Re: software raid1

[Carl Karsten <carl@personnelware.com>]

Carl Karsten wrote:
> Marco Gerards wrote:
>> Carl Karsten <carl@personnelware.com> writes:
>>
>>> 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.
>>
>> RAID is more than just mirroring.  You can have striping, a RAID setup
>> with striping and a parity disk, etc.
>>
>> And besides that, even mirroring can do more advanced stuff.  Like
>> speeding up loading of kernels by loading from both disks and being
>> more robust against broken disks.  But I do not know if this can be
>> realized.
> 
> Having everything would be great - but I think just achieving the 
> simplest case of fault tolerance would be a very good return on investment.
> 
> I think I have achieved it by doing (search web again...) what someone 
> did last year: http://www.linuxsa.org.au/mailing-list/2003-07/1270.html
> 
> My short version:
> 
> 1. Basic Linux install: / and everything (therefor /boot) on hda1 (no raid)
> 
> boot a live CD and run this script:
> 
> #mkraid.sh
> 
> # Change the partition type of hda1 to fd (raid autodetect)
> echo ,,fd,|sfdisk --force /dev/hda -N1
> 
> # Copy the partition table from hda to hdc:
> sfdisk --no-reread -d /dev/hda | sfdisk --no-reread --force /dev/hdc
> 
> # Set up md device
> mdadm --create /dev/md0 --level=raid1 --raid-devices=2 /dev/hda1 /dev/hdc1
> 
> # Make hdc bootable:
> grub << EOF
> root (hd1,0)
> setup (hd1)
> EOF
> 
> This runs.  I think it even works.  

I was wrong.
md0 is hda1 + hdc1, but even after changing boot/grub/menu.lst and fstab entries 
from hda1 to md0, hda1 still gets mounted on /, but RO and then nothing works. 
when I boot to a live CD, it looks like md0 should go, but I am missing somehting.

btw - "I also need a scipt line to change hda1 to md0." is for manu.lst and 
fstab.  suggestions welcome.

I am starting on step 1 once again.

Carl