Author
======

Mark Pustjens
pustjens@dds.nl


Introduction
============

This guide describes how I implemented software raid under RedHat 7.2 and 7.3.

This guide assumes the installation of raid1 (mirroring) on an existing RedHat 
system. For more information about RedHat go to `www.redhat.com'. I also
assume GRUB as the bootloader.

The original disk used in this guide is `/dev/hda`, the new disk to create the
mirror on is `/dev/hdc`.

The following partitions are assumed on the origional disk:

    /dev/hda1	/boot
    /dev/hda2	/
    /dev/hda3	swap
    /dev/hda4	/home

For the creating of this document I relied heavilly on the 
[Software Raid Howto](http://www.ostenfeld.dk/~jakob/Software-RAID.HOWTO/Software-RAID.HOWTO.html).
This document is also available for download on [my website](http://unkie.org/wiki/misc/software raid)

Disclaimer
==========

Although RAID seems stable for me, and stable for many other people, it may 
not work for you. If you lose all your data, your job, get hit by a truck, 
whatever, it's not my fault, nor the developers'. Be aware, that you use the 
RAID software and this information at your own risk! There is no guarantee 
whatsoever, that any of the software, or this information, is in anyway correct
nor suited for any use whatsoever. Back up all your data before experimenting 
with this. Better safe than sorry. 

Preperation
===========

To be able to set up mirroring, you'll need an extra harddisk, besides the one
with the running RedHat installation. This harddisk needs to be at least 
equal or larger in storage capacity than the origional harddisk.

For performance and safety reasons it's recommended the harddisks are connected 
to seperate ide-busses. 

* Two harddisks sharing a single ide-bus will result in less performance. 
* Since it is very possible a crashing harddisk will also take down you're
  ide-bus, you should put the disks on seperate ide-busses.

Since RedHat comes with a kernel with raid as kernel modules, you need to build
a new kernel, with raid in the kernel itself, not as module. If you don't do
this, you won't be able to boot from raid.

Also upgrade your GRUB. Use an official GRUB release, nog a RedHat package.

Also _very_important_ is to _make_backups_ of your data, because something might
go wrong.

Step 1: Preparing the new harddisk
==================================

The first thing we should do is prepare the new, empy harddisk for raid1. This
has to be done as root user. First dump the partition information of the 
origional harddisk to a file:

    # sfdisk -d /dev/hda > hda-partition-info.txt

This command will create a new file in the current location, with information
about the partitions on the origional disk.

Since we want raid, we need to edit this file a bit. So open the file with
your favorite editor (eg: `pico` or `vim`). Everywhere is says `Id=83` or 
`Id=82`, change the number to `fd`. The `Id` field in this file defines the 
partition type.  Where `83` means a standard linux partition, `fd` means a 
`linux raid auto` partition. `82` means a swap partition.

When you're done don't forget to save the file and continue.

Now create the partitions on the mirror disk by issuing the following command:

    # sfdisk /dev/hdc < hda-partition-info.txt

Now the new partition table will be written to disk, and the kernel will resync
the drive. 

Step 2: Creating a raidtab
==========================

Now the correct partitions are created, we can start writing a `raidtab` file.
The `raidtab` file has to be saved under `/etc`. Open up a new empty file with
your favorite editor, eg:

    # vi /etc/raidtab

The `raidtab` file is essential for the creation and activation of raid devices
during system startup.

This file shoud have an entry of the following form for each partition. I will
provide one example. You will have to write the others yourself.

    raiddev /dev/md0
    raid-level                     1
    nr-raid-disks                  2
    persistent-superblock          1
    chunk-size                     128
    device                         /dev/hda1
    failed-disk                    0
    device                         /dev/hdc1
    raid-disk                      1

The `raiddev /dev/md0` defines which device file this raid partition will be.
With multiple raid partitions, just increase the 0 for each new entry.

The `raidlevel` entry defines which raid type you want. I want mirroring, so
I type `1`. Other raid levels are, of course, possible. Read the
`Software Raid Howto` if you want more info. However, I only have 2 disks, so
only a few options are available.

The `nr-raid-disk` tells how many disks there are in the raid device. We have
2 disks, so we put in 2.

The `persistant-superblock` field should allways be 1. This options makes sure
the raid information is stored on the partition itself, not only in file. 

The `chunk-size` field has no infuence on raid1, but should allways be present

The `device` entries define which partitions on which disks will be part of
this raid device. The `device` entry should allways be followed by a
`failed-disk` or `raid-disk` entry.

For `/dev/hda1` we set `failed disk`. This marks the drive as `bad`. It will
not be used when creating/activating this raid device. This way, our origional
disk will not be overwritten until we mark it as a good disk. Also note the
number after the entry. This defines which `device` entry we are talking 
about.

Now we have defined one device as bad, we also define one as good. The good 
disk in my case is the disk i want the mirror on: `/dev/hdc1`.

After this, you should add entries like the example above for each partition.
Also don't forget to save the file when you're done.

Step 3: Creating the raid devices
=================================

Now we should create the raid devices for each entry in `/etc/raidtab`. Do 
this by issuing the following command for each raid device:

    # mkraid /dev/md0

After creation the raid devices will be automaically be activated. You can
confirm this by issuing the command:

    # cat /proc/mdstat

Step 4: Creating filesystems on the raid devices
================================================

On each raid device you'll have to create a filesystem. I want an ext3
filesystem, so i issue the following command for each raid device:

    # mkfs.ext3 /dev/md0

See `man mke2fs` for information about the options this command takes.

