Re: RAID Questions

["Jesús M. Navarro" <jesus.navarro@undominio.net>]

Hi, Miles:

On Friday 09 July 2010 22:10:46 Miles Fidelman wrote:
> Tom H wrote:
> > On Tue, Jul 6, 2010 at 8:15 PM, Miles Fidelman
> >
> > <mfidelman@meetinghouse.net>  wrote:
> >> shell>  grub-install hd0
> >> shell>  grub-install hd1
> >
> > With these grub-install invocations, you will not be able to boot in
> > degraded mode.
> >
> > You have to set both sda and sdb to hd0 but you cannot do that in
> > device.map. You have to use the grub shell:
>
> I'm pretty sure you're wrong on this - at least it's worked for me in
> the past.
>
> You DON'T have to set both sda and sdb to hd0

hd0 is the first disk "seen" by the BIOS.  If your first disk fails badly your 
second disk will become hd0.  If you don't set up grub on the second disk 
to "think" that it is the first disk you won't be able to boot up (it'll try 
to find hd1 to load the system from it and it won't find it).

>
> I'm pretty sure that things will break badly if grub things that both
> disks are hd0.

Probably, but grub doesn't "think" that both disks are hd0 but you will have 
grub installed on the boot sectors of two different disks.  When you boot up 
your BIOS will load grub's first stage from only one of the disks, the first 
one on normal conditions, or the second one when the first fails, but then 
the second one will become the first and only! (so it will be hd0 under such 
conditions).

Note that this won't cover the case when the first disk is broken but not 
broken enough, so it's still detected by the BIOS: in this case only "real 
hardware RAID" will allow you to boot without previous mangling.

Cheers.