Re: grub2/grub-pc install not possible on mdadm

[Chris Murphy]

On Feb 28, 2012, at 12:58 PM, Stefan Priebe wrote:

> Am 28.02.2012 20:52, schrieb Chris Murphy:
>> 
>> The partition table should be outside of RAID, and define the member devices 
>> (and partitions) that make up the array. But inside the RAID, other than 
>> RAID 1, the file system itself is obscured and GRUB probably can't find it's 
>> various modules in e.g. /boot/grub2/
>> 
>> Seems like core.img needs to already have mdraid1x.mod already baked into 
>> it, in order for GRUB2 to understand RAID, and find the rest of its modules 
>> to load. But my understanding is that to boot off RAID, it had to be RAID1. 
>> I didn't think it was possible to have /boot on say RAID 0, 5, or 6 but I 
>> could be confused. The manual isn't exactly clear what RAID levels 
>> mdraid1x.mod supports.
> 
> Correct - i'm using raid 1 so it should work.


Well with RAID1 I haven't even needed GRUB to have RAID support. Even GRUB 
Legacy will work correctly. So as I'm thinking about it, I'm not sure exactly 
what mdraid09.mod and mdraid1x.mod do? When GRUB comes up and starts to boot 
off /boot, it's basically booting it as a non-dm/md device. Once initrd is 
loaded, then md devices become active, and are mounted per fstab. Obviously 
grub is well over and done with.

One thing is that if for some reason something were to be modified on one of 
the RAID1 members during boot, would cause md to resync the array, which for a 
normal 500MB RAID1 /boot partition would take a few seconds. Not a big deal if 
that happened at every boot.

So yeah, I'm a bit flummoxed on what the GRUB2 RAID modules do. Briefly 
googling isn't getting me far.

The one trick for RAID1, already mentioned, is you need GRUB2 installed in and 
just after the MBR on both drives (or the BIOS Boot partition, also on each 
drive, for GPT disks).

Chris Murphy