This step will take a long time to complete, so get a cup of coffee, or
a glass of beer.

Step 5: Copying the origional partitions to raid
================================================

Now the raid devices are activated and have file systems on them, we need to
copy the data on the origional partitions to the raid devices. During this
and the next steps, make sure no data is altered. You can do this by bringing
the system down to runlevel 1. Or you could also make sure no user is able to
log in, by unplugging the networking cable.

When you're sure of the above, proceed with the following.

First you need to create a mountpoint where we will temporarilly mount the raid
devices. I created a mountpoint like this:

    # mkdir /mnt/tmp

The actions described below should be performed for each and every partition to
be mirrored.

Mount the raid device:

    # mount /dev/md0 /mnt/tmp

Change the path to the mountpoint of the origional partition:
(In my case: `/boot`, `/`, and `/home`. swap partitions dont need to be copied).

    # cd /boot

Copy all data the origional partition to the mounted raid device (make sure you
dont make _any_ typing errors):

   # find . -xdev | cpio -pm /mnt/tmp 

This command finds all the files in the current working directory (`.`), without
descending to other filesystems/partitions.

The previous 3 actions will take a lot of time, depending on how much data is
present on the partitions.

Step 6: Modifying /etc/fstab
============================

Now you should open `/etc/fstab` with your favorite editor (dont forget to 
backup). 

In this file you should change the first field of every row to correspond with
it's raid device. So my raidtab would look something like this:

before:

    /dev/hda1      /boot		ext3	defaults	1 1
    /dev/hda2      /			ext3	defaults	1 2
    /dev/hda3      /swap		ext3	defaults	1 2
    /dev/hda4      /home		swap	defaults	0 0

after:

    /dev/md0       /boot		ext3	defaults	1 1
    /dev/md1       /			ext3	defaults	1 2
    /dev/md2       /swap		ext3	defaults	1 2
    /dev/md3       /home		swap	defaults	0 0

When you're done don't forget to save the file.

Step 7: Reboot
==============

Now it's time to reboot. When rebooting, open the grub command shell. On a
standard RedHat installation that means you push the up and down button in the
os selection menu. By pressing up/down the timeout will go away. Now select your
kernel with raid support. Then press `e` to edit this entry. Change or add the
kernel argument `root` to point to the raid device. In my case this would be:

    raid=/dev/md1

When done press `b' to boot this kernel with these options. If everything boots
up correctly, continue with the following step, otherwise make sure you did
everything like described.

Step 8: Fdisk the origional disk
================================

Now you have rebooted, you have your root device on a raid partition. You can
veryfy this by issuing the `mount` command. It should list `/dev/mdX` as root
device. You can also type:

    # cat /proc/mdstat

To check the status of your raid devices. Right now they should be in degraded 
mode.  This because we marked the origional disk as a failed disk.

Now use fdisk on your origional harddisk:

    # fdisk /dev/hda

You must change the partition types of all the partitions you have mirrored to 
`fd'.

Or you could use the save partition information we used earlier to create 
partitions on our new disk:

    # sfdisk /dev/hdc < hda-partition-info.txt

This should do the job. We are now ready to add the partitions on the 
origional disk to the raid device.

Step 9: Completing the raid device
==================================

The following actions should be performed for each downgraded raid device.
You can check the status of the raid devices using:

    # cat /proc/mdstat

Add the partitions on the origional disk (`/dev/hda`) to the raid devices
(`/dev/mdX`):

    # raidhotadd /dev/md0 /dev/hda1

Now the disk in the raid device will be `synced`. That is, the data on the
new disk will be mirrored to the origional disk. If you now check the status
you will see how for the sync is complete, and the speed.

You could best wait for the current raid device to be synced. After that
proceed with the oher devices.

This step will take a long time, depending of how large your partitions are.

Step 10: Update the raidtab
===========================

Now all raid devices are no longer in degraded mode, and they are all in sync
we can update the file `/etc/raidtab`. (verify the status of the devices
before proceeding.

Open `/etc/raidtab` with your favorite editor. Look up all the `failed-disk`
lines, and change them into `raid-disk` lines. That's all! You now have one
thing left to do.

Step 11: Installing GRUB
========================

Now a very important step: installing GRUB. This we have to do manually. (if
you know an other way, let me know).

While it is true that we can install GRUB by changing `grub.conf`, grub does
not know how to handle with raid devices. It will install GRUB on only one
device, which will be a problem when the first disk failes and you have to boot
from the second disk.

To install GRUB, start the GRUB commandline shell. You will get a prompt. Enter
the following commands (after tailoring to your situation).

    > device (hd0) /dev/hda
    > root (hd0,0)
    > kernel /vmlinuz-2.4.19-athlon-smp ro root=/dev/md1 vga=791
    > setup

Now grub is installed on the first harddisk (/dev/hda). To install on the second
harddisk (/dev/hdc), repeat these commands, but change `device (hd0) /dev/hda`
to `device (hd0) /dev/hdc`.

Now you are all done. Perhaps you could reboot once more to see if it will all
work like planned. Instructions for testing the redundancy are provided in the
`Software Raid Howto`, a link to this document is provided at the top of this
document.

Special Thanks
==============

I would like to thank Linus Torvalds, for creating the basis of such a fine
operation system. 

Also would i like to thank Richard Stallman for founding the GNU Foundation.
Without him, GNU/Linux as we know it would probably not exist.

Thanks to Jakob Østergaard, for writing the Software Raid HOW-TO. (from which
i borrowed the disclaimer).