Debian Soft Raid

How to switch a Debian system on software RAID 1 (mirroring)

Here is how to switch your root (/) filesystem on RAID 1:

Have 2 same disks, let's say hda and hdc (yep, put them on different IDE controllers!)
Create a specific small partition for /boot at the very beginning of the first disk (hda) because some (most?) bootloaders don't understand RAID.
Mine is hda1->/boot hda2->swap hda3->/
Install Debian as usual on hda
Format hdc with a same partition as the / on hda, it'll be the RAID mirror of /
My second disk (same vendor, same size) didn't have the same geometry (C/H/S) but after dd if=/dev/zero of=/dev/hdc bs=512 count=1 fdisk used the same geometry...
Now it is the same as hda: hdc1->/boot-img hdc2->swap hdc3->/

apt-get install initrd-tools raidtools2 mdadm (decline offer to start RAID at boot time)

Create /etc/raidtab:

raiddev                 /dev/md0
raid-level              1
nr-raid-disks           2
nr-spare-disks          0
persistent-superblock   1
device                  /dev/hdc3
raid-disk               0
device                  /dev/hda3
failed-disk             1

So the actual / partition is declared as "broken" for the RAID
Create the RAID:

mkraid /dev/md0 (it will say disk1: failed)
mkfs.ext3 /dev/md0
mount -v /dev/md0 /mnt/root

Copy the / content:

cd /
find . -xdev | cpio -pm /mnt/root

Prepare to reboot on the RAID:
Edit /etc/mkinitrd/mkinitrd.conf:

ROOT=probe -> ROOT=/dev/md0

mkinitrd -o /boot/initrd.img-raid

Edit /mnt/root/etc/fstab:

/dev/md0 / ext3 defaults,errors=remount-ro 0 1

Edit /etc/lilo.conf:

image=/boot/vmlinuz...
label=LinuxRAID
root=/dev/md0
read-only
initrd=/boot/initrd.img-raid

umount /dev/md0
raidstop /dev/md0
lilo
reboot

Restore the "broken" RAID:

cat /proc/mdstat: we see only one disk
raidhotadd /dev/md0 /dev/hda3

Now the system is synchonizing the "new" RAID partition

watch cat /proc/mdstat

Prepare for next reboot:
Edit /etc/fstab:

failed-disk -> raid-disk
mkinitrd -o /boot/initrd.img-raid
lilo
reboot

Automatic watching

dpkg-reconfigure mdadm -> accept mdadm survey daemon and give user who should get alert emails

Simulating RAID 0 (striping) for the swap:
Simply give the same priority to both swap partitions:

/dev/hda2 swap swap defaults,pri=1 0 0
/dev/hdc2 swap swap defaults,pri=1 0 0

Troubleshooting

Don't simply dd the MBR from hda to hdc otherwise lilo will complain about a timestamp error, actually that's because now both disks got the same ID number.
You can mount the initrd image to check if it contains well the RAID instructions:

mount /boot/initrd.img-raid /mnt/disk -o loop,ro

/mnt/disk/script should contain a last line with mdadm
check the end of the line, first time only /dev/hdc3 is mentioned, second time /dev/hda3 should also be present (or the system will be mounted again in degraded mode)
If reboot fails: Boot on a Knoppix

modprobe raid1
mdadm --assemble /dev/md0 /dev/hdd3 /dev/hda3
mount /dev/md0 /mnt/xxx
chroot /mnt/xxx
mount also /proc /boot etc
mkinitrd -o /boot/initrd.img-raid <kernel version>

Useful commands

• Diagnostics

mdadm -D /dev/mdXX
mdadm -E /dev/hdXX
cat /proc/mdstat

• Add

mdadm /dev/mdXX -a /dev/hdXX