Gentoo LVM2 installation

1. Introduction

This guide is based on an example with two IDE hard disks. It means that you will more than likely need to change the drive, partition names and partition sizes to match your own setup and needs.

Initial requirements 

If you do a fresh install of Gentoo, you will need to use a bootable CD with LVM2 support such as a Gentoo LiveCD. You can find the LiveCD for an x86 architecture on our mirrors under /releases/x86/2004.0/livecd/universal. Other architectures might be supported as well.

It you install LVM2 on a currently running system with some spare hard disk space, you will need to enable the LVM2 module (dm-mod). This module is available in gentoo-sources, in development-sources and in gentoo-dev-sources. Compiling your kernel and getting LVM2 to work is covered later in this guide.

Not all 2.4 kernels provided by Gentoo support LVM2!

Partitions 

Our example system has 2 IDE hard disks and will be partitioned as follows:

• /dev/hda1 -- /boot
• /dev/hda2 -- (swap)
• /dev/hda3 -- /
• /dev/hda4 -- Will be used by LVM2
• /dev/hdb1 -- Will be used by LVM2

OK, time to start...

2. Installation

Follow the handbook, but with the following amendments to chapter 4. Preparing the Disks:

Use fdisk as described in the handbook, but use the partition scheme mentioned above as an example. It is only an example, adapt it to your own needs.

Create a small physical /boot partition (hda1). In this example, /boot will be not managed by LVM2. This partition will contain your bootloader and your kernel(s). A 64MB partition should be well enough for quite a few kernel generations.

Create a swap partition (hda2) and activate it.

# mkswap /dev/hda2
# swapon /dev/hda2

Create a / (root) partition (hda3). If you are interested in trying to put your root partition under LVM management (which we do not recommend), see the resources section at the end of this guide for a link to a mini-howto on how to do this. The size of the root partition need not be large if you will keep /opt /usr /home /var and /tmp in an LVM2 Volume Group (vg). In this case, 150M is sufficient.

Assuming the /boot, swap and root partitions do not use the whole physical disk, create a fourth partition on this disk and set it to type 8e (Linux LVM). If you have more physical drives you would like to use with LVM, create one partition on each and give them the same type (8e).

Load the LVM2 dm-mod module. For some reason, this module has been compiled into the kernel 2.6 (named smp) on the Gentoo LiveCD. If you used this kernel instead of the default 2.4 (named gentoo), you can skip this step or ignore the warning you will get.

# modprobe dm-mod

Scan and activate LVM:

(Avoid scanning your cdrom)
# mkdir -p /etc/lvm
# echo 'devices { filter=["r/cdrom/"] }' >/etc/lvm/lvm.conf
# vgscan
  Reading all physical volumes.  This may take a while...
  No volume groups found

Prepare the partitions.

# pvcreate /dev/hda4 /dev/hdb1
  No physical volume label read from /dev/hda4
  Physical volume "/dev/hda4" successfully created
  No physical volume label read from /dev/hdb1
  Physical volume "/dev/hdb1" successfully created

Setup a volume group. A volume group is the result of combining several physical units into a single logical device.

In our example, /dev/hda1, /dev/hda2 and /dev/hda3 are the /boot, swap and root partitions so we need to combine /dev/hda4 and /dev/hdb1. It can be done with a single command, but, as an example, we will create our volume group and extend it.

(Create a volume group named vg)
# vgcreate vg /dev/hda4
  /etc/lvm/backup: fsync failed: Invalid argument (Ignore this warning)
  Volume group "vg" successfully created
(Extending an existing volume group)
# vgextend vg /dev/hdb1
  /etc/lvm/backup: fsync failed: Invalid argument (Ignore this warning, again and later as well)
  Volume group "vg" successfully extended

Create the logical volumes. Logical volumes are the equivalent of partitions you would create using fdisk in a non LVM2 environment. In our example, we create the following partitions:

Since we are going to use LVM2, we should not worry too much about partition sizes because they can always be expanded as needed.

