Chapter 10: Manage Logical Volumes

🎯 Introduction to LVM

Logical Volume Manager (LVM) provides flexible disk management. Unlike traditional partitioning, LVM allows you to resize volumes, create snapshots, and move data between physical disks without downtime.

LVM Architecture

Physical Disks (/dev/sdb, /dev/sdc)
    ↓
Physical Volumes (PV) - pvcreate
    ↓
Volume Group (VG) - vgcreate (pool of storage)
    ↓
Logical Volumes (LV) - lvcreate (virtual partitions)
    ↓
File Systems (mkfs, mount)

LVM Components

Component	Description	Example
Physical Volume (PV)	Physical disk or partition used by LVM	/dev/sdb1, /dev/sdc
Physical Extent (PE)	Smallest unit of storage (default 4MB)	4MB blocks
Volume Group (VG)	Pool of PVs	vg_data, rhel
Logical Volume (LV)	Virtual partition created from VG	/dev/vg_data/lv_web
Logical Extent (LE)	PE mapped to LV	Same size as PE

LVM Benefits

Flexible Sizing: Resize volumes online (extend XFS/ext4, shrink ext4)
Snapshots: Point-in-time backups for testing/rollback
Striping: Spread data across disks for performance
Mirroring: Data redundancy across physical volumes
Migration: Move data between disks without downtime

📘 LVM vs Traditional Partitions

Traditional: Fixed sizes, hard to resize, limited flexibility
LVM: Dynamic sizing, snapshots, pooled storage, easier management

💽 Physical Volumes (PV)

Creating Physical Volumes

# Initialize partition or disk as PV
sudo pvcreate /dev/sdb1
sudo pvcreate /dev/sdc

# Initialize multiple at once
sudo pvcreate /dev/sdb1 /dev/sdc /dev/sdd1

# Output example:
  Physical volume "/dev/sdb1" successfully created.

Viewing Physical Volumes

# List all PVs
sudo pvs

# Output:
  PV         VG     Fmt  Attr PSize  PFree
  /dev/sda2  rhel   lvm2 a--  19.00g    0 
  /dev/sdb1  vgdata lvm2 a--   5.00g 2.00g

# Detailed information
sudo pvdisplay

# Specific PV
sudo pvdisplay /dev/sdb1

# Show PE information
sudo pvs -v
sudo pvscan

Removing Physical Volumes

# Remove PV from VG first (pvmove if data exists)
sudo pvremove /dev/sdb1

# Output:
  Labels on physical volume "/dev/sdb1" successfully wiped.

💡 PV Best Practices

Use whole disks or partitions with Linux LVM type (8e for MBR, lvm flag for GPT). Label partitions with parted before creating PVs for clarity.

🗄️ Volume Groups (VG)

Creating Volume Groups

# Create VG from one PV
sudo vgcreate vg_data /dev/sdb1

# Create VG from multiple PVs
sudo vgcreate vg_data /dev/sdb1 /dev/sdc

# Specify PE size (default 4MB)
sudo vgcreate -s 8M vg_data /dev/sdb1

# Output:
  Volume group "vg_data" successfully created

Viewing Volume Groups

# List all VGs
sudo vgs

# Output:
  VG      #PV #LV #SN Attr   VSize  VFree
  rhel      1   2   0 wz--n- 19.00g    0 
  vg_data   2   1   0 wz--n-  9.99g 7.00g

# Detailed information
sudo vgdisplay

# Specific VG
sudo vgdisplay vg_data

# Show PVs in VG
sudo vgs -o +pv_name

Extending Volume Groups

# Add PV to existing VG
sudo vgextend vg_data /dev/sdd1

# Output:
  Volume group "vg_data" successfully extended

# Verify
sudo vgs vg_data

Reducing Volume Groups

# Remove PV from VG (must be empty or use pvmove first)
sudo vgreduce vg_data /dev/sdd1

# Remove missing/failed PVs
sudo vgreduce --removemissing vg_data

Removing Volume Groups

# Remove VG (must delete all LVs first)
sudo vgremove vg_data

# Output:
  Volume group "vg_data" successfully removed

Renaming Volume Groups

# Rename VG
sudo vgrename vg_data vg_backup

# Or by path
sudo vgrename /dev/vg_data vg_backup

📦 Logical Volumes (LV)

