Linux – Getting Grub2 to recognize a Raid 10 boot/root

grubgrub2linuxmdadmsoftware-raid

I've been trying to get my raid to boot from grub2 for about 2 days now and I don't seem to be getting closer. The problem appears to be that it doesn't recognize my raid at all. It doesn't see (md0) etc. I'm not sure Why or how to change this. I'm using mdadm, 2 device (essentially a raid1) raid10,f2, which is currently degraded.

I have tried adding the raid and mdraid modules with grub install along with others. I've tried several variation on grub-install such as grub-install --debug --no-floppy --modules="biosdisk part_msdos chain raid mdraid ext2 linux search ata normal" /dev/md0

I've been searching the net for an answer to what I haven't done but no luck. On my other drive which I plan on removing the raid is initialized and mounted fine on boot, but it's not the boot/root for that setup. My grub.cfg isn't recognized by grub since it can't read the raid partition so I'm not posting that. md0 is not listed in my /boot/grub/device.map.

Best Answer

Don't forget GRUB2 is still in a development stage, so if you want to use it... it is at your own risk.

Given the above, I think you'd be best advised to get the latest source and compile to see if that fixes any issues you may be seeing. You should be using version 1.9.6 at least, as the docs say, and make sue your raid is built using the v0.90 mdadm metadata superblock.

For example, to create a "-l 10" RAID1 device for the /boot file system, with "-n 4" disks and "-x 0" spares:

mdadm --create -e 0.90 -n 4 -x 0 -l 1 /dev/md2 /dev/sda2 /dev/sdb2 [/dev/sdc2 /dev/sdd2 ...]

And in testing with mdadm v3.2.3, grub 1.99-21ubuntu3, only the boot file system needed metadata 0.90, not the root. And raid10 didn't work after a disk was removed, so example is raid1 above. (another question related to raid10 here)

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For anybody else who ends up suffering the error 15 grief that I did, it turns out that the device naming scheme in grub (hd0, hd1, hd2...) ended up being different between when grub boots and when grub is running after the system is up and running. I spent a week with root (hd2,0) because that's what grub told me the drive I wanted was called. But when I dropped to the grub shell on bootup I was surprised to find out that what was hd2 when the machine is up, is hd1 on boot. So I changed the menu.lst to use root (hd1,0) and it started working. I hope to save somebody else lots of hair pulling with that one.

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You need to install GRUB to the MBR of both drives, and you need to do it in a way that GRUB considers each disk to be the first disk in the system.

GRUB uses its own enumeration for disks, which is abstracted from what the Linux kernel presents. You can change which device it thinks is the first disk (hd0), by using a "device" line in the grub shell, like so:

device (hd0) /dev/sdb

This tells grub that, for all subsequent commands, treat /dev/sdb as the disk hd0. From here you can complete the installation manually:

device (hd0) /dev/sdb
root (hd0,0)
setup (hd0)

This sets up GRUB on the first partition of the disk it considers to be hd0, which you've just set as /dev/sdb.

I do the same for both /dev/sda and /dev/sdb, just to be sure.

Edited to add: I always found the Gentoo Wiki handy, until I did this often enough to commit it to memory.

Best Answer

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