CoreOS mdadm RAID1 for ROOT

README.md

CoreOS mdadm RAID1 for ROOT

• Setup SoftwareRAID for the CoreOS root partition without data-loss.
• Only for ext4!
• The Trick is to use the right volume label and fs-types :)
• We assume CoreOS is on /dev/sdaX and the second RAID Device is /dev/sdb
• Drawback: only / gets mirrored, Node goes down when sda fails

1. Boot into any recovery system like Grml

2. If not already done: install CoreOS

3. Backup /dev/sda9

   mkdir -p /root/sda9
   mount /dev/sda9 /mnt || exit 1
   rsync -a /mnt/* /root/sda9
   umount /dev/sda9

4. Prepare the partition layout

    # delete sda9
    sgdisk /dev/sda --delete=9
   
    # create new /dev/sda9 partition (max size)
    START=`sgdisk /dev/sda -f`
    END=`sgdisk /dev/sda -E`
    sgdisk /dev/sda --new=9:$START:$END --type=9:fd00
    sleep 0.5
    partprobe /dev/sda
   
    # Remove partition table from sdb
    sgdisk --clear -g /dev/sdb || exit 1
   
    # create a partition sdb9 with the size of sda9
    PARTDATA=( $(sgdisk -i 9 /dev/sda | grep 'Partition size' || exit 1) )
    SECTORS=$(expr ${PARTDATA[2]} + 2048)
    sgdisk /dev/sdb -a 2048 --new=9:2048:$SECTORS --type=9:fd00 || exit 1
    sleep 0.5
    partprobe /dev/sdb

5. Create the RAID

    mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda9 /dev/sdb9
    mkfs.ext4 -I 128 -L ROOT /dev/md0

6. Copy the Data

    mount /dev/md0 /mnt
    "rsync -a /root/sda9/* /mnt" || exit 1
    umount /mnt

7. Wait until resync is finished

   watch -n 1 cat /proc/mdstat

8. reboot!

coreos doesn't seem to init mdadm on boot, as a result the boot fails and you're dropped into the dracut shell (after a large delay as it tries to look for ROOT by-label which isn't there. It should just be returning as soon as it isn't found, though...).

Running a mdadm --assemble --scan on that shell and exiting out allows the normal boot to continue.

Any way to automate this?

Edit (Answering my own question):

Pass rd.auto to the kernel with https://coreos.com/os/docs/latest/other-settings.html#adding-custom-kernel-boot-options

Thanks to kayrus@freenode for assisting me with this.