Installing with Software RAID or LVM

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Revision as of 13:17, 18 September 2010 by Markg85 (talk | contribs) (Troubleshooting)
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Warning: Installing a system with RAID is a complex process which can destroy data. Be sure to backup all data beforehand. It is also good practice to ensure that only the drives involved in the installation are attached while performing the install.

This document is up-to-date with all Arch versions as of 2008.06 'Overlord'. It may not be applicable to previous releases of Arch Linux.


RAID (Redundant Array of Independent Disks) is designed to prevent data loss in the event of a hard disk failure. There are different "levels" of RAID. RAID 0 (striping) isn't really RAID at all, because it provides no redundancy. It does, however, provide a speed benefit. The example will utilize RAID 0 for swap, on the assumption that a desktop system is being used, where the speed increase is worth the possibility of system crash if one of your drives fails. On a server, a RAID 1 or RAID 5 array is more appropriate. The size of a RAID 0 array block device is the size of the smallest component partition times the number of component partitions.

RAID 1 is the most straightforward RAID level: straight mirroring. As with other RAID levels, it only makes sense if the partitions are on different physical disk drives. If one of those drives fails, the block device provided by the RAID array will continue to function as normal. The example will be using RAID 1 for everything except swap. Note that RAID 1 is the only option for the boot partition, because bootloaders (which read the boot partition) do not understand RAID, but a RAID 1 component partition can be read as a normal partition. The size of a RAID 1 array block device is the size of the smallest component partition.

RAID 5 requires 3 or more physical drives, and provides the redundancy of RAID 1 combined with the speed and size benefits of RAID 0. RAID 5 uses striping, like RAID 0, but also stores parity blocks distributed across each member disk. In the event of a failed disk, these parity blocks are used to reconstruct the data on a replacement disk. RAID 5 can withstand the loss of one member disk.

ATTENTION: Do not be lulled into a false sense of security. Using RAID does not mean that you do not need backups - read the CAVEATS section below!


LVM (Logical Volume Management) makes use of the device-mapper feature of the Linux kernel. It provides a system of specifying partitions independently of the layout of the underlying disk. What this means for you is that you can extend and shrink partitions (subject to the filesystem you use allowing this) and add and remove partitions without worrying about whether you have enough contiguous space on a particular disk, without getting caught up in the problems of fdisking a disk that is in use (and wondering whether the kernel is using the old or new partition table) and without having to move other partition out of the way.

This is strictly an ease-of-management issue: it doesn't provide any addition security. However, it sits nicely with the other two technologies we are using.

Note that LVM is not used for the boot partition (because of the bootloader problem).


Security (redundancy)

Again, RAID does not provide a guarantee that your data is safe. If there is a fire, if your computer is stolen or if you have multiple hard drive failures, RAID won't protect you. So make backups. Whether you use tape drives, DVDs, CDROMs or another computer, keep a copy of your data out of your computer (and preferably offsite) and keep it up to date. Get into the habit of making regular backups. If you organize the data on your computer in a way that separates things you are currently working on from "archived" things that are unlikely to change, you can back up the "current" data frequently, and the "archived" data occasionally.

General Approach

For starters, note that this document seeks primarily to give you a good example walkthrough of how to install Arch with Software RAID or LVM support for a typical case. It won't try to explain all the possible things you can do -- it's more to give you an example of something that will work that you can then tweak to your own purposes.

In this example, the machine I'm using will have three similar IDE hard drives, at least 80GB each in size, installed as primary master, primary slave, and secondary master, with my installation CD-ROM drive as the secondary slave. I will assume these can be reached as /dev/sda, /dev/sdb, and /dev/sdc, and that the cdrom drive is /dev/cdrom.

We'll create a 100MB /boot partition, a 2048MB (2GB) swap partition and a ~ 78GB root partition using LVM. The boot and swap partitions will be RAID1, while the root partition will be RAID5. Why RAID1? For boot, it's so you can boot the kernel from grub (which has no RAID drivers!), and for swap, it's for redundancy, so that your machine will not lose its swap state even if 1 or 2 drives fail.

