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Re: Software Raid On Boot



On Thu, 2006-02-16 at 08:50, Nigel Wade 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.

But for all of those operations, up to the time that the
kernel and initrd are completely loaded, all access to the
disk must be though BIOS calls since grub doesn't know
about disk drivers.

> 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).

Note that this is different with scsi, since if your first drive
fails, the next one becomes /dev/sda instead.  And in many cases
a broken IDE drive will simply cause the machine to fail to
boot until it is removed (something you can't simulate by
removing the drive).  While you have the case open you might
as well move the cable to what was hdc and make it become hda.
But, you are right that it is a good idea to know how to boot
to rescue mode from CD and fix things if there is any problem.

-- 
  Les Mikesell
   lesmikesell gmail com



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