Linux – How to boot GRUB2 so it mounts “root” on a different drive (remote server without KVM switch)

bootgrubgrub2linuxraid

Summary:
I created a copy of the root file system on a RAID array (with one disk on it), I tried to get the system to boot off that newly copied drive.

Background:

  • 2 disks of the same size in server (sda, sdb)
  • sdb not being used
  • I tried to transition the whole thing to a RAID1 mirror
  • current active partitions:
    • sda1 – boot
    • sda2 – swap
    • sda3 – root (mounted to "/")

I am not sure if it's possible to make a full RAID1 system able to boot off any of the drives, since I do not have KVM access (I can only tell them to help me out of a jam via a trouble ticket)

What has been done so far:

  • Created partitions on sdb to match sda
  • Created new raid1 array (with 1 disk)
  • /dev/md3 consists of 1 disk: /dev/sdb3
  • mounted /dev/md3 /mnt/md3
  • cp -ax / /mnt/md3
  • So now I have matching copies of data on / and /dev/md3

Can I just edit GRUB2 to make /dev/md3 the root? Everything should be OK, right?

I need to be absolutely sure, since I have no KVM access. I looked at /boot/grub/grub.cfg and I saw this entry:

menuentry 'Ubuntu, with Linux 2.6.32-28-generic-pae' --class ubuntu --class gnu-linux --class gnu --class os {
    recordfail
    insmod ext2
    set root='(hd0,1)'
    search --no-floppy --fs-uuid --set 18de6bbd-e46d-4f89-a2c9-fa2e7fa718b7
    linux   /vmlinuz-2.6.32-28-generic-pae root=/dev/sda3 ro   
    initrd  /initrd.img-2.6.32-28-generic-pae
}

So, note the "root=/dev/sda3" part. Can I just replace that with "root=/dev/md3", then reboot??

Best Answer

Assuming that the filesystem is directly on /dev/md3 (and you're not partitioning it or using LVM), yes, setting root=/dev/md3 is what you want. Things you should check though:

  1. Rebuild the initrd and check that it contains RAID support (you may need to coax it since the current root isn't on RAID, by adding raid1 to /etc/initramfs-tools/modules).
  2. Make sure that booting from a degraded array[0] is enabled (Ubuntu disables this by default: set BOOT_DEGRADED=true in /etc/initramfs-tools/conf.d/mdadm)
  3. Duplicate the GRUB entry and set a fallback entry in GRUB so that it (hopefully) boots the non-RAID disk if the RAID disk fails to boot (add a set fallback="2" line to /etc/grub.d/40_custom, where the number is the number of the menuentry counted from 0).
  4. Possibly also use grub-set-default --once instead of setting the default explicitly so that rebooting the machine is sufficient to boot the non-RAID disk.

[0]: Your array may actually not be degraded, but you probably want this anyway.

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