Each RAID1 redundant partition will have three physical partitions, all the same size, one on each of the drives. The total storage capacity will be the size of a single one of these physical partitions. A RAID1 redundant partition with 3 physical partitions can lose any two of its physical partitions and still function.

Each RAID5 redundant partition will also have three physical partitions, all the same size, one on each of the drives. The total storage capacity will be the combined size of two of these physical partitions, with the third drive being consumed to provide parity information. A RAID5 redundant partition with 3 physical partitions can lose any one of its physical partitions and still function.

Get the Arch Installer CD

Please note that in order to use LVM, you need the lvm2 and dev-mapper packages installed, otherwise you will be unable to see any LVM partitions on reboot.


Just to give you an idea of how all this will work, I'll outline the steps. The details for these will be filled in below.

  1. Boot the Installer CD
  2. Partition the Hard Drives
  3. Create the RAID Redundant Partitions
  4. Create and Mount the Main Filesystems
  5. Setup LVM and Create the / (root) LVM Volume
  6. Install and Configure Arch
  7. Install Grub on the Primary Hard Drive
  8. Unmount Filesystems and Reboot
  9. Install Grub on the Alternate Boot Drives
  10. Archive your Filesystem Partition Scheme


Boot the Installer CD

First, load all your drives in the machine. Then boot the Arch Linux installation CD.

At the syslinux boot prompt, hit enter: we want to use the SCSI kernel, which has support for RAID and LVM built in.

So far, this is easy. Don't worry, it gets harder.

Partition the Hard Drives

If your hard drives are already prepared and all you want to do is activate RAID and LVM jump to Activate existing RAID devices and LVM volumes.

We'll use cfdisk to do this partitioning. We want to create 3 partitions on each of the three drive:

Partition 1 (/boot): 100MB, type FD, bootable
Partition 2 (swap): 2048MB, type FD
Partition 3 (RAID): <Rest of the drive>, type FD

Note that in general, in cfdisk, you can use the first letter of each Bracketed Option to select it; however, this is not true for the Write command, you have to hold SHIFT as well to select it.

First run:

# cfdisk /dev/sda

Create each partition in order:

  1. Select New.
  2. Hit Enter to make it a Primary partition.
  3. Type the appropriate size (in MB), or for Partition 3, just hit enter to select the remainder of the drive.
  4. Hit Enter to choose to place the partition at the Beginning.
  5. Select Type, hit enter to see the second page of the list, and then type fd for the Linux RAID Autodetect type.
  6. For Partition 1 on each drive, select Bootable.
  7. Hit down arrow (selecting the remaining free space) to go on to the next partition to be created.

When you're done, select Write, and confirm y-e-s that you want to write the partition information to disk.

Then select Quit.

Repeat this for the other two drives:

# cfdisk /dev/sdb
# cfdisk /dev/sdc

Create the same exact partitions on each disk. If a group of partitions of different sizes are assembled to create a redundant RAID partition, it will work, but the redundant partition will be in multiples of the size of the smallest one, leaving the rest of the allocated drive space to waste.

You can also use sfdisk to clone the partition table from your first drive to the other drives. Use:

# sfdisk -d /dev/sda > table

to dump the partition from the first drive into table, and then

# sfdisk /dev/sdb < table
# sfdisk /dev/sdc < table

to write the partition table to the other disks.

Load the RAID Modules

Before using mdadm, you need load the modules for the RAID levels you'll be using. In this example, we're using levels 1 and 5, so we'll load those. You can ignore any modprobe errors like "cannot insert md-mod.ko: File exists". Busybox's modprobe can be a little slow sometimes.

# modprobe raid1
# modprobe raid5

Create the RAID Redundant Partitions

Now that you've created all the physical partitions, you're ready to set up RAID. The tool you use to create RAID arrays is mdadm.

