Debian Soft Raid

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

Intro

Here are just some quick notes to update this page as now everything can be done relatively easily with just mdadm.
A very interesting page: http://www200.pair.com/mecham/raid/raid1-degraded-etch.html
It helps getting familiar with the concepts.

Creating degraded array

Here we're missing /dev/hda3 so we start with only /dev/hdd3: 

mdadm --create /dev/md0 --raid-devices=2 --level=raid1 missing /dev/hdd3

And to get it properly set after reboot, we can create mdadm.conf: 

echo DEVICE partitions > /etc/mdadm/mdadm.conf
mdadm --examine --scan >> /etc/mdadm/mdadm.conf

Edit and check the file manually...
Then prepare for reboot

Preparing for reboot

During the configuration, every time we want to reboot, we've to make sure to:

  • have the intended partition layout (fdisk)
  • have the intended partitions mounted on the intended mountpoints (mount)
  • have /etc/fstab reflecting the current mounts
  • have /etc/mdadm/mdadm.conf reflecting the current Raid arrays
  • have an initrd reflecting the current situation:
dpkg-reconfigure linux-image-...

This is true also after having added the 2nd partition to a raid1 array

Diagnostic

Some useful commands to inspect the raid situation: 

# From what's currently assembled:
cat /proc/mdstat
mdadm --detail --scan
mdadm --detail /dev/md1
# From what's available as raid partitions
mdadm --examine --scan
mdadm --examine /dev/hda5

Repairing a degraded array

Later when we'll be able to integrate /dev/hda3 we'll do: 

mdadm /dev/md0 --add /dev/hda3

Then prepare for reboot

/boot

Here is one example:
Initially /boot was not on raid1 but as now it's possible with grub I did so.
I had /boot=/dev/hda1 and /boot-img=/dev/hdd1 and I did sth like: 

umount /dev/hdd1
mdadm --create /dev/md1 --raid-devices=2 --level=raid1 missing /dev/hdd1
mount /dev/md1 /boot-img
cp -a /boot/* /boot-img
umount /boot
umount /boot-img
mdadm /dev/md1 --add /dev/hda1
vi /etc/fstab #/dev/md0 /boot ... and delete /boot-img entry
grub-install "(hd1)"
grub-install "(hd0)"
dpkg-reconfigure linux-image-...
mdadm --examine --scan |grep md1>> /etc/mdadm.conf
reboot

Changing super-minor

During the process I wanted to change the number associated to an array (/dev/mdX):
Suppose /dev/md3 = /dev/hda5+/dev/hdd5
And we want /dev/md2 = /dev/hda5+/dev/hdd5 

mdadm --stop /dev/md3
mdadm --assemble /dev/md2 /dev/hda5 /dev/hdd5

Then we've to update again /etc/mdadm/mdadm.conf

Grub

I moved from lilo to grub and installed the first stage on both drives: 

grub-install "(hd1)"
grub-install "(hd0)"

I edited /etc/kernel-img.conf to have the hooks for Debian kernel automatic installation: 

postinst_hook = /usr/sbin/update-grub
postrm_hook = /usr/sbin/update-grub
do_bootloader = no

I edited /boot/grub/menu.lst and added a fallback directive: 

default         0
fallback        1

When executing update-grub it creates the following entry: 

title           Debian GNU/Linux, kernel 2.6.21-2-vserver-k7
root            (hd0,0)
kernel          /vmlinuz-2.6.21-2-vserver-k7 root=/dev/md0 ro 
initrd          /initrd.img-2.6.21-2-vserver-k7
savedefault

And I added manually the following one: 

title           Debian GNU/Linux, kernel 2.6.21-2-vserver-k7 (hd1)
root            (hd1,0)
kernel          /vmlinuz-2.6.21-2-vserver-k7 root=/dev/md0 ro 
initrd          /initrd.img-2.6.21-2-vserver-k7
savedefault

But I don't know how to make it happening automatically via update-grub, anyway in case of a failure of the first harddrive I'll probably have to reboot manually and Grub is rich enough to allow reconfiguration on-the-fly.
That's the major reason why I moved away from lilo.

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

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
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