Converting to RAID - An Idiots Guide
(note: I'd have called this
Raid for Dummies, but IDG publishers would take exception.)
by David A. Bandel (david at gmail dot com)
Revised: Oct 2006
This document is designed to update the fine RAID write-ups already on
the Linux SXS site. As such, I make no attempt to duplicate that
Following is a summary of the steps I took to convert several running
(read production) systems to RAID. While I chose to convert to
RAID 1, the process for RAID 4, 5, 6, or 10 is identical, it just requires more disks.
Preparing for the conversion.
Converting a production system means, above all else, protecting the
data you already have. So while not necessary, a verified backup
is always a good idea. And if you're comfortable with that backup
and can spare the downtime, then it might be easier to just set your
system up with RAID from scratch.
First, building and testing a new kernel with the proper RAID support
built-in is absolutely
essential. As of this writing, I am using a Debian 2.6.17
kernel with all support built as modules and am
using an initrd.
The important point for this kernel is that you either build in the RAID
support or have it in your initrd.img. Debian's kernel and initrd.img will boot correctly out of the box, I imagine most other modern distros can as well.
If you choose to build in RAID support (the safe way to go), in your choice of configuration menus select "Device
Drivers" --> "Multi-device support (RAID and LVM)" and ensure that
RAID support and at least the RAID level you want to use is built in,
not a module. You might also want to choose Multipath I/O support
if your system can make use of it (choose Yes if unsure).
Build and boot from your new kernel. If you're using an initrd.img, you may have to modprobe your raid module this time. You should be able to cat
Second, you'll need a second hard disk at least as large as the one you want
to RAID (a third for RAID 4 or 5, a fourth for RAID 6). Identical disks will make this
task much easier. Assuming you have your current disk installed
as /dev/hda, install the second disk as /dev/hdc (and if a third, as
/dev/hde, a fourth as /dev/hdg). Using one disk per controller (even to the point of
buying new controllers) is well worth the extra effort for the increase
in speed. This document will assume you've installed as per this
paragraph. SATA and SCSI do not suffer from the shared IRQ problem so how they are connected does not matter.
Once your new disk is installed and running, open fdisk /dev/hda,
option p and note your partition sizes and locations. Then run df
and note your partition sizes and mount points. One of my systems
looked like this:
The above corresponded to a 20Mb disk that originally held the
So I installed an 80Mb disk (no 20Mb disks were to be had) and
partitioned it as follows:
|/dev/hdc1||1||685||5502231||fd||Linux raid autodetect|
|/dev/hdc2||686||1370||5502262+||fd||Linux raid autodetect|
|/dev/hdc3||1371||2050||5462100||fd||Linux raid autodetect|
|/dev/hdc5||2051||4175||17069031||fd||Linux raid autodetect|
|/dev/hdc6||4176||4520||2771181||fd||Linux raid autodetect|
|/dev/hdc8||4521||4525||40131||fd||Linux raid autodetect|
The above partitions correspond to:
Other partitions correspond to partitions on /dev/hde and were RAIDED
The important points to pay attention to are:
1. The new RAID partition must be the same size or _smaller_ than
the corresponding non-RAIDED partition it will mirror.
2. The new partitions must use type fd - Linux raid autodetect.
3. Your kernel or initrd.img must have RAID support.
4. Ensure you have the RAID device files. If you're using
udev, you should see them in /dev/md/, otherwise you might have to
create them. You can create them with this command:
for i in 0 1 2 3 4 5 6 7 8 9 10; do mknod /dev/md$i b 9 $i; done
Ensure you have installed the latest mdadm utility. You will
_NOT_ need raidtools2 (and particularly not raidtools version 1).
We will not need to build /etc/raidtab.
Once your new RAID disk has been partitioned and all but the swap
partition are type Linux raid autodetect (fd), the hard part is
done. N.B. I chose not to RAID swap. However you may if you
like. I kept two separate swap partitions, one on each disk, for
extra speed. Your choice, RAID if you like, but you'll take a
slight performance hit.
Setting up the RAID
Now, the easy part. Activate the RAID partitions. For this
step we'll activate a degraded RAID array. That is, it won't be a
full RAID yet. Our friend is mdadm:
/dev/md0 -c 128 -l 1 -n 2 /dev/hdc8 missing
The above will create the RAID array /dev/md0 using a chunk size of
128Mb, level 1 (RAID 1), with 2 devices. The first device is
/dev/hdc8, the other device is missing (we'll add it later).
Once the device is created, we need to format it with our favorite
filesystem. My filesystem of choice is XFS, but you get to pick
your own poison:
mkfs.xfs -f /dev/md0
Mount the device and copy the appropriate information to it:
mount /dev/md0 /foo
Do the same for each of the other partitions, creating the degraded
RAID, formatting the device, and copying the information you want to
each new device.
tar c /boot | tar xvC /foo
Some notes about the copying process:
Some directories are static. They can be copied at anytime.
Others are not. To handle the ones that are not (notably /home
and /var) I took the following precautions:
before copying /home, I ran `touch /etc/nologin` then booted everyone
from the system
before copying /var, I went to runlevel 1
Once all was copied over (and I was in runlevel 1), I removed
/etc/nologin, then modified etc/fstab on the new RAID device to reflect
the RAID device mounts.
I reboot the system and at the boot: prompt gave the new kernel name
and root=/dev/md2 (where my root filesystem was located).
Congratulations, you're now running on a RAID system (albeit
degraded). So let's fix that part. You can `cat
/proc/mdstat` at any time to see your progress. You can only add
one device at a time, then you'll have to wait for the kernel to bring
that device fully on-line before you can go on to the next
device. Not to worry, the kernel will handle the scheduling part
First, open /dev/hda with fdisk and change all the partittions you want
to have added to the array to type fd - linux raid autodetection.
Then, for each degraded array, add the appropriate non-array device to
mdadm /dev/md0 -a /dev/hda1
Once each device has been rebuilt, you can see in /proc/mdstat that all
Note: If you get an error message to the effect that a device
cannot be added because it doesn't have enough space, that's because
you're trying to add a device that's smaller than the current RAID
device. Fix that and all will be well. This is why I
emphasized that the RAID devices needed to be the same size or smaller
(not larger) than your current non-RAIDed devices in the first part
The final step comes in making sure you can boot from your raid
array. I'm sorry for all those folks using GRUB, but last I heard
(please correct this document if I'm wrong), only LILO will boot from a
raid partition. So dump GRUB and install LILO.
Ensure your /etc/lilo.conf has the correct setup:
LILO will write boot information to each of the individual raid devices
boot sectors, so if either /boot or your root partition are on failed
disks, you'll still be able to boot.
Enjoy your newly raided system (but continue to make backups).