To create /dev/md0 (/):

# mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sda3 /dev/sdb3 /dev/sdc3

To create /dev/md1 (/boot):

# mdadm --create /dev/md1 --level=1 --raid-devices=3 --metadata=0.90 /dev/sda1 /dev/sdb1 /dev/sdc1

If you want to use Grub 0.97 (default in the Arch Linux 2010.05 release) on RAID 1, you need to specify an older version of metadata than the default. Add the option "--metadata=0.90" to the above command. Otherwise Grub will respond with "Filesystem type unknown, partition type 0xfd" and refuse to install. This is supposedly not necessary with Grub 2.

To create /dev/md2 (swap):

# mdadm --create /dev/md2 --level=1 --raid-devices=3 /dev/sda2 /dev/sdb2 /dev/sdc2

At this point, you should have working RAID partitions. When you create the RAID partitions, they need to sync themselves so the contents of all three physical partitions are the same on all three drives. The hard drives lights will come on as they try to sync up. You can monitor the progress by typing:

# cat /proc/mdstat

You can also get particular information about, say, the root partition by typing:

# mdadm --misc --detail /dev/md0

You don't have to wait for synchronization to finish -- you may proceed with the installation while syncronization is still occurring. You can even reboot at the end of the installation with synchronization still going.

Setup LVM and Create the / (root) LVM Volume

This is where you create the LVM volumes. LVM works with abstract layers, check out LVM and/or its documentation to discover more. What you will be doing in short:

  • Turn block devices (e.g. /dev/sda1 or /dev/md0) into Physical Volume(s) that can be used by LVM
  • Create a Volume Group consisting of Physical Volume(s)
  • Create Logical Volume(s) within the Volume Group

Note: If you are using an Arch Linux install CD <= 0.7.1, you have to create and mount a sysfs partition on /sys, to keep lvm from getting cranky. Otherwise you can skip this mounting of sysfs, unless you run into trouble. If you forget to do this, instead of giving you an intelligent error message, lvm will simply Segmentation fault at various inconvenient times.

To mount the sysfs partition, do:

# mkdir /sys
# mount -t sysfs none /sys

Let's get started:

Make sure that the device-mapper module is loaded:

# modprobe dm-mod

Now you need to do is tell LVM you have a Physical Volume for it to use. It's really a virtual RAID volume (/dev/md0), but LVM doesn't know this, or really care. Do:

# pvcreate /dev/md0

This might fail if you're using raid or creating PV on an existing Volume Group. If so you might want to add -ff option.

LVM should report back that it has added the Physical Volume. You can confirm this with:

# pvdisplay

Now it's time to create a Volume Group (which I'll call array) which has control over the LVM Physical Volume we created. Do:

# vgcreate array /dev/md0

LVM should report that it has created the Volume Group array. You can confirm this with:

# vgdisplay

Next, we create a Logical Volume called root in Volume Group array which is 50GB in size:

# lvcreate --size 50G --name root array

LVM should report that it created the Logical Volume root. You can confirm this with:

# lvdisplay

The LVM volume should now be available as /dev/mapper/array-root. Or something similar, LVM will also be able to tell you which when you issue the display command.

Activate existing RAID devices and LVM volumes

If you already have RAID partitions created on your system and you've also set up LVM and all you want is enabling them follow this simple procedure. This might come in handy if you're switching distros and don't want to lose data in /home for example.

First you need to enable RAID support. RAID1 and RAID5 in this case.

modprobe raid1
modprobe raid5

Activate RAID devices: md1 for /boot and md0 for LVM where two logical volumes will reside.

mdadm --assemble /dev/md0 /dev/sda3 /dev/sdb3 /dev/sdc3
mdadm --assemble /dev/md1 /dev/sda1 /dev/sdb1 /dev/sdc1

RAID devices should now be enabled. Check /proc/mdstat.

