Upgrade grub to grub2 with MD RAID

Question

I have a machine running Debian 6. Because of some recent changes, I tried upgrading to Grub2 (yes, grub legacy was installed by default). Everything went well (chainloading, reboot) until I started "upgrade-from-grub-legacy". The machine has 2 harddisks which contain two software raid1 - /dev/md0, a small one containing /boot and /dev/md1 using the remaining capacity.

Parted shows me:

Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system     Flags
 1      512B    512MB   512MB   primary  ext2            raid
 2      513MB   2560MB  2047MB  primary  linux-swap(v1)
 3      2560MB  954GB   951GB   primary  ext3            raid

and for the second disk:

Disk /dev/sdb: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system     Flags
 1      512B    512MB   512MB   primary  ext2            raid
 2      513MB   2560MB  2047MB  primary  linux-swap(v1)
 3      2560MB  954GB   951GB   primary  ext3            raid

The dialog for "upgrade-from-grub-legacy" shows:

[ ] /dev/sda (1000204 MB; Hitachi_HDS721010CLA332)
[ ] /dev/sdb (1000204 MB; Hitachi_HDS721010CLA332)
[ ] /dev/md0 (511 MB; ???)
[ ] /dev/md1 (951308 MB; ???)

but no matter which I select, it fails saying:

/usr/sbin/grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding won't be possible!.
/usr/sbin/grub-setup: error: embedding is not possible, but this is required when the root device is on a RAID array or LVM volume.

As far as I could guess from various posts, there should be a boot_grub Flag somewhere.

Can I use Grub2 in my current configuration, if yes, how and is it safe to reboot in the current stage?

Why did you create partitions that start at `512B`? That is very unusual. — Zoredache, Sep 13 '12 at 16:45
@Zoredache I did not create those, they were preformatted by the hoster - and btw I think it is next to impossible for me to resize the array. — ThE_-_BliZZarD, Sep 13 '12 at 16:47

score 1 · Accepted Answer · answered Sep 14 '12 at 12:38

Okay, I figured it out.

The problem was that grub2 needs space to save the stage2 loader to - and in my configuration, there was none.

The solution was to shrink Partition 1 on both disks using resize2fs, parted and mdadm by taking out one disk at a time from the RAID.

I shrank 16MB of the front of Partition 1, retried "upgrade-from-grub-legacy", installed to /dev/sda AND /dev/sdb, because the machine may boot from either one. Everything went well, reboot - and it was done.

So, in summary: Yes, I can use Grub2, but I had to rearrange the partitions to make space for stage2. Yes, it was safe to reboot as long as "upgrade-from-grub-legacy" had not finished because the chainloader with grub1 was still in place and working.

Thanks for sharing but, consider following: There are too many bugs in grub2 related to bootable mdraid-based partitions. Sometimes it just won't work. Google :) — GioMac, Sep 14 '12 at 14:19

Upgrade grub to grub2 with MD RAID

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