Logical Volume Management [LVM]

What is LVM & Why Use It?

Traditional partitioning strictly binds filesystems to physical disks (e.g., /dev/sda1).
Logical Volume Manager (LVM) creates an abstraction layer between physical hardware and the operating system.

Core Advantages:

Zero-Downtime Expansion: Resize partitions on the fly without rebooting Server.
Storage Pooling: Combine multiple physical disks into a single massive virtual drive.
Snapshots: Create point-in-time backups for safe updates or data recovery.
Agility: Easily allocate, shrink, or move storage as application needs change.

🏗️ LVM Architecture

LVM operates in a strict, three-tier hierarchy: Hardware Disk → Physical Volume (PV) → Volume Group (VG) → Logical Volume (LV) → Filesystem

Physical Volumes (PV): The raw building blocks. These are physical disks (e.g., /dev/sdb) or standard partitions initialized for LVM use.
Volume Groups (VG): The storage pool. PVs are grouped together to create a single, large storage reservoir. Logic: Always leave some unallocated free space in a VG for future snapshots or LV expansion.
Logical Volumes (LV): The usable partitions. Carved out of the VG, these act as virtual hard drives that you format (ext4, xfs) and mount to your directory structure.

1️⃣ Physical Volume (PV)

A Physical Volume is:

A whole disk (e.g. /dev/sdb)
OR a partition (e.g. /dev/sdb1)
Initialized for LVM

Create a PV

sudo pvcreate /dev/sdb

View PVs

sudo pvs

Detailed:

sudo pvdisplay

2️⃣ Volume Group (VG)

A Volume Group:

Combines one or more PVs
Acts like a storage pool

Create VG

sudo vgcreate my_vg /dev/sdb

Add another disk to existing VG:

sudo vgextend my_vg /dev/sdc

View VGs

sudo vgs

Detailed:

sudo vgdisplay

3️⃣ Logical Volume (LV)

A Logical Volume:

Created inside a VG
Works like a normal partition
You format and mount it

Create LV (10GB)

sudo lvcreate -L 10G -n my_lv my_vg

Or use all free space:

sudo lvcreate -l 100%FREE -n my_lv my_vg

View LVs

sudo lvs

Detailed:

sudo lvdisplay

📦 Format and Mount Logical Volume

After creating LV:

Format

sudo mkfs.ext4 /dev/my_vg/my_lv

Create mount directory

sudo mkdir /mnt/mydata

Mount

sudo mount /dev/my_vg/my_lv /mnt/mydata

Permanent mount (fstab)

Add to /etc/fstab:

/dev/my_vg/my_lv   /mnt/mydata   ext4   defaults   0  0

📈 Resizing in LVM (🔥)

This is why LVM is powerful.

🔼 Extend Logical Volume

1️⃣ Extend LV by 5GB

sudo lvextend -L +5G /dev/my_vg/my_lv

2️⃣ Resize filesystem

For ext4:

sudo resize2fs /dev/my_vg/my_lv

For xfs:

sudo xfs_growfs /mnt/mydata

🔽 Reduce Logical Volume (Careful ⚠️)

⚠️ Always shrink filesystem first.

1️⃣ Unmount

sudo umount /mnt/mydata

2️⃣ Check filesystem

sudo e2fsck -f /dev/my_vg/my_lv

3️⃣ Shrink filesystem

sudo resize2fs /dev/my_vg/my_lv 5G

4️⃣ Reduce LV

sudo lvreduce -L 5G /dev/my_vg/my_lv

📸 LVM Snapshots

Used for:

Backups
Safe upgrades
Testing changes

Create Snapshot

sudo lvcreate -L 2G -s -n my_snapshot /dev/my_vg/my_lv

Remove Snapshot

sudo lvremove /dev/my_vg/my_snapshot

🗑️ Remove LVM Components

Remove LV

sudo lvremove /dev/my_vg/my_lv

Remove VG

sudo vgremove my_vg

Remove PV

sudo pvremove /dev/sdb

Category	Command	Purpose
Monitoring/Info	`lsblk`	List block devices and their tree structure.
	`pvs` / `vgs` / `lvs`	Quick summary of PVs, VGs, or LVs.
	`pvdisplay` / `vgdisplay` / `lvdisplay`	Detailed metadata for PVs, VGs, or LVs.
	`watch -n 2 df -h`	Continuously monitor disk space every 2 seconds.
	`blkid <path>`	Get the UUID of a block device (needed for fstab).
Physical Volumes	`pvcreate <disk>`	Initialize disk for LVM use.
	`pvremove <disk>`	Wipe LVM metadata from a disk.
Volume Groups	`vgcreate <name> <pv>`	Create a new pool from physical volume(s).
	`vgextend <name> <pv>`	Add a new physical disk to an existing pool.
	`vgremove <name>`	Delete a volume group.
Logical Volumes	`lvcreate -L 5G -n <name> <vg>`	Create a new logical volume.
	`lvextend -L +5G <lv_path>`	Increase LV size by 5GB.
	`lvextend -l +100%FREE <lv> -r`	Use all free VG space AND automatically resize filesystem.
	`lvreduce -L 5G <lv_path>`	Shrink LV size to 5GB (Shrink FS first!).
	`lvchange -an <lv>` / `-ay <lv>`	Deactivate (`-an`) or Activate (`-ay`) an LV.
	`lvremove <lv_path>`	Delete a logical volume.
Filesystems	`mkfs.ext4 <lv_path>`	Format the LV with the ext4 filesystem.
	`mount <lv_path> <mount_dir>`	Mount the filesystem to a directory.
	`umount <mount_dir>`	Unmount the filesystem.
	`mount -a`	Test `/etc/fstab` configuration to ensure safe reboots.
	`resize2fs <lv_path>`	Expand/shrink ext4 filesystem to match LV size.
	`e2fsck -f <lv_path>`	Force check filesystem for errors (required before shrinking).
Snapshots	`lvcreate -s -L 1G -n <name> <lv>`	Create a 1GB point-in-time snapshot.
	`lvconvert --merge <snap_path>`	Rollback the original LV to the snapshot’s state.
Testing/Utils	`cat /dev/zero > <file>`	Fill a file with zeros to test 100% disk usage.
	`truncate -s 0 <file>`	Empty a file instantly to free space.
	`apt clean`	Clear local package cache to free emergency space.