If you haven't loaded kernel LVM support do so now.

modprobe dm-mod

Startup of LVM requires just the following two commands:

vgchange -ay

You can now jump to [3] Set Filesystem Mountpoints in your menu based setup and mount created partitions as needed.

Create and Mount the Filesystems

When you are using a setup that is newer then 2008.03; this step is optional!

Example using ReiserFS (V3):

To create /boot:

# mkreiserfs /dev/md1

To create swap space:

# mkswap /dev/md2

To create /:

# mkreiserfs /dev/array/root

Now, mount the boot and root partitions where the installer expects them:

# mount /dev/array/root /mnt
# mkdir /mnt/boot
# mount /dev/md1 /mnt/boot

We've created all our filesystems! And we're ready to install the OS!

Install and Configure Arch

This section doesn't attempt to teach you all about the Arch Installer. It leaves out some details here and there for brevity, but still seeks to be basically follow-able. If you're having trouble with the installer, you may wish to seek help elsewhere in the Wiki or forums.

Now you can continue using the installer to set-up the system and install the packages you need. Here's the walkthrough:

  • Type /arch/setup to launch the main installer.
  • Select < OK > at the opening screen.
  • Select 1 CD_ROM to install from CD-ROM (or 2 FTP if you have a local Arch mirror on FTP).
  • If you have skipped the optional step (Create and Mount the Filesystems) above, and haven't created a fileystem yet, select 1 Prepare Hard Drive > 3 Set Filesystem Mountpoints and create your filesystems and mountpoints here
  • Now at the main menu, Select 2 Select Packages and select all the packages in the base category, as well as the mdadm and lvm2 packages from the system category. Note: mdadm & lvm2 are included in base category since arch-base-0.7.2.
  • Select 3 Install Packages. This will take a little while.
  • Note: Because the installer builds the initrd using /etc/mdadm.conf in the target system, you should update that file with your RAID configuration. The original file can simply be deleted because it contains comments on how to fill it correctly, and that is something mdadm can do automaticly for you. So let's delete the original and have mdadm create you a new one with the currect setup:
    Press Alt-F2 to get a new terminal an log in, then do
rm /mnt/etc/mdadm.conf
mdadm --examine --scan >> /mnt/etc/mdadm.conf
  • Select 4 Configure System:

Add the dm_mod module to the MODULES list in /etc/mkinitcpio.conf.

Add the mdadm and lvm2 hook to the HOOKS list in /etc/mkinitcpio.conf (before 'filesystems', NOT after). See Configuring mkinitpcio using RAID for more details.

Edit your /etc/rc.conf. It should contain a USELVM entry already, which you should change to:


Please Note: The rc.sysinit script that parses the USELVM variable entry will accept either yes or YES, however it will not accept mixed case. Please be sure you've got your capitalization correct.

Edit your /etc/fstab to contain the entries:

/dev/array/root         /       reiserfs        defaults        0       1
/dev/md2                swap    swap            defaults        0       0
/dev/md1                /boot   reiserfs        defaults        0       0

At this point, make any other configuration changes you need to other files.

Then exit the configuration menu.

Since you will not be installing Grub from the installer, select 7 Exit Install to leave the installer program.

Old style:

Then specify the raid array you're booting from in /mnt/boot/grub/menu.lst like:

 # Example with /dev/array/root for / & /dev/md1 for /boot:
   kernel /kernel26 root=/dev/array/root ro  md=1,/dev/sda1,/dev/sdb1,/dev/sdc1 md=0,/dev/sda3,/dev/sdb3,/dev/sdc3

Nowadays (2009.02), with the mdadm hook in the initrd it it no longer necessary to add kernel parameters concerning the RAID array(s).

The arrays can be assembled on boot by the kernel using that hook and the contents of /etc/mdadm.conf, which is included in the initrd image when it's build. (See Configuring mkinitpcio using RAID )

An example of a GRUB boot configuration for booting of a RAIDed root like this:

# (0) Arch Linux
title  Arch Linux
root   (hd0,0)
kernel /vmlinuz26 root=/dev/md0 ro
initrd /kernel26.img

Install Grub on the Primary Hard Drive

This can also be done from the installer just fine now (2009.08 and should also work for 2009.02)

This is the last and final step before you have a bootable system!

