Beyond Linux� From Scratch (systemd Edition) - Version 12.0

Chapter 5. File Systems and Disk Management

About Logical Volume Management (LVM)

LVM manages disk drives. It allows multiple drives and partitions to be combined into larger volume groups, assists in making backups through a snapshot, and allows for dynamic volume resizing. It can also provide mirroring similar to a RAID 1 array.

A complete discussion of LVM is beyond the scope of this introduction, but basic concepts are presented below.

To run any of the commands presented here, the LVM2-2.03.22 package must be installed. All commands must be run as the root user.

Management of disks with lvm is accomplished using the following concepts:

physical volumes

These are physical disks or partitions such as /dev/sda3 or /dev/sdb.

volume groups

These are named groups of physical volumes that can be manipulated by the administrator. The number of physical volumes that make up a volume group is arbitrary. Physical volumes can be dynamically added or removed from a volume group.

logical volumes

Volume groups may be subdivided into logical volumes. Each logical volume can then be individually formatted as if it were a regular Linux partition. Logical volumes may be dynamically resized by the administrator according to need.

To give a concrete example, suppose that you have two 2 TB disks. Also suppose a really large amount of space is required for a very large database, mounted on /srv/mysql. This is what the initial set of partitions would look like: 

Partition  Use    Size      Partition Type
/dev/sda1  /boot  100MB     83 (Linux)
/dev/sda2  /       10GB     83 (Linux)
/dev/sda3  swap     2GB     82 (Swap)
/dev/sda4  LVM    remainder 8e (LVM)
/dev/sdb1  swap     2GB     82 (Swap)
/dev/sdb2  LVM    remainder 8e (LVM)

First initialize the physical volumes: 

pvcreate /dev/sda4 /dev/sdb2
[Note]

Note

A full disk can be used as part of a physical volume, but beware that the pvcreate command will destroy any partition information on that disk.

Next create a volume group named lfs-lvm: 

vgcreate lfs-lvm /dev/sda4  /dev/sdb2

The status of the volume group can be checked by running the command vgscan. Now create the logical volumes. Since there is about 3900 GB available, leave about 900 GB free for expansion. Note that the logical volume named mysql is larger than any physical disk. 

lvcreate --name mysql --size 2500G lfs-lvm
lvcreate --name home  --size  500G lfs-lvm

Finally the logical volumes can be formatted and mounted. In this example, the jfs file system (jfsutils-1.1.15) is used for demonstration purposes. 

mkfs -t ext4 /dev/lfs-lvm/home
mkfs -t jfs  /dev/lfs-lvm/mysql
mount /dev/lfs-lvm/home /home
mkdir -p /srv/mysql
mount /dev/lfs-lvm/mysql /srv/mysql

It may be needed to activate those logical volumes, for them to appear in /dev. They can all be activated at the same time by issuing, as the root user: 

vgchange -a y

A LVM logical volume can host a root filesystem, but requires the use of an initramfs (initial RAM file system). The initramfs proposed in the section called “About initramfs” allows to pass the lvm volume in the root= switch of the kernel command line.

If not using an initramfs, there is a race condition in systemd preventing mounting logical volumes through /etc/fstab. You must create a “mount” unit (see systemd.mount(5)) as in the following example, which mounts the /home directory automatically at boot: 

cat > /etc/systemd/system/home.mount << EOF
[Unit]
Description=Mount the lvm volume /dev/lfs-lvm/home to /home

[Mount]
What=/dev/lfs-lvm/home
Where=/home
Type=ext4
Options=default

[Install]
WantedBy=multi-user.target
EOF
[Note]

Note

The name of the unit must be the name of the mount point with the `/' character replaced by `-', omitting the leading one.

Next the unit must be enabled with: 

systemctl enable home.mount

For more information about LVM, see the LVM HOWTO and the lvm man pages. A good in-depth guide is available from RedHat�, although it makes sometimes reference to proprietary tools.