Simple Notes

Core Workflow: Creating LVM Storage

Step 1: Initialize Physical Volumes: sudo pvcreate /dev/sdb /dev/sdc

Step 2: Pool into a Volume Group: sudo vgcreate data_vg /dev/sdb /dev/sdc

Step 3: Carve out a Logical Volume

# Create a specific size LV
sudo lvcreate -L 10G -n my_lv data_vg

# OR use 100% of the available free space
sudo lvcreate -l 100%FREE -n my_lv data_vg

Step 4: Format and Mount

sudo mkfs.ext4 /dev/data_vg/my_lv
sudo mkdir -p /mnt/mydata
sudo mount /dev/data_vg/my_lv /mnt/mydata

(To make permanent, add to /etc/fstab: /dev/data_vg/my_lv /mnt/mydata ext4 defaults 0 0)

Resizing Operations (The Power of LVM)

1. Resize Volume Group (VG)

Scenario A: Adding a new disk to vg_1

# Initialize the new disk
sudo pvcreate /dev/nvme4n1

# Extend the Volume Group
sudo vgextend vg_1 /dev/nvme4n1

Scenario B: Resizing an existing EBS volume If you increased from 50GB to 70GB in AWS:
Terminal window
```
# Update LVM metadata to recognize the new physical size
sudo pvresize /dev/nvme1n1
```

2. Resize Logical Volume (LV)

Extending Storage (Safe & Online)
1. Add underlying space (if VG is full): sudo vgextend data_vg /dev/sdd
2. Extend the LV: sudo lvextend -L +5G /dev/data_vg/my_lv (or l +100%FREE to use all space).
3. Extend the Filesystem to match: sudo resize2fs /dev/data_vg/my_lv (Use xfs_growfs for XFS).

# 1. Extend the LV
sudo lvextend -L +10G /dev/vg_1/lv_data1

# 2. Resize the Filesystem (Online)
# For XFS (Amazon Linux default):
sudo xfs_growfs /mnt/data1

# For EXT4:
# sudo resize2fs /dev/vg_1/lv_data1

# Check LV size
sudo lvs

# Check usable disk space
df -h /mnt/data1

Reducing Storage (High Risk ⚠️)

Rule: Always shrink the filesystem BEFORE shrinking the Logical Volume to prevent data corruption.
1. Unmount: sudo umount /mnt/mydata
2. Check File System: sudo e2fsck -f /dev/data_vg/my_lv
3. Shrink File System: sudo resize2fs /dev/data_vg/my_lv 5G
4. Shrink LV: sudo lvreduce -L 5G /dev/data_vg/my_lv
Permanent Mount
- Update /etc/fstab Append these lines to the end of /etc/fstab:
  Terminal window
```
/dev/vg_1/lv_data1  /mnt/data1  xfs  defaults  0  0
/dev/vg_2/lv_data2  /mnt/data2  xfs  defaults  0  0
```
- Verify the Configuration
  
  Crucial Step: Test the fstab entries immediately. If there is a syntax error, the system may fail to boot.
  Terminal window
```
# Unmount current sessions
sudo umount /mnt/data1 /mnt/data2

# Attempt to mount everything defined in fstab
sudo mount -a

# Check if they are back
df -h | grep data
```
- Logic: mount -a reads the fstab file. If it returns no errors, your configuration is safe for reboot.

Snapshots & Recovery

Snapshots capture the exact state of an LV at a specific moment. They require free space in the VG.

Create Snapshot: sudo lvcreate -s -L 2G -n my_snap /dev/data_vg/my_lv
Restore Data (Merge):
1. Unmount original LV: sudo umount /mnt/mydata
2. Merge snapshot: sudo lvconvert --merge /dev/data_vg/my_snap
3. Reactivate LV: sudo lvchange -ay /dev/data_vg/my_lv
4. Remount: sudo mount /dev/data_vg/my_lv /mnt/mydata

Real-World & Interview Application

Scenario: You have two 500GB drives, but need an 800GB database partition.
LVM Logic: Initialize both as PVs -> Combine into one 1TB VG -> Create an 800GB LV -> Mount.
Interview Question: “A production server’s disk is at 100%. How do you fix it without downtime?”
Answer: Insert new disk -> pvcreate -> vgextend -> lvextend -> resize2fs. Total uptime maintained.

DUMMY Disks and Partitions