Re: software raid1

[Peter Jones]

On Tue, 2006-03-21 at 18:43 -0600, Carl Karsten wrote:
> I have been messing around with software raid1 - mirroring 2 drives - 
> including 
> the boot sectors. The goal is to be able to remove either drive and still 
> have a 
> functioning system.
> 
> I realize that this is "not supported" given that "Add support for software 
> RAID" is on the http://grub.enbug.org/TodoList but I have heard it can be 
> done 
> with some trickery: install grub on both drives and rely on the fact that 
> /dev/hda1 and /dev/hda2 and /dev/md0 are all "the same thing" as long as md0 
> is 
> the only one that gets written to.
> 
> I did run into one problem:  if you have ide master and slave, and you pull 
> the 
> master, the slave dissapears and the box/bios won't see it so it won't boot 
> from 
> it.  so I am guessing I have to make each drive the master on a seperate ide 
> bus 
> so that pulling one won't take out the other.

If you swap the drives, you can boot the second one.  Sometimes this is
possible with only a BIOS change, sometimes you have to re-cable them.
Depends on your BIOS.

It should be possible, if they're both bootable in the BIOS, to install
grub on the MBR of each half, and then swapping the drives shouldn't be
required, you'd just have to make BIOS boot the second (or, rather,
non-failed) disk.

> Before I spend too much time trying to make this work, anyone know of a HowTo 
> that outlines this?

Not exactly -- I've got patches for GRUB Legacy's grub-install if you're
interested in looking at them.  They're in the Fedora grub package, or I
can mail them to you if you need.

(Also, if Marco or Yoshinori are interested in such a patch for Legacy,
I could prepare it for submission upstream.)

-- 
  Peter