How to correctly install GRUB on a soft RAID 1?

Question

In my setup, I have two disks that are each formatted in the following way:

(GPT)
1)  1MB   BIOS_BOOT
2)  300MB LINUX_RAID   
3)  *     LINUX_RAID   

The boot partitions are mapped in /dev/md0, the rootfs in /dev/md1. md0 is formatted with ext2, md1 with XFS. (I understand that formatting has to be done on the md devices and not on sd - please tell me if this is wrong).

How do I setup GRUB correctly so that if one drive fails, the other will still boot? And by extension, that a replacement drive will automatically include GRUB, too? If this is even possible, of course.

Answer 1

If the two disks are /dev/sda and /dev/sdb, run both grub-install /dev/sda and grub-install /dev/sdb. Then both drives will be able to boot alone.

Make sure that your Grub configuration doesn't hard-code disks like (hd0), but instead searches for the boot and root filesystems' UUIDs.

I'm not aware of support in Grub to declare two disks as being in a RAID-1 array so that grub-install would automatically write to both. This means you'll need to run grub-install again if you replace one disk; it's one more thing to do in addition to adding new members to the RAID arrays.

Answer 2

I've done this on quite a few servers over the past while and found issues sometimes, usually what I do is run the following

sudo grub-install /dev/sdb
sudo update-grub /dev/sdb

The problems I've run into are usually displayed with error messages like

update-grub … update-grub failed with no such disk …

To resolve this i've run

sudo mv /boot/grub/device.map /boot/grub/device.map.old 
sudo update-grub 

and this seems to have resolved the issue - as a test in some cases I have removed /sdv/sda and just used /dev/sdb to boot from but this is not ideal and most likely not possible in most production environments!

Answer 3

My experience installing CentOS 5 and CentOS 6 is that during the installation phase I configured the RAID-1 at that time - the installation does give you the choice to do this. What transpired was the following: 1. Defined /dev/sda1 as 1 GB as a RAID partition 2. Defined /dev/sdb1 as 1 GB as a RAID partition 3. Created RAID device /dev/md0 made up of /dev/sda1 and /dev/sdb1 4. Created the /boot partition using /dev/md0 5. Created additional partitions using steps 1 - 3. 6. Implemented LVM on partitions created in step 5 by following instructions.

After the initial system boot, in the grub.conf I altered the entry /dev/sda1 to /dev/md0. I then copied the entry the splashimage line the has (hd0,0) and added a # at the beginning and changed (hd0,0) to (hd1,0). Looking at the above answers showing how to install grub to /dev/sdb follow them. Modify the fstab so that the boot mount point uses /dev/md0 (by doing this, when you do a yum update a there is something updated on the /boot partition it will be written to both /dev/sda1 and /dev/sdb1). If you have a /dev/sda hard drive failure, you would have to boot using the rescue and comment out the (hd0,0) line and uncomment the (hd1,0) line. Your system should be bootable