Creating Logical Volumes

# Create 2GB LV
sudo lvcreate -L 2G -n lv_web vg_data

# Create LV using all free space
sudo lvcreate -l 100%FREE -n lv_backup vg_data

# Create LV with specific extents
sudo lvcreate -l 512 -n lv_logs vg_data

# Output:
  Logical volume "lv_web" created.

LV Naming

# LVs are accessed via:
/dev/VG_NAME/LV_NAME
/dev/vg_data/lv_web

# Or via mapper:
/dev/mapper/VG_NAME-LV_NAME
/dev/mapper/vg_data-lv_web

Viewing Logical Volumes

# List all LVs
sudo lvs

# Output:
  LV      VG      Attr       LSize  Pool Origin Data%
  lv_web  vg_data -wi-a-----  2.00g                   
  root    rhel    -wi-ao---- 17.00g                   
  swap    rhel    -wi-ao----  2.00g                   

# Detailed information
sudo lvdisplay

# Specific LV
sudo lvdisplay /dev/vg_data/lv_web

# Show all attributes
sudo lvs -a -o +devices

Creating Filesystem on LV

# Create XFS filesystem
sudo mkfs.xfs /dev/vg_data/lv_web

# Create ext4 filesystem
sudo mkfs.ext4 /dev/vg_data/lv_logs

# Mount
sudo mkdir /web
sudo mount /dev/vg_data/lv_web /web

# Verify
df -h /web

Removing Logical Volumes

# Unmount first
sudo umount /web

# Remove LV
sudo lvremove /dev/vg_data/lv_web

# Confirm:
Do you really want to remove active logical volume vg_data/lv_web? [y/n]: y
  Logical volume "lv_web" successfully removed

Procedure: Complete LVM Setup

Create partition with LVM type:

sudo parted /dev/sdb mkpart primary 1MiB 5GiB
sudo parted /dev/sdb set 1 lvm on

Create Physical Volume:
```
sudo pvcreate /dev/sdb1
```
Create Volume Group:
```
sudo vgcreate vg_data /dev/sdb1
```
Create Logical Volume (2GB):
```
sudo lvcreate -L 2G -n lv_web vg_data
```
Create filesystem:
```
sudo mkfs.xfs /dev/vg_data/lv_web
```

Mount persistently:

sudo mkdir /web
echo "/dev/vg_data/lv_web /web xfs defaults 0 2" | sudo tee -a /etc/fstab
sudo mount -a

Verify:
```
df -h /web
sudo lvs
sudo vgs
```

📈 Extending Storage

Extending Logical Volumes

# Extend by specific size
sudo lvextend -L +1G /dev/vg_data/lv_web

# Extend to specific total size
sudo lvextend -L 5G /dev/vg_data/lv_web

# Extend using all free space
sudo lvextend -l +100%FREE /dev/vg_data/lv_web

# Extend and resize filesystem in one command
sudo lvextend -r -L +1G /dev/vg_data/lv_web

# Output:
  Size of logical volume vg_data/lv_web changed from 2.00 GiB to 3.00 GiB.
  Logical volume vg_data/lv_web successfully resized.

Extending File Systems

# After extending LV, extend the filesystem:

# XFS (must be mounted)
sudo xfs_growfs /web

# ext4 (can be mounted or unmounted)
sudo resize2fs /dev/vg_data/lv_web

# Check filesystem after extending
df -h /web

Extending Volume Groups

# If VG is out of space, add more PVs:

# Create new PV
sudo pvcreate /dev/sdc1

# Extend VG
sudo vgextend vg_data /dev/sdc1

# Now extend LV
sudo lvextend -L +5G /dev/vg_data/lv_web
sudo xfs_growfs /web

Procedure: Extend LV and Filesystem

Check current size:
```
df -h /web
sudo lvs /dev/vg_data/lv_web
```
Check VG free space:
```
sudo vgs vg_data
```

Extend LV by 2GB:

sudo lvextend -L +2G /dev/vg_data/lv_web

Extend filesystem (XFS):
```
sudo xfs_growfs /web
```
Verify:
```
df -h /web
sudo lvs
```

💡 Quick Extend

Use lvextend -r to automatically resize the filesystem after extending the LV. This works for both XFS and ext4: sudo lvextend -r -L +2G /dev/vg_data/lv_web

📉 Reducing Storage

⚠️ Critical Warning

XFS cannot be reduced! Only ext4 supports shrinking. Always backup data before reducing. Unmount the filesystem first. Risk of data loss if done incorrectly!

