[CentOS] question on software raid-1

[James Fidell]

Quoting Jerry Geis (geisj at pagestation.com):
> I have a system that is RAID -1 configured as
> 
> /dev/md0 is /dev/hda1 /dev/hdb1
> /dev/md1 is /dev/hdb3 /dev/hdb3
> 
> it seems as though /dev/hda has failed....
> 
> I have another disk (identical model) that I can replace hda with.
> 
> I know about the commands fdisk to repartion and raidhotadd /dev/md0 
> /dev/hda1
> and raidhotadd /dev/md1 /dev/hda3 (to be ran after the system boots).
> 
> BUT... how do I now get the system to boot since hda is not there to boot...
> I can certainly execute the fdisk and raidhotadd after it boots... How 
> do I get it to boot.
> 
> The hardware is a D865 intel motherboard with 2 seagate 120G drives.

Assuming the machine is still running, you need to install grub on the
second disk.  From memory, this goes something like:

  # grub
  grub> root (hd1,0)
  grub> setup (hd1)
  grub> quit

You *should* then be able to boot from the second drive with the first
drive replaced. It's possible your BIOS may need tweaking to allow that
though.

The alternative is to replace the first drive and then boot off the
install CD using

  linux rescue

then rebuild the mirrors and re-install grub on each once the /boot
filesystem is fully synchronised:

  # grub
  grub> root (hd0,0)
  grub> setup (hd0)
  grub> root (hd1,0)
  grub> setup (hd1)
  grub> quit

and the machine should in theory now boot from either disk.

As I said, this is from memory -- I installed a kernel update the other
night that somehow caused the machine not to boot from the (mirrored)
/boot parition any more and I recovered it by doing something very
similar.

James