Bug#764798: grub2: Grub rescue shell with RAID 6 mdadm over 8 disks

[Colin Watson]

On Sat, Oct 11, 2014 at 03:00:11AM -0500, Mike B wrote:
> Package: grub2-common
> Version: 2.02~beta2-11
> Severity: critical
> File: grub2
> Justification: breaks the whole system

Do you know if there was a previous version of GRUB 2 where this worked?
For example, how does the version in testing behave?  (This is for
practical reasons: if this has just always been broken in GRUB 2, then
it shouldn't block migration of 2.02~beta2 to testing.)

> I initially was running an 8-disk RAID 6 mdadm.  Everything worked
> fine.  The system was upgraded to 14 disks RAID 6 online.  Eventually
> the system was rebooted and now grub drops to rescue complaining of
> invalid UUID.
> 
> Upon further inspection, grub is only seeing 8 disks (ls) instead of
> 14 and cannot assemble the root mdadm device.

OK, so that implies that this doesn't have much directly to do with RAID
handling: the problem is probably simply that GRUB can't see all 14
disks.

> I've tested using VMWare and confirmed that 8-disk RAID 6 mdadm arrays
> work fine with grub for root, but 9 disk and above break the system
> and drop to rescue shell.  

A quick test in qemu shows that GRUB can see more than eight disks
there.  Could you please share details of your VMware setup so that I
might stand a chance of reproducing this?

Thanks,

-- 
Colin Watson                                       [cjwatson at debian.org]