

# Any /etc/mtab that exists here is old, so we delete it to start over:
/bin/rm -f /etc/mtab*
# Remounting the / partition will initialize the new /etc/mtab:
/sbin/mount -w -o remount /

Last edited by Erik_FL; 09-04-2006 at 02:24 PM.
Old 09-05-2006, 08:47 AM   #2
Registered: Jan 2005
Location: Midwest USA, Central Illinois
Distribution: SlackwareŽ
Posts: 11,397
Blog Entries: 3

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I will look a little deeper.


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