Re: software raid1

[Carl Karsten]

Marco Gerards wrote:
> Carl Karsten <address@hidden> 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 have installed Ubuntu-server 3 times 
today.  I lost count of how many times I have run various versions of the 
script.  I currently boot a live CD - I haven't figured out how to do it on a 
'running hda' even though it should let me.   I also need a scipt line to change 
hda1 to md0.

grub Q: is the grub << EOF... the 'normal/best' way?

Also, is there some way to display how grub has been configured?  doing 
something like that and then rebooting the box to see if it worked doesn't seem 
'best' - it is good to verify it works, but I would like something a bit more 
like: here is what is setup on hda and hdc: grub-display-setup-command

So I do all this, and now I can update md0/boot and the /boot on both /hda and 
hdc gets updated because of the raid, so normal booting of hda keeps working. 
if hda fails, I can just yank the cable and the box will boot from hdc.  To 
replace the failed hda, move the hdc to hda, add a new hdc and run the script.

The "yank the cable" step is not quite as elegant as a raid card (which I am 
assuming will take hda off line if it start erroring) but given how easy it is 
to recover, seems worth the trouble.

Carl k