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Re: software raid1

From: Carl Karsten
Subject: Re: software raid1
Date: Mon, 27 Mar 2006 23:20:31 -0600
User-agent: Thunderbird 2.0a1 (Windows/20060327)

Carl Karsten wrote:
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 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

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:

My short version:

1. Basic Linux install: / and everything (therefor /boot) on hda1 (no raid)

boot a live CD and run this script:

# 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)

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.


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