Reducing ext4 Logical Volumes

# IMPORTANT: Unmount first!
sudo umount /backup

# Check filesystem
sudo e2fsck -f /dev/vg_data/lv_backup

# Reduce filesystem first (to 3GB)
sudo resize2fs /dev/vg_data/lv_backup 3G

# Then reduce LV (to 3GB)
sudo lvreduce -L 3G /dev/vg_data/lv_backup

# WARNING prompt:
  WARNING: Reducing active logical volume to 3.00 GiB.
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce vg_data/lv_backup? [y/n]: y

# Remount
sudo mount /dev/vg_data/lv_backup /backup

# Verify
df -h /backup
sudo lvs

Safe Reduction Steps

Backup data (critical!)
Unmount the filesystem
Check filesystem integrity (e2fsck -f)
Reduce filesystem first (resize2fs)
Reduce LV to same or larger size
Remount and verify

Alternative: lvreduce with -r

# Reduce LV and filesystem together (ext4 only)
sudo umount /backup
sudo lvreduce -r -L 3G /dev/vg_data/lv_backup

# This automatically:
# 1. Checks filesystem
# 2. Reduces filesystem
# 3. Reduces LV

📘 Why XFS Can't Shrink

XFS is optimized for performance and scalability, not shrinking. If you need to reduce an XFS volume, you must: backup data → destroy LV → create smaller LV → restore data.

📸 LVM Snapshots

Snapshots provide point-in-time copies of logical volumes. Perfect for backups before system changes, testing, or rollback scenarios.

Creating Snapshots

# Create snapshot (20% of original LV size for changes)
sudo lvcreate -L 1G -s -n lv_web_snap /dev/vg_data/lv_web

# Or use percentage
sudo lvcreate -l 20%ORIGIN -s -n lv_web_snap /dev/vg_data/lv_web

# Output:
  Logical volume "lv_web_snap" created.

Viewing Snapshots

# List snapshots
sudo lvs

# Output shows snapshot with origin:
  LV           VG      Attr       LSize  Pool Origin Data%
  lv_web       vg_data owi-aos---  5.00g                   
  lv_web_snap  vg_data swi-a-s---  1.00g      lv_web 0.02

# Detailed snapshot info
sudo lvdisplay /dev/vg_data/lv_web_snap

Mounting Snapshots

# Mount snapshot for backup
sudo mkdir /snapshot
sudo mount -o ro /dev/vg_data/lv_web_snap /snapshot

# Backup from snapshot
sudo tar -czf /backup/web_backup.tar.gz -C /snapshot .

# Unmount
sudo umount /snapshot

Restoring from Snapshots

# Unmount original LV
sudo umount /web

# Merge snapshot back to original (reverts changes)
sudo lvconvert --merge /dev/vg_data/lv_web_snap

# Remount (after reboot or LV reactivation)
sudo mount /dev/vg_data/lv_web /web

Removing Snapshots

# Remove snapshot (keeps original LV)
sudo lvremove /dev/vg_data/lv_web_snap

# Confirm:
Do you really want to remove active logical volume vg_data/lv_web_snap? [y/n]: y

Procedure: Safe System Update with Snapshot

Create snapshot before update:

sudo lvcreate -L 2G -s -n root_snap /dev/rhel/root

Perform system update:
```
sudo dnf update -y
```

Test the system. If problems occur, rollback:

# Boot to rescue mode or single user
sudo umount /
sudo lvconvert --merge /dev/rhel/root_snap
sudo reboot

If update successful, remove snapshot:
```
sudo lvremove /dev/rhel/root_snap
```

Snapshot Best Practices

Size snapshots at 20-30% of origin LV for typical changes
Monitor snapshot usage (lvs shows Data% column)
Remove snapshots after use to free space
Snapshot fills = becomes invalid and is removed automatically
Use snapshots for backups, not long-term storage

⚠️ Snapshot Space

If snapshot runs out of space (Data% reaches 100%), it becomes invalid and is automatically removed. Monitor with lvs and extend if needed: sudo lvextend -L +500M /dev/vg_data/lv_web_snap

📝 Practice Questions

Question 1: What is the correct order for creating LVM storage?

