Root RAID HOWTO cookbook

8. Appendix A. - Bohumil Chalupa's md0 shutdown

Bohumil Chalupa's post to the linux raid list on the work around for the raid1 + 5 mdstop problem. His solution does not address the possibility of the raid device being corrupt at shutdown. So I have added a simple status comparison to a good reference status at boot. This allows the operator to intervene if something is wrong with a disk in the array. The description of this is in the main body of this document.

> From: Bohumil Chalupa <bochal at apollo.karlov.mff.cuni.cz>
>  
> I can now boot initrd and use linuxrc to start the RAID1 array,
> then successfully switch root to /dev/md0.
> 
> I don't know, however, any way how to cleanly _stop_ the array.
 
Well. I have to answer myself :-)
 
> Date: Mon, 29 Dec 1997 02:21:38 -0600 (CST)
> From: Edward Welbon <welbon at bga.com>
> Subject: Re: dismounting root raid device
> 
> For md devices other than raid0, there is probably state that needs to
> be saved that is only known once all writes have completed.  Such state
> of course can't be saved to root once it is mounted readonly.  In that
> case, you would have to be able to mount a writeable filesystem "X"
> on the readonly root and be able to write to "X" (I recall doing this
> during "rescue" operations, but not as an automated procedure).
> 
> The filesystem "X" would presumably be a boot device from which the raid
> (during linuxrc exection via initrd) would pickup it's initial state from.
> Fortunately raid0 isn't required to write out any state (though it would
> be pleasant to be able to write the check sums to mdtab after an mdstop).
> Eventually, I will fiddle with this but it doesn't seem difficult though
> the "devil" is always in the "details".
 
Yes, that's it.
I had this idea in mind for some time already, but had no time to try it.
Yesterday I did, and it works. 

With my RAID1 (mirror), I don't save any checksums or raid superblock data.
I only save an information on the "real" boot partition, that the root md
volume was remounted readonly during shutdown. Then, during boot, the
linuxrc script runs mkraid --only-superblock  when it finds this
information; otherwise, it runs ckraid.
This means, that the raid superblock information is not updated during
shutdown; it's updated at the boot time. 
It is not very clean, I'm afraid,  :-(   but it works.

I'm using Slackware and initrd.md by Edward Welbon to boot the root raid
device. 
As far as I remember now, the only modified files are
mkdisk and linuxrc, and /etc/rc.d/rc.6 shutdown script.
And lilo.conf, of course.

I'm appending the important parts.

Bohumil Chalupa

--------------- my.linuxrc follows -----------------
#!/bin/sh
# we need /proc
/bin/mount /proc 
# start up the md0 device. let the /etc/rc.d scripts get the rest of them
# we should do as little as possible here
# ________________________________________
# root raid1 shutdown test & recreation
# /start must be created on the rd image in my.mkdisk
echo "preparing md0: mounting /start"
/bin/mount /dev/sda2 /start -t ext2
echo "reading saved md0 state from /start"
if [ -f /start/root.raid.ok ]; then
 echo "raid ok, modyfying superblock"
 rm /start/root.raid.ok
 /sbin/mkraid /etc/raid1.conf -f --only-superblock
else
 echo "raid not clean, runing ckraid --fix"
 /sbin/ckraid --fix /etc/raid1.conf
fi
echo "unmounting /start"
/bin/umount /start
# _________________________________________
#
echo "adding md0 for root file system"
/sbin/mdadd /dev/md0 /dev/sda1 /dev/sdb1 
echo "starting md0"
/sbin/mdrun -p1 /dev/md0
# tell kernel we want to switch to /dev/md0 as root device, the 0x900 value
# is arrived at via 256*major_device_number + minor_device number.
echo "setting real-root-dev"
/bin/echo 0x900>/proc/sys/kernel/real-root-dev
#  unmount /proc so that the ram disk can be deallocated.
echo "unmounting /proc"
/bin/umount /proc
/bin/echo "We are hopefully ready to mount /dev/md0 (major 9, minor 0) as
root"
exit
--------------- end of my.linuxrc ----------------------------------


----------- extract from /etc/rc.d/rc.6 follows -----------------
  # Turn off swap, then unmount local file systems.
  echo "Turning off swap."
  swapoff -a
  echo "Unmounting local file systems."
  umount -a -tnonfs
  # Don't remount UMSDOS root volumes:
  if [ ! "`mount | head -1 | cut -d ' ' -f 5`" = "umsdos" ]; then
    mount -n -o remount,ro /
  fi

  # Save raid state
  echo "Saving RAID state"
  /bin/mount -n /dev/sda2 /start -t ext2
  touch /start/root.raid.ok
  /bin/umount -n /start

-------------- end of excerpt from rc.6 ------------------------


------------------ part of my.mkdisk follows ----------------------
#
#  now we have the filesystem ready to be populated, we need to 
#  get a few important directories.  I had endless trouble till
#  I created a pristine mtab.  In my case, it is convenient that
#  /etc/mdtab is copied over, this way I can activate md with
#  a simple "/sbin/mdadd -ar" in linuxrc.
#
cp -a $ROOT/etc $MOUNTPNT 2>cp.stderr 1>cp.stdout
rm -rf $MOUNTPNT/etc/mtab
rm -rf $MOUNTPNT/etc/ppp*
rm -rf $MOUNTPNT/etc/termcap
rm -rf $MOUNTPNT/etc/sendmail*
rm -rf $MOUNTPNT/etc/rc.d
rm -rf $MOUNTPNT/etc/dos* 
cp -a $ROOT/sbin $ROOT/dev $ROOT/lib $ROOT/bin $MOUNTPNT 2>>cp.stderr
1>>cp.stdout
# _____________________________________________________________________
#  RAID: will need mkraid and ckraid
cp -a $ROOT/usr/sbin/mkraid $ROOT/usr/sbin/ckraid $MOUNTPNT/sbin
2>>cp.stderr 1>>cp.stdout
# ---------------------------------------------------------------------
#  it seems that init wont come out to play unless it has utmp.   this can
#  probably be pruned back alot.  no telling what the real bug was 8-).
#
mkdir $MOUNTPNT/var $MOUNTPNT/var/log $MOUNTPNT/var/run $MOUNTPNT/initrd
touch $MOUNTPNT/var/run/utmp $MOUNTPNT/etc/mtab
chmod a+r $MOUNTPNT/var/run/utmp $MOUNTPNT/etc/mtab
ln -s /var/run/utmp $MOUNTPNT/var/log/utmp
ln -s /var/log/utmp $MOUNTPNT/etc/utmp
ls -lstrd $MOUNTPNT/etc/utmp $MOUNTPNT/var/log/utmp $MOUNTPNT/var/run/utmp
#
#  since I wanted to change the mount point, I needed this though
#  I suppose that I could have done a "mkdir /proc" in linuxrc.
#
mkdir $MOUNTPNT/proc
chmod 555 $MOUNTPNT/proc
#
#  ------------------------------------------------------
#  we'll mount the real boot device to /start temporarily
#  to check the root raid state saved at shutdown time
#
mkdir $MOUNTPNT/start
#  -------------------------------------------------------
#
#  need linuxrc  (it is, after all, the point of this exercise).
#
if [ -x ./my.linuxrc ]; then
  cp -a ./my.linuxrc $MOUNTPNT/linuxrc
  chmod 777 $MOUNTPNT/linuxrc
else
   ln -s /bin/sh $MOUNTPNT/linuxrc
fi
#
----------------- part of my.mkdisk ends -----------------