LVM

Objectives

• Understand the abstraction layers of LVM.
• Learn to create and resize logical volumes.
• Understand the benefits of LVM for server administration.

1. The LVM Architecture

Logical Volume Management (LVM) provides a layer of abstraction between your physical storage and the operating system. Unlike traditional partitioning, which is static and rigid, LVM allows for flexible storage management.

The Construction Analogy

To demystify the layers, think of LVM like building with bricks:

• PV (Physical Volume): These are your Bricks. Individual disks or partitions that you prepare to be used by LVM.
• VG (Volume Group): This is the Pile of Bricks. You throw all your PVs into one big pool of storage.
• LV (Logical Volume): This is the Wall. You build specific "walls" (partitions) by taking bricks out of the pile. You can always add more bricks to the pile or make a wall taller later.

Visual Representation

The flow of data storage in LVM looks like this:

Physical Disks  --->  Physical Volumes (PV)  --->  Volume Group (VG)  --->  Logical Volumes (LV)  --->  Filesystem
(e.g., /dev/sdb)      (pvcreate /dev/sdb)          (Storage Pool)           (lvcreate)                  (mkfs.ext4)

Why this abstraction? Because the filesystem is no longer tied to the physical boundaries of a single disk, you can resize volumes, span them across multiple disks, and take "snapshots" of your data effortlessly.

2. Managing LVM: A Step-by-Step Tutorial

In this tutorial, we will prepare a new 20GB disk (/dev/sdb) for use as a web data partition.

Step 1: Create Physical Volume

First, we tell LVM that /dev/sdb is available for use.

sudo pvcreate /dev/sdb

Output: Physical volume "/dev/sdb" successfully created.

Step 2: Create Volume Group

Next, we add our "brick" to a new "pile" (storage pool) named data_vg.

sudo vgcreate data_vg /dev/sdb

Output: Volume group "data_vg" successfully created.

Step 3: Create Logical Volume

Now, we carve out a 10GB "wall" named web_data from our pool.

sudo lvcreate -L 10G -n web_data data_vg

• -L 10G: Specifies the size.
• -n web_data: Specifies the name of the LV.
• data_vg: The pool to take space from.

Step 4: Format the Logical Volume

The LV behaves just like a standard partition. We must format it before use.

sudo mkfs.ext4 /dev/data_vg/web_data

Step 5: Mount the Logical Volume

Finally, create a mount point and attach the volume.

sudo mkdir -p /var/www
sudo mount /dev/data_vg/web_data /var/www

Inspecting your work

You can verify each layer using these "scan" commands:

• sudo pvs: Shows summary of Physical Volumes.
• sudo vgs: Shows summary of Volume Groups.
• sudo lvs: Shows summary of Logical Volumes.

3. Dynamic Resizing

One of LVM's greatest strengths is the ability to grow volumes on the fly.

Scenario: The web data is filling up!

We need to add 5GB of space to our /var/www partition.

Step 1: Extend the Logical Volume

sudo lvextend -L +5G /dev/data_vg/web_data

Important: This only expands the "block device" (the container). The filesystem inside (the Ext4 structure) still thinks it's 10GB.

Step 2: Resize the Filesystem

To tell the filesystem to fill the new space:

# For Ext4 filesystems:
sudo resize2fs /dev/data_vg/web_data

# For XFS filesystems:
# sudo xfs_growfs /var/www

The "Online" Advantage

This entire process can be performed online while the volume is mounted and in use. There is no downtime for your users, which is critical for production servers.

4. Why LVM for the "Phoenix Server"?

In our "Phoenix Server" architecture, LVM is not just a tool; it's a strategy for longevity and recovery.

1. Extreme Flexibility: If /var/log starts filling up due to an application error, you can instantly grow it by taking space from a less-used volume, avoiding a system crash.
2. Resilience through Separation: By putting / (root) and /home on separate Logical Volumes, you can completely reinstall or upgrade the OS (formatting the root LV) while leaving the user data on the /home LV completely untouched.
3. Physical Abstraction: If you run out of space in your data_vg, you can simply plug in a new physical hard drive, run pvcreate and vgextend, and your storage pool grows without you ever having to unmount your filesystems.

Resources

• LVM HOWTO
• Red Hat: Configuing LVM