As an overview, the basic concept is to copy over the grub bootloader files into /boot/grub, mount a procfs and a device tree inside of /mnt, then chroot to /mnt so you're effectively inside your new system. Once in your new system, you will run grub to install the bootloader in the boot area of your first hard drive.

Copy the GRUB files into place and get into our chroot:

# cp -a /mnt/usr/lib/grub/i386-pc/* /mnt/boot/grub
# sync
# mount -o bind /dev /mnt/dev
# mount -t proc none /mnt/proc
# chroot /mnt /bin/bash

At this point, you may no longer be able to see keys you type at your console. I'm not sure of the reason for this (NOTE: try "chroot /mnt /bin/<shell>"), but it you can fix it by typing reset at the prompt.

Once you've got console echo back on, type:

# grub

After a short wait while grub does some looking around, it should come back with a grub prompt. Do:

grub> root (hd0,0)
grub> setup (hd0)
grub> quit

That's it. You can exit your chroot now by hitting CTRL-D or typing exit.


The hard part is all over! Now remove the CD from your CD-ROM drive, and type:

# reboot

Install Grub on the Alternate Boot Drives

Once you've successfully booted your new system for the first time, you will want to install Grub onto the other two disks (or on the other disk if you have only 2 HDDs) so that, in the event of disk failure, the system can be booted from another drive. Log in to your new system as root and do:

# grub
grub> device (hd0) /dev/sdb
grub> root (hd0,0)
grub> setup (hd0)
grub> device (hd0) /dev/sdc
grub> root (hd0,0)
grub> setup (hd0)
grub> quit

Archive your Filesystem Partition Scheme

Now that you're done, it's worth taking a second to archive off the partition state of each of your drives. This guarantees that it will be trivially easy to replace/rebuild a disk in the event that one fails. You do this with the sfdisk tool and the following steps:

# mkdir /etc/partitions
# sfdisk --dump /dev/sda >/etc/partitions/disc0.partitions
# sfdisk --dump /dev/sdb >/etc/partitions/disc1.partitions
# sfdisk --dump /dev/sdc >/etc/partitions/disc2.partitions


For LVM management, please have a look at LVM

Mounting from a Live CD

If you want to mount your RAID partition from a Live CD, use

mdadm --assemble /dev/md0 /dev/sda3 /dev/sdb3 /dev/sdc3

(or whatever mdX and drives apply to you)

Note: Live CDs like SystemrescueCD assemble the RAID arrays automatically at boot time if you used the partition type fd at the install of the array)

Removing device, stop using the array

You can remove a device from the array after you mark it as faulty.

mdadm --fail /dev/md0 /dev/sdxx

Then you can remove it from the array.

mdadm -r /dev/md0 /dev/sdxx

Remove device permanently (for example in the case you want to use it individally from now on). Issue the two commands described above then:

mdadm --zero-superblock /dev/sdxx

After this you can use the disk as you did before creating the array.

Warning: If you reuse the removed disk without zeroing the superblock you will LOSE all your data next boot. (After mdadm will try to use it as the part of the raid array). DO NOT issue this command on linear or RAID0 arrays or you will LOSE all your data on the raid array.

