Software Raid On Boot

[Nigel Wade]

John Summerfied wrote:
> Rejean Proulx wrote:
> 
>> Does software raid work on a boot device? 
> 
> 
> I believe "not exactly." If you are using mirroring, it can be done: 
> google for how to, I think Dell has published on this.
> 
> Remember, the BIOS has to load the grub including its menu, the kernel 
> and initrd.

The BIOS only job is to allow execution of the code in the MBR of the boot 
drive. Grub stage1 is written into the boot sector, including the block location 
on disk of the next stage it must bootstrap. If the filesystem is efs it can 
bootstrap stage2 directly. For other filesystems it needs an intermediate 
filesystem_stage_1_5 to bootstrap stage2 for it. Stage2 displays the menu, and 
handles bootstrapping of the OS.

> 
> The trick is to make /boot appear in just the right place.
> 
> There is nothing to prevent you combining (say) hda and hdc, being like 
> drives and taking a slice of each as (say) ext2 of the same size, 
> installing grub & the kernel to hda1, grub also to /dev/hdc1 and at 
> convenient times (say on shutdown and kernel installation) just running 
> dd to mirror the partition.
> 

You can make a proper mirror of them (RAID1). All that's necessary for grub to 
boot is that it can access the filesystem without using md. For a mirror that's 
perfectly possible as the filesystem of a mirror is available via either 
partition which makes up the raid. So, if your RAID1 comprises /dev/hda1 and 
/dev/hdc1 (to make /dev/md0) then grub can boot from /dev/hda1. When the OS 
boots it mounts /dev/md0 as /boot and the filesystem is mirrored from that point.

The only thing you have to remember with grub is that grub.conf identifies the 
grub boot device as, for example, /dev/hda1. So if hda fails you cannot boot 
from hdc as grub.conf isn't configured for it. You need to move hdc to become 
hda, or use a rescue CD to edit grub.conf (and you must have grub installed 
somewhere on hdc).

-- 
Nigel Wade, System Administrator, Space Plasma Physics Group,
             University of Leicester, Leicester, LE1 7RH, UK
E-mail :    nmw at ion.le.ac.uk
Phone :     +44 (0)116 2523548, Fax : +44 (0)116 2523555