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Logical Volume Management (LVM) is a powerful technology that allows for flexible storage device management. It provides users with the ability to pool and abstract the physical layout of storage devices, enabling easy administration and allocation of logical units. LVM2, the current iteration of LVM, utilizes the device mapper Linux kernel framework to combine existing storage devices into groups and allocate logical volumes from the combined space as needed.

Advantages of LVM

LVM offers several advantages that make it a popular choice for storage management. One of the key advantages is increased abstraction, flexibility, and control. With LVM, logical volumes can be assigned meaningful names such as “databases” or “root-backup”. These logical volumes can also be resized dynamically as space requirements change, and can even be migrated between physical devices within the pool on a running system.

LVM also provides advanced features like snapshotting, striping, and mirroring. Snapshotting allows for the creation of point-in-time copies of logical volumes, which can be used for backups or testing purposes. Striping improves performance by spreading data across multiple physical devices, while mirroring provides data redundancy by maintaining identical copies of data on multiple devices.

LVM Architecture and Terminology

Before diving into the administrative commands and operations of LVM, it’s important to familiarize yourself with the architecture and terminology used in LVM.

LVM Storage Management Structures

LVM organizes storage devices through several layers of abstractions. The basic layers used by LVM are:

1. Physical Volumes: Physical volumes (PVs) are the raw building blocks for higher levels of abstraction in LVM. They are regular storage devices or disk-like devices, such as RAID arrays, and are identified by the prefix “pv…”. LVM writes a header to these devices to allocate them for management.
2. Volume Groups: Volume groups (VGs) are storage pools that combine physical volumes into a unified logical device. They abstract the characteristics of the underlying devices and are identified by the prefix “vg…”.
3. Logical Volumes: Logical volumes (LVs) are the primary components that users and applications interact with. They are functionally equivalent to partitions on a physical disk but offer much more flexibility. Logical volumes are identified by the prefix “lv…” or “lvm…”.

By combining physical volumes into volume groups, administrators can unify the storage space available on a system. They can then segment the volume group into arbitrary logical volumes, which act as flexible partitions.

Understanding Extents

LVM organizes volumes into fixed-size chunks called extents. Extents represent the smallest unit of allocation by LVM and are the key to its flexibility and power. Logical extents, maintained by LVM, map to physical extents on the underlying physical volumes.

LVM allows for copying and reorganizing physical extents without interrupting users, enabling seamless expansion or shrinkage of logical volumes. Extents can be added or removed from a logical volume, allowing for dynamic resizing.

Common Use Cases

Now that we have an understanding of the terminology and structures used in LVM, let’s explore some common use cases that demonstrate the power and flexibility of LVM.

Use Case: Creating Logical Volumes from Physical Disks

In this use case, we will create four logical volumes using two physical disks. We will utilize the pvcreate, vgcreate, and lvcreate commands to achieve this.

Step 1: Marking the Physical Devices as Physical Volumes

First, we need to identify the physical devices that LVM can manage. We can do this by running the lvmdiskscan command:

sudo lvmdiskscan

The output will display all available block devices that LVM can interact with. Make sure to double-check that the devices you intend to use with LVM do not have any important data already written to them, as using these devices within LVM will overwrite their current contents.

Once you have identified the devices you want to use, mark them as physical volumes within LVM using the pvcreate command:

sudo pvcreate /dev/sda /dev/sdb

Verify that LVM has registered the physical volumes by running pvs:

sudo pvs

Step 2: Adding the Physical Volumes to a Volume Group

Now that we have created physical volumes from the devices, we can create a volume group. Most of the time, you will have a single volume group per system for maximum flexibility in allocation. In this example, we will create a volume group named “LVMVolGroup” and add both physical volumes to it:

sudo vgcreate LVMVolGroup /dev/sda /dev/sdb

To check the volume group information, run pvs again:

sudo pvs

Step 3: Creating Logical Volumes from the Volume Group Pool

With the volume group created, we can now allocate logical volumes from it. In this example, we will create four separate logical volumes: “projects”, “www”, “db”, and “workspace”.

To create the logical volumes, use the lvcreate command, specifying the volume group and the desired size for each logical volume:

sudo lvcreate -L 10G -n projects LVMVolGroup
sudo lvcreate -L 5G -n www LVMVolGroup
sudo lvcreate -L 20G -n db LVMVolGroup
sudo lvcreate -l 100%FREE -n workspace LVMVolGroup

You can check the volume group information again to verify that the logical volumes have been created:

sudo vgs -o +lv_size,lv_name

Step 4: Formatting and Mounting the Logical Volumes

Now that we have created the logical volumes, we can format them with a file system and mount them as normal block devices.

Format each logical volume with the Ext4 file system:

sudo mkfs.ext4 /dev/mapper/LVMVolGroup-projects
sudo mkfs.ext4 /dev/mapper/LVMVolGroup-www
sudo mkfs.ext4 /dev/mapper/LVMVolGroup-db
sudo mkfs.ext4 /dev/mapper/LVMVolGroup-workspace

Create mount points for each logical volume:

sudo mkdir -p /mnt/projects
sudo mkdir -p /mnt/www
sudo mkdir -p /mnt/db
sudo mkdir -p /mnt/workspace

Mount the logical volumes to the appropriate mount points:

sudo mount /dev/mapper/LVMVolGroup-projects /mnt/projects
sudo mount /dev/mapper/LVMVolGroup-www /mnt/www
sudo mount /dev/mapper/LVMVolGroup-db /mnt/db
sudo mount /dev/mapper/LVMVolGroup-workspace /mnt/workspace

To make the mounts persistent, add them to the /etc/fstab file:

sudo nano /etc/fstab

Add the following lines to the end of the file:

/dev/mapper/LVMVolGroup-projects /mnt/projects ext4 defaults,nofail 0 0
/dev/mapper/LVMVolGroup-www /mnt/www ext4 defaults,nofail 0 0
/dev/mapper/LVMVolGroup-db /mnt/db ext4 defaults,nofail 0 0
/dev/mapper/LVMVolGroup-workspace /mnt/workspace ext4 defaults,nofail 0 0

Save and exit the file. The logical volumes will now be mounted automatically at boot.

Conclusion

Logical Volume Management (LVM) is a powerful technology that allows for flexible storage device management. By pooling and abstracting the physical layout of storage devices, LVM provides increased abstraction, flexibility, and control. It offers advanced features like snapshotting, striping, and mirroring, making it a popular choice for storage management.

In this article, we explored the architecture and terminology used in LVM, as well as some common use cases. We learned how to create logical volumes from physical disks, mark physical devices as physical volumes, add physical volumes to a volume group, and format and mount logical volumes.

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Christian Wells