Stop using an array:

1. Umount target array
2. Stop the array with: mdadm --stop /dev/md0
3. Repeat the three command described in the beginning of this section on each device.
4. Remove the corresponding line from /etc/mdadm.conf

Adding a device to the array

Adding new devices with mdadm can be done on a running system with the devices mounted. Partition the new device "/dev/sdx" using the same layout as one of those already in the arrays "/dev/sda".

sfdisk -d /dev/sda > table
sdfisk /dev/sdx < table

Assemble the RAID arrays if they are not already assembled:

mdadm --assemble /dev/md1 /dev/sda1 /dev/sdb1 /dev/sdc1
mdadm --assemble /dev/md2 /dev/sda2 /dev/sdb2 /dev/sdc2
mdadm --assemble /dev/md0 /dev/sda3 /dev/sdb3 /dev/sdc3

First, add the new device as a Spare Device to all of the arrays. We will assume you have followed the guide and use separate arrays for /boot RAID 1 (/dev/md1), swap RAID 1 (/dev/md2) and root RAID 5 (/dev/md0).

mdadm --add /dev/md1 /dev/sdx1
mdadm --add /dev/md2 /dev/sdx2
mdadm --add /dev/md0 /dev/sdx3

This should not take long for mdadm to do. Check the progress with:

cat /proc/mdstat

Check that the device has been added with the command:

mdadm --misc --detail /dev/md0

It should be listed as a Spare Device.

Tell mdadm to grow the arrays from 3 devices to 4 (or however many devices you want to use):

mdadm --grow -n 4 /dev/md1
mdadm --grow -n 4 /dev/md2
mdadm --grow -n 4 /dev/md0

This will probably take several hours. You need to wait for it to finish before you can continue. Check the progress in /proc/mdstat. The RAID 1 arrays should automatically sync /boot and swap but you need to install Grub on the MBR of the new device manually. Installing_with_Software_RAID_or_LVM#Install_Grub_on_the_Alternate_Boot_Drives

The rest of this guide will explain how to resize the underlying LVM and filesystem on the RAID 5 array.

Note: I am not sure if this can be done with the volumes mounted and will assume you are booting from a live-cd/usb

If you are have encrypted your LVM volumes with LUKS, you need resize the LUKS volume first. Otherwise, ignore this step.

cryptsetup luksOpen /dev/md0 cryptedlvm
cryptsetup resize cryptedlvm

Activate the LVM volume groups:

vgchange -ay

Resize the LVM Physical Volume /dev/md0 (or e.g. /dev/mapper/cryptedlvm if using LUKS) to take up all the available space on the array. You can list them with the command "pvdisplay".

pvresize /dev/md0

Resize the Logical Volume you wish to allocate the new space to. You can list them with "lvdisplay". Assuming you want to put it all to your /home volume:

lvresize -l +100%FREE /dev/array/home

To resize the filesystem to allocate the new space use the appropriate tool. If using ext2 you can resize a mounted filesystem with ext2online. For ext3 you can use resize2fs or ext2resize but not while mounted.

You should check the filesystem before resizing.

e2fsck -f /dev/array/home
resize2fs /dev/array/home

Read the manuals for lvresize and resize2fs if you want to customize the sizes for the volumes.




If you are getting error when you reboot about "invalid raid superblock magic" and you have additional hard drives other than the ones you installed to, check that your hard drive order is correct. During installation, your RAID devices may be hdd, hde and hdf, but during boot they may be hda, hdb and hdc. Adjust your kernel line in /boot/grub/menu.lst accordingly. This is what happened to me anyway.

Recovering from a broken or missing drive in the raid

You might get the above mentioned error also when one of the drives breaks for whatever reason. In that case you will have to fore the raid to still turn on even with one disk short. Type this (change where needed):

mdadm --manage /dev/md0 --run

Now you should be able to mount it again with something like this (if you had it in fstab):

mount /dev/md0

Now the raid should be working again and available to use, however with one disk short! So, to add that one disc partition it the way like described above in #Partition_the_Hard_Drives. Once that's done you can add the new disk to the raid by doing:

mdadm --manage --add /dev/md0 /dev/sdd1

If you type:

cat /proc/mdstat

you probably see that the raid is now active and rebuilding.

You also might want to update your /etc/mdadm.conf file by typing:

mdadm --examine --scan > /etc/mdadm.conf

That should be about all steps required to recover your raid. It certainly worked for me when i had lost a dive due to a partition table corruption.

Additional Resources