# lvcreate -L10G -nusr  vg
  Logical volume "usr" created (Further similar messages not displayed)
# lvcreate -L5G  -nhome vg
# lvcreate -L5G  -nopt  vg
# lvcreate -L10G -nvar  vg
# lvcreate -L2G  -ntmp  vg
(As an example, let's extend a logical volume with 5 extra Gbytes)
# lvextend -L+5G /dev/vg/home

Create filesystems on the logical volumes the same way you would on a regular partition. We use ext3 on the logical volumes but any filesystem of your choice will work:

# mke2fs -j /dev/vg/usr
# mke2fs -j /dev/vg/home
# mke2fs -j /dev/vg/opt
# mke2fs -j /dev/vg/var
# mke2fs -j /dev/vg/tmp

Mount your partitions as described in the handbook and mount your LVM2 logical volumes as if they were partitions. Replace the usual /dev/hdxx with /dev/vg/logical_volumename.

(Make sure you have mounted your root partition as described in the handbook first)
# mkdir /mnt/gentoo/usr
# mount /dev/vg/usr /mnt/gentoo/usr
# mkdir /mnt/gentoo/home
# mount /dev/vg/home /mnt/gentoo/home
# mkdir /mnt/gentoo/opt
# mount /dev/vg/opt /mnt/gentoo/opt
# mkdir /mnt/gentoo/var
# mount /dev/vg/var /mnt/gentoo/var
# mkdir /mnt/gentoo/tmp
# mount /dev/vg/tmp /mnt/gentoo/tmp

When configuring your kernel, make sure to configure your kernel to support LVM2 (not all 2.4 kernels do). Select the LVM2 module as follows:

Multi-device support (RAID and LVM)  --->
  [*] Multiple devices driver support (RAID and LVM)
  < >  RAID support
(Note that LVM is not selected on purpose, this was for LVM1)
  < >  Logical volume manager (LVM) support
  <M>  Device-mapper support
  < >   Mirror (RAID-1) support

Device Drivers  --->
 Multi-device support (RAID and LVM)  --->
   [*] Multiple devices driver support (RAID and LVM)
   < >   RAID support
   <M>   Device mapper support

The compiled module is called dm-mod.ko

After you have built your kernel and installed its modules, add the following line to your /etc/modules.autoload.d/kernel-{KV} where {KV} represents your kernel version (2.4 or 2.6) so that the LVM2 module gets loaded when your machine is booted:

# nano -w /etc/modules.autoload.d/kernel-2.6
(Add the following line)
dm-mod

Now, install the lvm2 package.

# emerge lvm2
(Prevent lvm2 from probing your cdrom)
# echo 'devices { filter=["r/cdrom/"] }' >> /etc/lvm/lvm.conf

When editing your /etc/fstab file, follow the handbook and add your LVM2 logical volumes as needed. Again, here are a few lines needed for our example:

/dev/hda1     /boot   ext3    noauto,noatime 1 1
/dev/hda2     none    swap    sw             0 0
/dev/hda3     /       ext3    noatime        0 0
# Logical volumes
/dev/vg/usr   /usr    ext3    noatime        0 0
/dev/vg/home  /home   ext3    noatime        0 0
/dev/vg/opt   /opt    ext3    noatime        0 0
/dev/vg/var   /var    ext3    noatime        0 0
/dev/vg/tmp   /tmp    ext3    noatime        0 0

When you reach the end of the installation part of the handbook, don't forget to umount all your LVM2 logical volumes as well and for a good measure run the following command before you reboot:

# vgchange -an

Restart your machine and all partitions should be visible and mounted.

3. Resources

• The official LVM2 home page
• The LVM Howto
• Daniel Robbins's articles on LVM at IBM's DeveloperWorks: http://www-106.ibm.com/developerworks/linux/library/l-lvm/?dwzone=linux and http://www-106.ibm.com/developerworks/linux/library/l-lvm2.html?dwzone=linux
• How to boot your root FS off of LVM1: http://www.the-infinite.org/archive/docs/lvm/howto-boot-off-root-lv.txt

4. Acknowledgements

Thanks Thilo Bangert and Terje Kvernes for their help and comments on this document.