A) LV → VG → PV → filesystem
B) PV → VG → LV → filesystem
C) VG → PV → LV → filesystem
D) filesystem → LV → VG → PV

Answer: B) PV → VG → LV → filesystem
Correct order: 1) pvcreate (Physical Volume), 2) vgcreate (Volume Group), 3) lvcreate (Logical Volume), 4) mkfs (filesystem), 5) mount. Think: physical → pool → virtual → format → use.

Question 2: Which filesystem can be extended but NOT reduced?

A) ext4
B) ext3
C) XFS
D) Btrfs

Answer: C) XFS
XFS can only be extended (xfs_growfs), never reduced. Use ext4 if you need shrinking capability. To "reduce" XFS, you must backup → delete → create smaller → restore.

Question 3: What command extends both LV and filesystem in one step?

A) lvextend -L +2G /dev/vg/lv
B) lvextend -r -L +2G /dev/vg/lv
C) lvresize -L +2G /dev/vg/lv
D) vgextend -L +2G /dev/vg

Answer: B) lvextend -r -L +2G /dev/vg/lv
The -r (or --resizefs) option automatically resizes the filesystem after extending the LV. Without -r, you must manually run xfs_growfs or resize2fs after lvextend.

Question 4: What happens when an LVM snapshot runs out of space?

A) System crashes
B) Snapshot becomes read-only
C) Snapshot becomes invalid and is removed
D) Original LV is corrupted

Answer: C) Snapshot becomes invalid and is removed
When snapshot space fills (Data% = 100%), it becomes invalid and LVM automatically removes it. The original LV is not affected. Monitor with lvs and extend snapshot if needed: lvextend -L +500M /dev/vg/snap

Question 5: How do you create a 3GB logical volume named 'lv_data' in 'vg_app'?

A) lvcreate -n lv_data -L 3G vg_app
B) lvcreate -L 3G -n lv_data vg_app
C) Both A and B
D) vgcreate -L 3G lv_data vg_app

Answer: C) Both A and B
Both commands are valid. Options order doesn't matter: lvcreate -L SIZE -n NAME VG or lvcreate -n NAME -L SIZE VG. The VG name must be last. Use -l for extents, -L for size.

Question 6: What must you do before reducing an ext4 logical volume?

A) Nothing, just run lvreduce
B) Unmount, check filesystem, reduce filesystem first
C) Create a snapshot
D) Extend the VG

Answer: B) Unmount, check filesystem, reduce filesystem first
Safe reduction: 1) Backup data, 2) umount, 3) e2fsck -f, 4) resize2fs to smaller size, 5) lvreduce to same or larger size. Or use lvreduce -r to automate steps 3-5. Never reduce LV before reducing filesystem = data loss!

Manage Logical Volumes (LVM)

📋 Table of Contents

🎯 Introduction to LVM

LVM Architecture

LVM Components

LVM Benefits

💽 Physical Volumes (PV)

Creating Physical Volumes

Viewing Physical Volumes

Removing Physical Volumes

🗄️ Volume Groups (VG)

Creating Volume Groups

Viewing Volume Groups

Extending Volume Groups

Reducing Volume Groups

Removing Volume Groups

Renaming Volume Groups

📦 Logical Volumes (LV)

Creating Logical Volumes

LV Naming

Viewing Logical Volumes

Creating Filesystem on LV

Removing Logical Volumes

Procedure: Complete LVM Setup

📈 Extending Storage

Extending Logical Volumes

Extending File Systems

Extending Volume Groups

Procedure: Extend LV and Filesystem

📉 Reducing Storage

Reducing ext4 Logical Volumes

Safe Reduction Steps

Alternative: lvreduce with -r

📸 LVM Snapshots

Creating Snapshots

Viewing Snapshots

Mounting Snapshots

Restoring from Snapshots

Removing Snapshots

Procedure: Safe System Update with Snapshot

Snapshot Best Practices

📝 Practice Questions

Question 1: What is the correct order for creating LVM storage?

Question 2: Which filesystem can be extended but NOT reduced?

Question 3: What command extends both LV and filesystem in one step?

Question 4: What happens when an LVM snapshot runs out of space?

Question 5: How do you create a 3GB logical volume named 'lv_data' in 'vg_app'?

Question 6: What must you do before reducing an ext4 logical volume?