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GRUB2, GPT, mdadm & LVM

From: Sander Smeenk
Subject: GRUB2, GPT, mdadm & LVM
Date: Thu, 18 Jul 2013 13:56:21 +0200
User-agent: Mutt/1.5.21 (2010-09-15)


I'm having issues getting GRUB to boot from my setup. It drops to an
rescue shell stating it can't find the LVM root for my system. Similar
'stacked' setups (md raid5/6 lvm, root on lvm) work fine: difference
seems to be GPT.

Setup is:
16x 3TB disks with GPT partitions:
| 1   2048         4095   1024.0 KiB  EF02  BIOS boot partition
| 2   4096   5860533134   2.7 TiB     FD00  Linux RAID

Next i created an mdadm /dev/md0 with raid6:
| md0 : active raid6 sda2[0]  [ .. ]  sdp2[2]
|  41021865472 blocks super 1.2 level 6, 64k chunk, algorithm 2 [16/16] 
|  [==>..................]  resync = 10.1% (296747044/2930133248) 
finish=985.3min speed=44540K/sec

The entire /dev/md0 is a Physical Volume for LVM on which two Logical
Volumes were created, one for / and one for swap. This setup works just
fine when activated from within a LiveCD like GRML:
| # lvdisplay | grep "LV Name"
|   LV Name                host_root
|   LV Name                host_swap
| # mount /dev/mapper/fdi-host_root /tgt
| # ls /tgt/boot/
|  abi-3.8.0-26-generic config-3.8.0-26-generic
| grub/  initrd.img-3.8.0-26-generic vmlinuz-3.8.0-26-generic

During installation no errors are logged. GRUB embeds fine in the BIOS
boot partition and the generated grub.cfg shows 'insmod lvm' and 'insmod
raid6rec' amongst others. The BIOS starts GRUB just fine, it's GRUB
where it seems to fail:

During boot, GRUB starts and drops to 'grub rescue>' stating "Error:
disk lvm/fdi-host_root not found". Typing 'ls' in the rescue shell shows
the '(md/0)' device, but LVM is never started by GRUB.
Much more than 'ls' isn't available in the rescue shell. Typing 'insmod
lvm' just returns without error, typing 'insmod thisdoesnotexist'
complains GRUB can't find the disk 'lvm/fdi-host_root'.

Could someone please advise where i am going wrong?

I tried (re)creating the core.img like so:
| # grub-mkimage -o /boot/grub/core_lvm.img -O i386-pc \
|     part_gpt part_msdos mdraid09 mdraid1x raid6rec \
|     diskfilter lvm ext2

And put that in all the disks:
| # grub-bios-setup -b i386-pc/boot.img -c core_lvm.img /dev/sdX

Alas, to no avail.

| A bicycle can't stand alone; it is two tired. 
| 4096R/20CC6CD2 - 6D40 1A20 B9AA 87D4 84C7  FBD6 F3A9 9442 20CC 6CD2

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